physic test 4
14. Discuss the advisability of simply removing the sail in the preceding questions.
A better way to create this craft would be to not include the sail at all similar to water craft boats that have the large fans on the back. If no sail were present, then the fan would push on the air and air would push back on the fan propelling the craft backward.
10. How does the buoyant force on a fully submerged object compare with the weight of water displaced?
According to Archimedes' principle, the buoyant force of a fully submerged object in water will be exactly equal to the weight of water that is displaced.
3. A car is lifted a certain distance in a service station lift and therefore has potential energy relative to the floor. If it were lifted twice as high, how much potential energy would it have?
According to the definition of potential energy, P E = mgh, the potential energy is directly proportional to the height above a reference. If the height is doubled, then the potential energy must also be doubled
2. When a ball is hit with a given force, why does contact over a long time impart more speed to the ball?
According to the impulse momentum relationship, when force is exerted for a longer period of time, the change in momentum is greater and thus a greater final speed will be reached.
6. Compared with some original speed, how much work must the brakes of a car supply to stop a care that is moving four times as fast? How does the stopping distance compare? (Consider your answer for # 18)
According to the work energy theorem, the work done by the brakes is equal to the change in kinetic energy of the car. W = KEfinal- KEinitial Since the car comes to rest the final KE is equal to zero in both cases and thus W = - KEinitial. From what we learned in question 5, the work required to stop a 2 kg car moving at 1 m/s would be W = - 1 J and the work required to stop a 2 kg car moving at 4 m/s would be W = - 16 J Thus, it requires 16 times more work to stop the car that is moving four as fast. Now since the brakes apply the same force in both cases and work can also be expressed as W = F d, since the work has increased by a factor of 16 then the distance to stop the car that is traveling four times as fast would also be 16 times as large.
3. In terms of impulse and momentum, why are nylon ropes, which stretch considerably under tension, favored by mountain climbers?
Again, the same reason as above. If a climber falls they might be traveling quickly by the time the rope begins to stop them. When the nylon ropes stretch, they slow down the climber over a larger period of time thus changing the momentum of the climber over a great period of time. According the impulse momentum relationship by increasing the time to stop, the force required will be less.
14. Before boarding an airplane, you buy a bag of chips (or any item packaged in an airtight bag) and while in flight, you notice that the bag is puffed up. Explain why this occurs.
As you increase in altitude, atmospheric pressure decreases. The pressure inside the closed package was originally the same as the pressur e at a lower altitude. As atmospheric pressure decreases the pressure inside the
2. Which, if either, requires more work - lifting a 50 kg sack a vertical distance of 2 m or lifting a 25 kg sack a vertical distance of 4 m?
Both of these require the same amount of work. In order to lift a certain amount of mass, you must apply a force at least equal to the weight of the object, w = mg. Thus, the work done to lift each is (50 kg): W = (50 kg * 10 m/s2)(2 m) = 1000 J (25 kg): W = (25 kg * 10 m/s2)(4m) = 1000 J
3. Why is it a good idea to have your hand extended forward when you are getting ready to catch a fast-moving baseball with your bare hand?
By extending you hand forward, you can "give" with the ball and slow the fast-moving baseball down over a longer period of time. The change in momentum when the ball is caught will be the same regardless of how the ball is caught but will a larger time, the force applied on the ball will be smaller and thus the force the ball exerts on the hand will be smaller
6. In boxing, shy is it advantageous to roll with the punch?
By rolling with the punches, you are increasing the time of contact and thus reducing the force of the punch
10. Would the water level in a canal lock go up or down if a battleship in the lock were to sink?
Following the same reason as above, if a battleship was originally floating in a lock it must displace a weight of water that is equal to the weight of the battleship. Since water is less dense than iron (material of the battleship), a larger volume of water must be displaced to account for the mass of the battleship. The ship is made to displace a larger amount of water. Now, when the battleship sinks, the volume of water displaced is equal to the volume of the battleship. Since the object is sinking, the buoyant force must be less than the weight of the object and thus less water is displaced when the battleship sinks. If less water is displaced, then the water level in the lock would go down .
7. How does the volume of a completely submerged object compare with the volume of water displaced?
For a completely submerged object, the volume of water displaced will be equal to the volume of the object.
1. If your friend pushes a lawn mower four times as far as you do while exerting only half the force, which one of you does more work? How much more?
For this question, it is easiest to put some numbers in. Suppose that you push a lawn mower with a force of 1 N for a distance of 1 m. The work that you do is W = F d = (1 N)(1 m) = 1 J. Your friend push the mower 4 times a far (4 m while exerting ½ the force (1/2 N). The work your friend does isW = F d = (1/2 N) (4 m) = 2 J. Your friend doesn 2 times as much work under this circumstance.
2. Two people who weigh the same climb a flight of stairs. The first person climbs the stairs in 30 s, while the second person climbs them in 40 s. Which person does more work? Which person uses more power?
If both individuals weigh the same, then they both must apply a force at least equal to their weight to get up the stairs. So let's assume that they both apply the same force over the same distance, this means that they both do the same amount of work, W = F d. Since power = work / time, the person who climbs the stairs in less time uses more power.
13. Will your answer to the preceding question be different if the air is brought to a halt by the sail without bouncing?
If the air is brought to a halt without bouncing then the answer would be different. The ice sail craft would be set in motion backward. There would not be a force on the sail and thus only the force from the air on the fan would propel the craft
15. When vertically falling sand lands in a horizontally moving cart, the cart slows. Ignore any friction between the cart and the tracks. Explain why the cart slows down in terms of conservation of momentum.
If the car is moving forwards with some velocity then it has some momentum. As mass is added to the cart, the velocity of the cart must decrease in order to keep the total momentum of the system constant. p = m v. As the mass increases the velocity decreases
3. When a rifle with a longer barrel is fired, the force of expanding gases acts on the bullet for a longer distance. What effect does this have on the velocity of the emerging bullet? (Do you see why long-range cannons have such long barrels?)
If the expanding gas applies a force on the bullet over a longer distance, this means that more work is done on the bullet (W = Fd). Now, according to the work energy theorem, W = Kd final - KEinitial. Since the bullet starts from rest, KEinitial= 0 J, when more work is done by the longer barrel this means that it will impart more kinetic energy (more speed) to the bullet, KEfinal
1. Cite an example in which a force is exerted on an object without doing work on the object.
If the object is not in motion relative then work is not done. There must be some displacement of the object. Also, if the force is applied perpendicular to the motion then that force is not doing work on the object
6. A physics instructor demonstrates energy conservation by releasing a heavy pendulum bob, as shown in the sketch, allowing it to swing to and fro. What would happen if, in his exuberance, he gave the bob a slight shove as it left his nose? Explain.
If the physics instructor gave the pendulum bob some initial shove, he would be imparting some initial velocity and thus initial kinetic energy. The end result would be that when the pendulum returned to the position of the instructor it would still be moving with some velocity and would thus strike the instructor. If the pendulum was simply released (with no initial velocity) then the pendulum would return to the position that it was released and then turn around
9. If two equal-mass sacks are lifted equal distances in the same time, how does the power required for each compare? How does the power required compare for the case in which the lighter sack is moved its distance in half the time?
If the sacks have equal mass and are lifted equal distances, then the work required to lift both sacks is the same. Since power is p=w/t If they are both lifted in the same amount of time then the power will be the same. For the question with the lighter sack, we would need to know how much lighter the sack is. If the lighter sack has ½ the mass and is lifted in ½ the time then the power would be the same. This is due to the fact that the work would be ½ as much but also the time would be ½. If the lighter sack had more than ½ the mass of the original sacks then the power would be greater and if it has less than ½ the mass of the original sacks then the power would be less
5. In karate, why is a force that is applied for a short time more advantageous?
If we consider a karate "chop", like that used to break a stack of wood, the hand is moving quickly and then comes into contact with the stack of wood. The change in momentum of the hand is the same regardless of the time frame of the "chop". If the force is applied for a short period, the force applied will be greater,
4. Why would it be a poor idea to have the back of your hand up against the outfield wall when you catch along fly ball?
If we consider the answer to question 3, if your hand was up against the outfield wall when the ball is caught the hand does not have any time to "give" with the ball and reduce the momentum of the ball over a larger period of time thus reducing the force on the ball and in turn the force on the hand
11. The momentum of an apple falling to the ground is not conserved because the external force of gravity acts on it. But momentum is conserved in a larger system. Explain.
If we include the Earth as part of the system then momentum is conserved in the apple/Earth system. We just don't see the momentum of the Earth change. The Earth exerts a force on the apple and the apple exerts an equal and opposite force back on the Earth
8. If you throw a ball horizontally while standing on roller skates, you roll backward with a momentum that matches that of the bal. Will you roll backward if you go through the motions of throwing the ball, but instead hold on to it? Explain in terms of momentum conservation.
If you hold onto the ball, you will not roll backwards. The initial momentum of the ball & skater system is zero and if momentum is conserved then the total momentum after the two interact must also be zero. In order to impart momentum to the skater, the ball must be given some momentum. If the skater does not release the ball then the ball does not get momentum.
10. Can a machine multiple input force? Input distance? Input energy? (If your three answers are the same, seek help, for the last question is especially important.
In a simple machine, the input energy (work) is equal to the output energy (work). Since work = force x distance, this means that a simple machine can change the input force to a different output force and can also change the input distance to a different output distance. In general if the output force is greater than the input force, then the output distance will be less than the input distance
7. How much force is necessary to push a nearly weightless but rigid 1-L carton beneath the surface of water?
In order to push a 1-L carton "weightless" carton below the surface of water, we must displace 1-L of water. When the 1-L of water is displac ed an upward buoyant force equal to the weight of the fluid displaced will act on the carton. We would have to apply a force equal to the upward buoyant force in order to immerse the object in the water. Now the weight of 1-L of water can be determined by knowing the density of water, ρwater= 1 kg / L and thus we can solve for the mass of water in a volume of L he weight of this amount of water is w = mg = (1 kg)(10 m/s2) = 10 N According to Archimedes' principle, the buoyant force is equal to the weight of the fluid displaced and thus it is 10 N. Thus, we would need to apply a downward force equal to 10 N in order to completely immerse the 1-L carton. The fact that is weightless is just to let us know that there is no force from gravity that is helping to submerge the object.
11. If the equally massive cars of the previous question stick together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A?
In this case, the initial momentum of car A must be shared between the two cars after the collision. Thus, the speed of the combined system will be ½ the original speed of car A.
9. The examples of the two previous exercises can be explained both in terms of momentum conservation and in terms of Newton's third law. Explain your answers to 7 & 8 in terms of Newton's 3rd law.
Inboth of the above cases, when one object exerts a force on a second object, the second object exerts an equal but opposite force back on the first according to Newton's 3rdLaw. So in the case of the person on the ice, when the person applies a force on the cloths, the cloths exert an equal force back on the person. In the case of the skater, when the skater exerts a force on the ball (throwing it), the ball exerts a force back on the skater but the skater must release the ball to apply the force.
10. Your friend says that the law of conservation of momentum is violated when a ball rolls down a hill and gains momentum. What do you say?
Momentum is conserved in the absence of any external forces acting on the objects. In other words, the objects can exert forces on one another but there can not be any other forces that don't cancel out on the objects. In the case of a ball rolling down a hill, gravity is an external force that is causing the ball to speed up. Since the gravitational force is not completely canceled by the support force (as would be the case if the ball was rolling on a flat surface) then momentum is not conserved.
5. If liquid pressure were the same at all depths, would there be a buoyant force on an object submerged in the liquid? Explain.
No, the buoyant force is a direct result of a difference in pressure between the bottom of the object and top of the object. (See question # 6 above in first section)
4. Two cars are raised to the same elevation on service station lifts. If one car is twice as massive as the other, how do their potential energies compare?
Once again, according to the definition of potential energy, PE = mgh, the potential energy is directly to the mass of the object. If the mass is doubled, then the potential energy is also doubled.
3. How does the pressure exerted by a liquid change with depth in the liquid? How does the pressure exerted by a liquid change as the density of the liquid changes?
Pressure increases with depth. As the density of the liquid increases, the pressure exerted by the liquid also increases. The pressure below the surface of salt water is greater than the pressure below fresh water since salt water is more dense that fresh water
2. Distinguish between force and pressure.
Pressure is the amount of force exerted per unit area. Force has direction and is a vector quantity. Pressure is a scalar quantity meaning that we just talk about the size of the pressure and not the direction
4. Automobiles were previously manufactured to be as rigid as possible, whereas today's autos are designed to crumple upon impact. Why?
Same reason as above, by allowing the autos to crumple the impact time has increased and thus the force has decreased.
9. What does it mean to say that momentum (or any quantity) is conserved?
Saying that momentum, or any quantity, is conserved means that it stays fixed for specific situations. This means that the value it starts off with will be the same as the value it ends with.
4. A baseball and golf ball have the same momentum. Which has the greater kinetic energy?
Since both the baseball and golf ball have the same momentum, p, but different mass, the baseball which has a greater mass would have a smaller KE and the golf ball which has a smaller mass would have a greater KE
12. The relative densities of water, ice, and alcohol are 1.0 g/cm3, 0.9 g/cm3, and 0.8 g/cm3 respectively. Ice cubes float in water but will they float higher or lower in a mixed alcoholic drink (which in general consist of alcohol and some water substance)? What can you say about a cocktail in which the ice cubes lie submerged at the bottom of the glass?
Since ice is less dense than waterbutisdenser than alcohol, if water and alcohol are mixed the density of the mixture will be less than the density of water. This means that ice will float lower in in the mixture of water and alcohol. As the density of the liquid decreases, the object will have to displace more liquid in order to remain floating.If the ice sinks, this means that there is more alcohol in the mixture as compared to water. If it was an even mixture the density would likely be 0.9g/cm3but if there is more alcohol than water then the density would be less than the density of the ice and the ice would sink
8. In the preceding question, in which case is the greatest impulse required?
Since impulse, (ΣF) t, is equal to the change in momentum, Δp, the greatest impulse will be case (3).
1. Which has greater momentum, an automobile at rest or a moving skateboard?
Since the automobile at rest has zero momentum, the moving skateboard has a larger momentum (even though it has a much smaller mass as compared to the automobile).
12. What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?
Since the efficiency is a measure of the output energy as compared to the input energy, this would be 100 % efficient. The closer the output energy is to the input energy, the closer the efficiency is to 100 %.
8. On a playground slide, a child has potential energy that decreases by 1000 J while her kinetic energy increases by 900 J. What other form of energy is involved, and how much?
Since the loss of PE is not equal to the gain in KE, some other form of energy must be involved. This is likely heat energy due to the friction between the surface of the slide and the child. Anytime friction is present, energy will not be conserved
10. Railroad car A rolls at a certain speed and makes a perfectly elastically collision with car B of the same mass. After the collision, car A is a rest. How does the speed of B after the collision compare the initial speed of A?
Since the railroad cars are identical and momentum is conserved. The momentum of car B must equal the momentum that car A started with. Car B will have the same speed as car A had before the collision. In elastic collisions when the cars are identical and one car starts from rest, after the collision, the car that was at rest will move at the speed of car that was moving before the collision.
2. In a deep dive, a whale is appreciably compressed by the pressure of the surrounding water. What happens to the whale's density?
Since the whale is compressed, its volume has decreased. Density is inversely proportional to volume and thus as the volume decreases the density increases
3. Which do you suppose exerts more pressure on the ground an elephant or a woman standing on spike heels? (Which will be more likely to make dents in a linoleum floor?)
Since the women's weight is distributed on a very small area, the woman likely exerts more pressure on the ground. The elephant weighs more but has large foot pads to distribution the weight over a larger area.
11. If a machine multiplies force by a factor of four, what other quantity is diminished, and by how much?
Since the work remains unchanged and W = F d. If the force increases by a factor of 4 then the distance, d, must diminish by a factor of 4 or in other words by ¼ the distance of the input
9. What is the difference between being immersed and being submerged?
Submerged refers to something that sinks will immersed refers to placing something below the surface of fluid. If an object is immersed, something might be holding it under the fluid but if the object is released it might float to the surface or sink
13. When the mass of a moving object is doubled with no change in speed, by what factor is its kinetic energy changed?
The KE doubles. Since KE = ½mv2, the KE is directly proportional to the mass of the object.
5. At what point in its motion is the KE of a pendulum bob at a maximum? At what point is its PE at a maximum? When its KE is half its maximum value, how much PE does it poses?
The KE is a maximum when the PE is at a minimum which occurs at the lowest position of the pendulum. The PE is a maximum at the highest position of the pendulum. At this point, the KE is zero. Both the KE and PE are ½ their maximum values when the pendulum is half way between its maximum position and minimum position.
12. Does the KE of a car change more when it goes from 10 to 20 km/h or when it goes from 20 to 30 km/h?
The KE of the a car changes more when it goes from 20 to 30 km/h. In the formula for KE= ½ mv2 the velocity is squared and thus the larger values for the velocity will have a bigger change. (20)2 - (10)2 = 400 - 100 = 300 (30)2 - (20)2 = 900 - 400 = 500
14. When the velocity of an object is doubled, by what factor is its kinetic energy changed?
The KE will increase by a factor of 4. The KE is directly proportional to the v2, therefore whatever happens to the velocity you must square that effect to see what happens to the KE. Double velocity to (2)2= 4
8. State Archimedes' principle.
The buoyant force acting on an object is equal to the weight of the fluid that is displaced.
11. A ship sailing from the ocean into a freshwater harbor sinks slightly deeper into the water. Does the buoyant force on it change? If so, does is increase or decrease?
The buoyant force acting on the ship does not change. When an object floats the buoyant force acting on the object must equal the weight of the object. Since the weight of the object didn't change the buoyant force remains the same. Now fresh water is less dense than salt water. So, when the ship enters fresh water, more water must be displaced to equal the weight of the object. Think about the fact that if something is less dense then it is less compact. Fresh water is less compact as compare to salt water. In order to get the same mass of fresh water as compared to salt water you must have a larger volume.
6. Why does buoyant force act upward on an object submerged in water?
The buoyant force comes from the difference in pressure at the bottom of the object as compared to the top of the object. Since the pressure is greater at a greater depth, the pressure at the bottom of the object is greater than the pressure at the top of the object. Pressure = Force/Area and thus there is a greater force on the bottom of the object directed upward as compared to the force acting on the top of the object acting downward
12. Does the buoyant force on a fully submerged object depend on the weight of the object or on the weight of the fluid displaced by the object? Does the force depend on the weight of the object or on its volume? Explain.
The buoyant force on a fully submerged object depends on the weight of the fluid that is displaced by the object. The weight of the fluid displaced will be affected by the volume of the object. Thus the buoyant force depends on the volume of the object. Two objects with the same mass but different volumes will displace different amounts of fluid and thus will experience different buoyant forces.
12. An ice sail craft is stalled on a frozen lake on a windless day. The skipper sets up a fan as shown. If all the wind bounces backward from the sail, will the craft be sent in motion? If so, in what direction?
The craft will not be set in motion. The fan pushes on the air and the air pushes pack on the fan. If the sail was not present, the ice sail craft would move backwards according to this picture. In this case, the air travels forward and strikes the sail pushing it forward but the sail pushes back on the air in the opposite direction. So there are two forces on the sail craft. A force from the air on the fan pushing it backward and a force from the air on the sail pushing it forward. These two gorces cancel and the ice sail craft will not move
6. Why is it inaccurate to say that heavy objects sink and that light objects float? Give exaggerated examples to support your answer.
The density of the object relative to the density of the fluid is what determines whether or not an object will float. An exaggerated example would be a penny sinks and yet a large log will float. The log obviously has a larger mass than the penny but it also has a smaller density.
4. Discounting the pressure of the atmosphere, if you swim twice as deep in water, how much more water pressure is exerted on your ears? If you swim in salt water, will the pressure be greater than in fresh water at the same depth? Why or why not?
The liquid pressure increase with depth and is proportional to how far below the surface you go. Thus, if you double the distance below the surface, the pressure on your ears would be doubled. (PLiquid= ρfluidgh). If you swim in salt water which is more dense, the pressure on your ears will be greater at the same depth since the liquid pressure also depends directly on the density of the fluid
1. Stand on a bathroom scale and read your weight. When you lift one foot up so you're standing on one foot, does the reading change? Does a scale read force or pressure?
The scale will read the same value if you lift up one foot. The scale reads force not pressure. When you lift up one foot, less area is in contact with the scale and thus the pressure on the scale has increased but the force is still the same
8. Compared to an empty ship, would a ship loaded with a cargo of Styrofoam sink deeper into water or rise in water? Explain.
The ship loaded with Styrofoam will sink deeper in the water. The buoyant force of a floating object is equal to the weight of the object. Since the loaded ship weighs more, the buoyant force must be greater and thus more water must be displaced.
4. When you are sun bathing and wading on a stony beach, why do the stones hurt your feet less when you get in deep water?
The stones hurt your feet less in deeper water. This is likely due to the fact that as you go deeper in the water, your body displaces more water and thus there is greater upward buoyant force acting on your body. You are not pushing down as hard on the rocks at in the deeper water due to the larger upward buoyant force.
1. What happens to the volume of a loaf of bread that is squeezed? What happens to the mass? What happens to the density?
The volume decreases. The mass remains the same. The density increases
5. How does water pressure one meter below the surface of a small pond compare to water pressure on meter below the surface of a huge lake?
The water pressure would be same in both cases because you are measuring the pressure in both cases at the same depth. The amount of water in the "container" doesn't affect the pressure only the depth below the surface.
9. A barge filled with scrap iron is in a canal lock. If the iron is thrown overboard, does the water level at the side of the lock rise, fall, or remain unchanged? Explain.
This is a bit more difficult. In general scrap iron will sink if place in the water. When it is on the barge, the barge is displacing enough volume of water to allow the scrap metal to float. Since iron is more dense than water, this means that the barge ship is displacing a volume of water that is greater than the iron in order to allow the iron to float on the barge. Once the iron is thrown into the water, the water displaced by the iron is less because now the only the volume of the iron is displaced. This means that the water level will actual drop when the iron is thrown in. Less water is displaced when the iron sinks as compared to when it was floating on the barge.
2. In terms of impulse and momentum, why do airbags in cars reduce the chances of injury in accidents?
This is very similar to the question above. According the impulse momentum relationship, by increasing the time to stop, the force required will be less.
7. If you push a crate horizontally with 100 N across a 10-m factory floor, and friction between the crate and the floor is a steady 70 N, how much kinetic energy is gained by the crate?
This question is asking for the change in kinetic energy. Since the work done by the net force on the crate is equal to the change in kinetic energy, we can find the work done by the forces. (W = KE final- KEinitial). By definition, the net work done is W= (ΣF)d In this case there is a horizontal force of 100 N forward and a 70 N frictional force acting backward. Thus the net force is (100N)+(-70N)= 30N the work done is then W = (30 N)(10 m) = 300 J. The gained kinetic energy is thus 300 J
5. A moving car has kinetic energy. If it speeds up until it is going four times as fast, how much kinetic energy does it have in comparison?
This question is easiest to consider by giving some numbers to mass and velocity. Let's assume the mass of the object is 2 kg and the starting speed is 1 m/s. The starting kinetic energy is then KE = ½ m v2 = ½ (2 kg)(1 m/s2)= 1 J Now, let's consider what happens when the speed increase to 4 times as fast so that v = 4 m/s. In this case the kinetic energy is KE = ½ m v2= ½ (2 kg)(4 m/s)2= 16 J The kinetic energy increases by a factor of 16. Basically, however must the velocity increases, you must square that value to see how the KE increase (4)2= 16.
15. Which if either has a greater momentum: a 1-kg ball moving at 2 m/s or a 2-kg bll moving at 1 m/s? Which has a greater kinetic energy
Two objects with identical momentum will have different KE based on the velocity of the object. The object with the larger velocity (and thus smaller mass) will have a greater KE.
7. Why does the force of gravity do work on a car that rolls down a hill, but no work when it rolls along a level part of the road.
When a car rolls down a hill, some of the motion is in the downward direction which is also the direction of the force. (Gravity is helping the car move). When part of the force is directed along the motion, work is done. In the case of a car traveling on a level road, the force of gravity is perpendicular to the motion and any force that is perpendicular to the motion does not do work on the object.
13. What weight of water is displaced by a 100-ton floating ship? What is the buoyant force that acts on the ship?
When an object floats it is in equilibrium and the downward force of gravity (weight of the object) is exactly equal to the upward buoyant force. Thus the buoyant force acting on a 100-ton floating ship is 100 -tons. According to Archimedes' principle, the buoyant force is equal to the weight of the fluid displaced and thus the weight of the water displaced will also be 100-tons.
6. A boxer can punch a heavy bag for more than an hour without tiring, but tires quickly when boxing with an opponent for a few minutes. Why? (Hint: When the boxer's fist is aimed at the bag, what supplies the impulse to stop the punches? When the boxer's fist is aimed at the opponent, what or who supplies the impulse to stop the punches that don't connect?)
When the boxer punches a heavy bag, the bag supplies the impulse to stop the punch. The boxer pushes on the bag but the bag also pushes back on the boxer. When the boxer aims at an opponent but the punch doesn't make contact, the boxer must supply the force to stop the punch. In the case of the heavy bag, the impact time to stop the punch is very short and thus the force is relatively large. On the other hand, when the boxer does not make contact with the opponent, the boxer must stop the punch over a longer period of time. The force is less than the case of the heavy bag but the force must be supplied by the boxer.
5. If you throw a raw egg against a wall, you'll break it; but, it you throw it with the same speed into a sagging sheet, it won't break. Explain, using concepts from this chapter.
When the egg is thrown against a wall, it stops very quickly in comparison to when the egg is thrown into a sagging sheet where the time to stop is larger. There will be less force on the egg that is caught in the sheet due to the time of impact being larger.
13. A half-filled bucket of water is on a spring scale. Will the reading on the scale increase or remain the same if a fish is placed in the bucket? (Will your answer be different if the bucket is initially filled to the brim when the fish is placed in the bucket?)
When the fish is placed in the half-filled bucket of water, the reading of scale will increaseby a factor of the fish's weight. If the bucket was filled to the top, when the fished is placed in the water some of the water will spill out. The amount of water that will spill out is exactly equal to the volume of the fish. Now we assume that the fish is denser than water, which means that the weight of the water that spills out will be less than the weight of the fish. (Both the fish and spilled water have the same volume but the fish is more compact and thus has a greater weight). The scale would still read an increase but it would not be as great as when the bucket was only filled half way. It would read a difference between the weight of the fish and weight of the water that spilled out.
7. A fully dressed person is at rest in the middle of a pond on perfectly frictionless ice and must get to shore. How can this be accomplished? Explain in terms of momentum conservation.
When the person is standing on the pond at rest, the initial momentum of the person is zero. If the person were to take off some of his/her cloths and toss the cloths quickly away from his/her body, the cloths would have momentum in one direction. Since momentum is conserved, the person would get equal and opposite momentum in the other direction. The person might not travel very fast but if the ice were frictionless, they would eventually reach the other end of the pond.
15. Why do you suppose that airplane windows are smaller than bus windows?
When traveling in a bus, atmospheric pressure changes very little as you go up and d own hills. The pressure inside the bus and outside the bus is easy to equalize. As a plane increases its altitude, the pressure decreases significantly. The pressure inside the airplane must be kept relatively constant since passengers can only withstand s light changes in atmospheric pressure. This means that the pressure inside the plane is very larger compared to outside the plane at high altitudes. Since Pressure = Force/Area we can write Force = Pressure x Area The windows on the plane will experience a force based on the difference in pressure between inside and outside the plane as well as the area of the window. By making the window area smaller, the force on the window is less.
11. If a 1-L container is immersed halfway in water, what is the volume of water displaced? What is the buoyant force on the container?
f the container is halfway immersed then the amount of volume of the container under the water is ½ L. This means the ½ L of water is displaced. It turns out that the density of water can be written as ρ water= 1 kg / L and thus we can solve for the mass of water in a volume of ½L. The weight of this amount of water is w = mg = (0.5 kg)(10 m/s 2) = 5 N According to Archimedes' principle, the buoyant force is equal to the weight of the fluid displaced and thus it is 5 N
1. To bring a supertanker to a stop, its engines are typically cut off about 25 km from port. Why is it so difficult to stop or turn a super tanker?
large momentum. Since the tanker needs to stop, the change in momentum will be very large. According the impulse momentum relationship, by increasing the time to stop, the force required will be less.
7. Which undergoes the greatest change in momentum: (1) a baseball that is caught, (2) a baseball that is thrown, or (3) a baseball that is caught and then thrown back, if all of the baseballs have the same speed just before being caught and just after being thrown?
loot at answer sheet
8. An apple hanging from a limb has potential energy because of its height. If the apple falls, what becomes of this energy just before it hits the ground?
the potential energy is converted into kinetic energy
