Physics exam 2

The motion of a very massive object can be minimally affected by what would seem to be a substantial force. Consider an oil supertanker with mass 3.0*10^8 kg. Suppose you strapped two jet engines (thrust) onto the sides of the tanker. Ignoring the drag of the water (which, in reality, is not a very good approximation), how long will it take the tanker, starting from rest, to reach a typical cruising speed of 6.0 m/s?

5 hours

A car has a maximum acceleration of 10.0 m/s2. What will the maximum acceleration be if the car is towing another car of the same mass?

5m/s2

You've just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these are forces acting on the rock? List all that apply. A. Gravity, acting downward B. The normal force, acting upward C. The force of the kick, acting in the direction of motion D. Friction, acting opposite the direction of motion E. Air resistance, acting opposite the direction of motion

A, B, and D. Friction and the normal force are the only contact forces. Nothing is touching the rock to provide a "force of the kick." You can ignore air resistance unless the problem specifically calls for it.

|| Two children fight over a 200 g stuffed bear. The 25 kg boy pulls to the right with a 15 N force and the 20 kg girl pulls to the left with a 17 N force. Ignore all other forces on the bear (such as its weight). At this instant, can you say what the velocity of the bear is? If so, what are the magnitude and direction of the velocity? At this instant, can you say what the acceleration of the bear is? If so, what are the magnitude and direction of the acceleration?

A.No b. 10 m/s2 to the left

Two rubber bands stretched to the standard length cause an object to accelerate at 2 m/s2. Suppose another object with twice the mass is pulled by four rubber bands stretched to the standard length. What is the acceleration of this second object? 1 m/s2 2 m/s2 4 m/s2 8 m/s2 16 m/s2

Answer: 2 m/s^2 because Acceleration is proportional to force, so doubling the number of rubber bands doubles the acceleration of the original object from 2 m/s2 to 4 m/s2. But acceleration is also inversely proportional to mass. Doubling the mass cuts the acceleration in half, back to 2 m/s2.

A boy is using a rope to pull a sled to the right. What are the directions of the tension force and the friction force on the sled, respectively? A. Right, right B. Right, left C. Left, right D. Left, left

B

A block has acceleration a when pulled by a string. If two identical blocks are glued together and pulled with twice the original force, their acceleration will be A. (1/4)a B. (1/2)a C. a D. 2a E. 4a

C

Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on the ocean floor. If a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water so that the thrust force on the scallop is . A. upward, upward B. upward, downward C. downward, upward D. downward, downward

C

Tennis balls experience a large drag force. A tennis ball is hit so that it goes straight up and then comes back down. The direction of the drag force is Always up. Up and then down. Always down. Down and then up.

Down then up

A falcon is hovering above the ground, then suddenly pulls in its wings and begins to fall toward the ground. Air resistance is not negligible. Indentify the forces on the bird.

Drag and weight

Drag

Friction at a surface is one example of a resistive force, a force that opposes or resists motion. This kind of resistive force—the force of a fluid on a moving object—is called drag and is symbolized as D. Like kinetic friction, drag points opposite the direction of motion.

Friction

Friction, like the normal force, is exerted by a surface. Unlike the normal force, however, the frictional force is always parallel to the surface, not perpendicular to it. (In many cases, a surface will exert both a normal and a frictional force.) The symbol for friction is f. 1. Kinetic friction, denoted by fk, acts as an object slides across a surface, it is always "opposed the motion". 2. Static friction, denoted by fs, is the force that keeps an object "stuck" on a surface and prevents its motion relative to the surface. It points in the direction necessary to prevent motion.

A beach ball is thrown straight up, and some time later it lands on the sand. Is the magnitude of the net force on the ball greatest when it is going up or when it is on the way down? Or is it the same in both cases? Explain. Air resistance should not be neglected for a large, light object.

Greater on the way up

A group of students is making model cars that will be propelled by model rocket engines. These engines provide a nearly constant thrust force. The cars are light- most of the weight comes from the rocket engine- and friction and drag are very small. As the engine fires, it uses fuel, so it is much lighter at the end of the run than at the end of the run than at the start. A student ignites the engine in a car, and the car accelerates. As the fuel burns and the car continues to speed up, the magnitude of the acceleration will A increase b stay the same C decrease

Increase

If an object is not moving, does that mean that there are no forces acting on it?

No because the weight and normal forces still apply

Spring force

Springs exert one of the most basic contact forces. A spring can either push (when compressed) or pull (when stretched). The symbol for Spring Force is Fsp.

An object moves in a straight line at a constant speed. Is it true that there must be no forces of any kind acting on this object?

The total force on the object is zero.

Internal injuries in vehicular accidents may be due to what is called the "third collision" The first collision is the vehicle hitting the external object. The second collision is the person hitting something on the inside of the car, such as the dashboard or windshield. This may cause external lacerations. The third collision, possibly the most damaging to the body, is when organs, such as the heart or brain, hit the ribcage, skull, or other confines of the body, bruising the tissues on the leading edge and tearing the organ from its supporting structures on the trailing edge. A) Why is there a third collision? In other words, why are the organs still moving after the second collision? B) If the vehicle was traveling at 60 mph before the first collision, would the organs be traveling more than, equal to, or less than 60 mph just before the third collision?

The troird collision is due to the organ movement after the 2nd collision. This collosion is caused due to the law of inertia. Before any collision car, body and organs are travelling with same speed. After 1st collision car experiences a force and comes to rest suddenly. But the driver is still moving with his body and internal organs intact. Hence after the 2nd collision the external body of the driver experiences a force through the windsheild or dashboard in order to make it stop as it has gained momentum which needs to be stopped due to external force. Similarly we can say that the organs have yet to experience a force in order to stop as they are still moving inside the body with same speed as the body before 2nd collision. Hence they experince the collision with the internal body or with the pipes or blood vessel to which they are connected to. This causes rupture or serious damage in order for the organs to come at rest from such such great speed. b.) Law of inertia - Every body will remain in the state of rest or of motion (in this case motion) until and unless any external force is provided to the system. The organs would be travelling with same speed i.e. 60kmph as they have yet to experience an external force in order to have any change in speed for the law of inertia to be valid.

Thrust

Thrust occurs when a jet or rocket engine expels gas molecules at high speed. Thrust is a contact force, with the exhaust gas being the agent that pushes on the engine. Thrust is a force opposite the direction in which the exhaust gas is expelled. There's no special symbol for thrust, so we will call it Fthrust.

A baseball player is sliding into second base. Identify the forces on the baseball player.

Weight, normal force by ground, kinetic friction force by ground

A skier is sliding down a 15° slope. Friction is not negligible. Identify the forces on the skier.

Weight, normal force by slope, kinetic friction force by slope

A mountain climber is hanging from a vertical rope, far above the ground and far from the rock face. Identify the forces on the mountain climber.

Weight, tension force by rope

Give an example of the motion of an object in which the frictional force on the object is the same direction as the motion.

When a person walks on the ground, he pushes the ground in the backwards direction and the frictional force actions in the direction of motion

Tension force

When a string or rope or wire pulls on an object, it exerts a contact force that we call the tension force, represented by T. The direction of the tension force is always in the direction of the string or rope.

Alyssa pushes to the right on a filing cabinet; the friction force from the floor pushes on it to the left.Because the cabinet doesn't move, these forces have the same magnitude. Do they form an action/reaction pair?

according to Newton's third law of motion: in every interaction between two objects, there are equal and opposite forces on each of them i.e. for every action , there is an equal (in magnitude) and opposite(in direction) reaction. these two objects form an action-reaction pair then most important point to notice here is the two forces i.e. the action force and the reaction force are acting on two different objects , not on the same object but in the given situation, the pushing force and the friction force are acting on the same object i.e the filing cabinet. hence they dont form an action-reaction pair.

Figure 4.29 shows block A sitting on top of block B. A constant force, F is exerted on block B, causing block B to accelerate to the right. Block A rides on block B without slipping. Which statement is true? A Block exerts a friction force on block A B Block B exerts a friction force on block A, directed to the right C Block B does not exert a friction force on block A

b

4.27 as shown in the chapter, scallops use jet propulsion to move from one place to another. Their shells make them denser than water , so they normally rest on the ocean floor. If a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water _____ so that the thrust force on the scallop is ____ A upward, upward B upward, downward C downward, upward D downward, downward

c

A frog is resting on a slope, what can you say about the friction force acting on the frog? a. There is no friction force b. There is a kinetic friction force directed pu the slope c. There is a static friction force directed up the slope d. There is a kinetic friction force directed down the slope e. there is a static friction force directed down the slope

c

Dave pushed his four-year old son, Thomas across the snow on a sled. As dave pushed, Thomas speeds up. Which statement is true? A the force of dave of Thomas is larger than the force of Thomas on dave B the force of thomas on dave is larger than the force of dave on thomas C both forces have the same magnitude D it depends on how hard dave pushes Thomas

c

a person gives a box a shove so that it slides up a ramp, then it reverses its motion and slides down. The direction of the force of friction is A always down the ramp B up the ramp and then down the ramp c always down the ramp d down the ramp and then up the ramp

d

a person is pushing horizontally on a box with a constant force, causing it to slide across the floor with a constant speed. If the person suddenly stops pushing on the box, the box will A immediately come to a stop b continue moving at a constant speed for a while, then gradually slow down to a stop C immediately change to a slower but constant speed D immediately begin slowing down and eventually stop

d

Normal force

the force exerted by a surface (the agent) against an object that is pressing against the surface. The symbol for the normal force is n. The normal force is perpendicular to the surface.

Suppose you are an astronaut in deep space, far from any source of gravity. You have two objects that look identical, but one has a large mass and the other a small mass. How can you tell the difference between the two?

the object with small mass will have high acceleration and moves towards the object with higher mass because the acceleration is inversely proportional to mass. Then, we can simply say that the object moving faster has small mass and the object moving slower than the other object has the higher mass.