COTC Physics 105 Exam 1 Review

Show that a machine that has an input of 100 J and an output of 40 J is 40% efficient.

(40 / 100) x 100% = 40%

What is the acceleration of a car that maintains a constant velocity of 100 km/h for 10 s? (Why do some of your classmates who correctly answer the preceding question get this question wrong?)

0 (because velocity is constant)

Comic-strip Superhero meets an asteroid in outer space and hurls it at 800 m/s, as fast as a speeding bullet. The asteroid is a thousand times more massive than Superhero. In the strip, Superhero is seen at rest after the throw. Taking physics into account, what would be the speed of his recoil?

0 = m x (vf) + 1000m x 800 m/s vf = -800,000 m/s

What is the net force on a bag pulled down by gravity with a force of 18 newtons and pulled upward by a rope with a force of 18 newtons?

0 N

What is the net force on an object in either static or dynamic equilibrium?

0 N

Calculate the acceleration of a 2000-kg, single-engine airplane as it begins its takeoff with an engine thrust of 500 N. (The unit N/kg is equivalent to m/s2.)

0.25 m/s2

Calculate the acceleration of a 300,000-kg jumbo jet just before takeoff when the thrust on the aircraft is 120,000 N.

0.4 m/s2

What is the weight of a 1-kilogram brick resting on a table?

10 N

Consider these measurements: 10 m, 10 m/s, and 10 m/s2. Which is a measure of speed, which of distance, and which of acceleration?

10 m/s is speed. 10 m is distance, 10 m/s2 is acceleration.

How far does a horse travel if it gallops at an average speed of 25 km/h for 30 min?

12.5 km

This question is typical on some driver's license exams: A car that was moving at 50 km/h skids 15 m with locked brakes. How far will the car skid with locked brakes if it was moving at 150 km/h?

135 m

Calculate the magnitude of the resultant of a pair of 100-km/h velocity vectors that are at right angles to each other.

141.4 km/h

Betty (mass 40 kg), standing on slippery ice, catches her leaping dog (mass 15 kg) moving horizontally at 3.0 m/s. Show that the speed of Betty and her dog after the catch is about 0.8 m/s.

15 x 3 = 55 x (V) V = 45 / 55 = 0.82 m/s

A car carrying a 75-kg test dummy crashes into a wall at 25 m/s and is brought to rest in 0.1 s. Show that the average force exerted by the seat belt on the dummy is 18,750 N.

18,750 N

A 2-kg blob of putty moving at 3 m/s slams into a 2-kg blob of putty at rest. Show how the speed of the two stuck-together blobs of putty immediately after colliding is 1.5 m/s.

2 x 3 = 4 x (V) V = 6 / 4 = 1.5 m/s

Forces of 3.0 N and 4.0 N act at right angles on a block of mass 2.0 kg. Show that the acceleration of the block is 2.5 m/s2.

2.5 m/s2

How much impulse stops a 50-kg carton that is sliding at 4 m/s when it meets a rough surface?

200 kg m/s

What is the momentum of a 50-kg carton that slides at 4 m/s across an icy surface?

200 kg m/s

What will be the speed of an airplane that normally flies at 200 km/h when it encounters a 80-km/h wind from the side (at a right angle to the airplane).

215.4 km/h

What impulse occurs when an average force of 10 N is exerted on a cart for 2.5 s?

25 Ns

Calculate the resultant of the pair of velocities 100 km/h north and 75 km/h south. Calculate the resultant if both of the velocities are directed northward.

25 km/h, north; 175 km/h, north

What is the average speed in kilometers per hour of a horse that gallops a distance of 15 km in a time of 30 min?

30 km/h (0.5 km/min)

If you push a crate horizontally with 100 N across a 10-m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?

300 J

A railroad diesel engine weighs four times as much as a freight car. If the diesel engine coasts at 5 km/h into a freight car initially at rest, show that the speed of the coupled engine and car is 4 km/h.

4m x 5 km/h + m x 0 m/s = (m + 4m) x V 4m x 5 km/h = 5m x V V = 4 km/h

Calculate the resultant of a horizontal vector with a magnitude of 4 units and a vertical vector with a magnitude of 3 units.

5

A 5-kg fish swimming at 1 m/s swallows an absentminded 1-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Show that the speed of the approaching smaller fish before lunch must have been 5 m/s.

5 kg x 1 m/s + 1 kg x v = 0 v = -5 m/s

A different scaffold that weighs 400 N supports two painters, one 500 N and the other 400 N. The reading in the left scale is 800 N. What is the reading in the right-hand scale?

500 N

Calculate the work done when a force of 5 N moves a book 1.2 m. (Remember that 1 N⋅m = 1 J.)

6 J

If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?

60%

Show that about 786 W of power is expended when a 500-N barbell is lifted 2.2 m above the floor in 1.4 s.

786 W

Belly-flop Bernie dives from atop a tall flagpole into a swimming pool below. His potential energy at the top is 10,000 J (relative to the surface of the pool). What is his kinetic energy when his potential energy is reduced to 1000 J?

9000 J

Calculate the kinetic energy of an 84-kg scooter moving at 10 m/s.

KE = 4,200 J

Show that when a 3.0-kg book is lifted 2.0 m its increase in gravitational potential energy is 60 J.

PE = mgh = 3 x 10 x 2 = 60 J

Calculate the work done in lifting a 500-N barbell 2.2 m above the floor. (What is the gain of potential energy of the barbell when it is lifted to this height?)

W = 1,100 J, which is also the gain in PE.

The second floor of a house is 6 m above the street level. How much work is required to lift a 300-kg refrigerator to the second-story level?

W = 18,000 J

What exactly is meant by a "freely falling" object?

a freely falling object is one that experiences *only* the force of gravity. as such, it falls with no air resistance.

What kind of speed is registered by an automobile speedometer: average speed or instantaneous speed?

a speedometer registers instantaneous speed.

A stone is shown at rest on the ground. (a) The vector shows the weight of the stone. Complete the vector diagram showing another vector that results in zero net force on the stone. (b) What is the conventional name of the vector you have drawn?

a) the other vector is upward as shown: b) this is called the normal force

You hold an apple over your head. (a) Identify all the forces acting on the apple and their reaction forces. (b) When you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces. Ignore air drag.

a) two force pairs act: earth's pull on apple (action) and apple's pull on earth (reaction); and hand pushes apple upward (action), and apple pushes hand downward (reaction). b) with no air drag, one force pair acts: earth's pull on apple, and apple's pull on earth.

(a) How much work is done when you push a crate horizontally with 100 N across a 10-m factory floor? (b) If the force of friction on the crate is a steady 70 N, show that the KE gained by the crate is 300 J. (c) Show that 700 J is turned into heat.

a) you do 1000 J of work b) KE change = net force x d = 30 N x 10 m = 300 J c) heat = work from friction = 70 N x 10 m = 700 J

What is the gain in speed per second for a freely falling object?

about 10 m/s

How does the direction of acceleration compare with the direction of the net force that produces it?

acceleration and net force are in the same direction.

Is acceleration proportional to net force, or does acceleration equal net force?

acceleration and net force are proportional to each other, not equal to each other.

Is acceleration directly proportional to mass, or is it inversely proportional to mass? Give an example.

acceleration is inversely proportional to mass.

Consider hitting a baseball with a bat. If we call the force on the bat against the ball the action force, identify the reaction force.

action: bat against the ball. reaction: ball against the bat.

The auto in the sketch moves forward as the brakes are applied. A bystander says that during the interval of braking, the auto's velocity and acceleration are in opposite directions. Do you agree or disagree?

agree. acceleration is opposite the direction the car is moving

What does it mean to say something is in mechanical equilibrium?

all of the forces on an object in mechanical equilibrium add up to zero.

If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?

as force is increased, distance is decreased by the same factor.

Why do we say that force is a vector quantity?

because it has a magnitude *and* a direction

For the same force, why does a long cannon impart more speed to a cannonball than a short cannon?

because the cannonball receives the force from the explosion for a longer period of time

To impart the greatest momentum to an object, should you exert the largest force possible, extend that force for as long a time as possible, or both? Explain.

both. change in momentum = F x t

Show that a 2,500,000-J change in kinetic energy occurs for an airplane that is moved 500 m in takeoff by a sustained force of 5000 N.

change in KE = 2,500,000 J

Show that a freely falling rock drops a distance of 45 m when it falls from rest for 3 s.

d = ½ x g x t^2 = ½ x 10 x 3^2 = 45 m

What two units of measurement are necessary for describing speed?

distance and time

Exactly what is it that enables an object to do work?

energy.

The sketch shows a painter's scaffold in mechanical equilibrium. The person in the middle weighs 500 N, and the tensions in each rope are 400 N. What is the weight of the scaffold?

from the equilibrium rule, Σ F = 0, the upward forces are 800 N, and the downward forces are 500 N + the weight of the scaffold. so the scaffold must weigh 300 N.

If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare? How about for the case in which the lighter sack is moved its distance in half the time?

if they are lifted in the same amount of time, the power will be the same. if one of the objects is lifted in half the time, the power will double

Why is it advantageous to roll with the punch in boxing?

if you roll with the punch, the impact occurs over a longer period of time, which means a less forceful punch.

What are the two ways to increase impulse?

impulse can be increased by increasing the force or increase the time of application of the force.

If a Mack truck and Honda Civic have a head-on collision, upon which vehicle is the impact force greater? Which vehicle experiences the greater deceleration? Explain your answers.

in accord with newton's third law, the force on each is of the same magnitude. by the effect of the force (acceleration) on each is different because of the different masses. the more massive mack truck experiences less of an acceleration than the civic.

What is the effect of air resistance on the acceleration of falling objects?

it decreases their acceleration

If a car is moving at 90 km/h and it rounds a corner, also at 90 km/h, does it maintain a constant speed? A constant velocity? Defend your answers.

it maintains a constant speed, but not a constant velocity

If the net force acting on a sliding block is somehow tripled, what happens to the acceleration?

it triples

A car is raised a certain distance in a service-station lift and therefore has potential energy relative to the floor. If it were raised twice as high, how much more potential energy would it have?

it would have twice the PE.

When you push horizontally on a crate on a level floor that doesn't slide, how great is the force of friction on the crate?

it's magnitude is equal to your push

Fill in the blanks: The Standard International unit for mass is the ______ . The Standard International unit for force is the______ .

kg, N

Which is more fundamental: mass or weight? Which varies with location?

mass is more fundamental - it's a property of an object weight can vary with location

Can you physically touch a person without that person touching you with the same amount of force?

no

As you read this in your chair, how fast are you moving relative to the chair? Relative to the Sun?

relative to the chair, your speed is zero. relative to the sun your speed is 30 km/s.

When a car is moving in reverse, backing from a driveway, the driver applies the brakes. In what direction is the car's acceleration?

since braking causes the car to slow down, the direction of the acceleration will be in the opposite direction that the car is moving

Lucy Lightfoot stands with one foot on one bathroom scale and her other foot on a second bathroom scale. Each scale reads 350 N. What is Lucy's weight?

since each scale reads 350 N, lucy's total weight is 700 N.

How does the force of friction for a sliding object vary with speed?

sliding friction doesn't vary much with the speed of the object

What is the main difference between speed and velocity?

speed has only a magnitude, velocity has magnitude and direction

What type of path does a moving object follow in the absence of a force?

straight line, constant speed

If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change?

the acceleration decreases to one-third.

What is the acceleration of a car moving along a straight road that increases its speed from 0 to 100 km/h in 10 s?

the acceleration is 10 km/h/s

State Newton's second law of motion.

the acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass. a = F / m

If the mass of a sliding block is somehow tripled at the same time the net force on it is tripled, how does the resulting acceleration compare with the original acceleration?

the acceleration won't change

What is the distance fallen for a freely falling object 1 s after being dropped from a rest position? What is the distance for a 4-s drop?

the distance of fall in 1 s is 5 m. for a 4 second fall the distance traveled is 80 m.

If you push on a crate with a force of 100 N and it slides at constant velocity, how great is the friction acting on the crate?

the force of friction is 100 N.

If you exert a horizontal force of 200 N to slide a crate across a factory floor at constant velocity, how much friction is exerted by the floor on the crate? Is the force of friction equal and oppositely directed to your 200-N push? If the force of friction isn't the reaction force to your push, what is?

the friction on the crate is 200 N, which cancels the 200 N push on the crate to yield zero net force. this accounts for the constant velocity (zero acceleration). although the friction force is equal and oppositely directed to applied force, the two do not make an action/reaction pair of forces. this is because both forces act on the same object (the crate). the reaction to your push on the crate is the crate pushing back on you. the reaction to the friction force of the floor on the crate is the opposite friction force of the crate on the floor.

Which is normally greater: static friction or sliding friction on the same object?

the maximum value of static friction is greater than sliding friction for a given object.

Which has a greater momentum: a heavy truck at rest or a moving skateboard?

the moving skateboard has a greater momentum because only it is moving.

What is the net force on an object that is pulled with forces of 80 newtons to the right and 80 newtons to the left?

the net force is 0.

What is the net force on a cart that is pulled to the right with 100 pounds of force and to the left with 30 pounds of force?

the net force is 70 lb. to the right.

Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, compare their gains of potential energy.

the one that is twice as massive will have twice the PE

What is the speed acquired by a freely falling object 5 s after being dropped from a rest position? What is the speed 6 s after?

the speed acquired in 5 s is 50 m/s. in 6 seconds it's 60 m/s

Consider a book that weighs 15 N at rest on a flat table. How many newtons of support force does the table provide? What is the net force on the book in this case?

the support force is 15 N. the net force on the block is 0.

The same stone is being accelerated vertically upward. (a) Draw force vectors to some suitable scale showing relative forces acting on the stone. (b) Which is the longer vector, and why?

the upward tension force is greater, resulting in an upward net force and upward acceleration.

If you stand next to a wall on a frictionless skateboard and push the wall with a force of 40 N, how hard does the wall push on you? If your mass is 80 kg, show that your acceleration is 0.5 m/s2.

the wall pushes on you with a force of 40 N.

Which 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?

the work is the same

When you stand at rest on a bathroom scale, how does your weight compare with the support force by the scale?

weight = support force

Here the stone is sliding down a friction-free incline. (a) Identify the forces that act on it, and draw appropriate force vectors. (b) Use the parallelogram rule to construct the resultant force on the stone (carefully showing that it has a direction parallel to the incline—the same direction as the stone's acceleration).

weight vector straight down, normal force perpendicular to the ground only part a of this question needs to be done

A stone is suspended at rest by a string. (a) Draw force vectors for all the forces that act on the stone. (b) Should your vectors have a zero resultant? (c) Why or why not?

weight vector straight down, tension vector straight up. the vectors should have the same magnitude since the stone's motion isn't changing (it's at rest) and therefore no net force acts on it

What will be the kinetic energy of a pile driver ram that starts from rest and undergoes a 10-kJ decrease in potential energy?

when falling, PE is converted to KE. when it hits the ground, the energy is converted to heat because of friction between the ground and the object

State Newton's third law of motion.

whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, and an impulse changes the momentum of that object. What do we call the quantity force multiplied by distance?

work

Earth pulls down on you with a gravitational force that you call your weight. Do you pull up on Earth with the same amount of force?

yes

If a car moves with a constant velocity, does it also move with a constant speed?

yes. a car moving with a constant velocity moves with a constant speed.

As you increase your push, will friction on the crate increase also?

yes. as you increase your push, static friction increases just as much.

A bowling ball at rest is in equilibrium. Is the ball in equilibrium when it moves at constant speed in a straight-line path?

yes. the ball moving at constant speed in a straight line is in dynamic equilibrium.

Can a machine multiply input force? Input distance? Input energy? (If your three answers are the same, seek help; the last question is especially important.)

yes. yes. no.

When are you most aware of your motion in a moving vehicle: when it is moving steadily in a straight line or when it is accelerating? If you were in a car that moved with absolutely constant velocity (no bumps at all), would you be aware of motion?

you are aware of changes in your speed, but not steady motion.

Once the crate is sliding, how hard do you push to keep it moving at constant velocity?

your push needs to have the same magnitude as the sliding friction force (which is less than the amount of force that was needed to get it moving)