Physics Exam 2 - Conceptual Questions

A ball drops some distance and loses 30 J of gravitational potential energy. Do NOT ignore air resistance. How much kinetic energy did the ball gain?

Less than 30 J.

A brick is resting on a rough inclined surface. The friction force acting on the brick, along the incline, is:

Less than the weight of the brick.

A ball attached to a string hangs from a rear-view mirror of a car. The car is driving in a circle at a constant speed. What forces act directly on the ball?

Tension and gravity.

A 615 N student on a scale in an elevator notices that the scale reads 645 N. From this the student knows that the elevator must be moving:

You cannot tell if it is moving upward or downward.

A stalled car is being pushed up a hill at constant velocity by 3 people. Net force on the car is:

Zero.

Two friction-less water slides have the same height, h, slide 1 is straight down, while slide 2 is curved down before leveling out. How does the velocity V1 of a swimmer reaching the bottom of slide 1 compare to the velocity V2 of a swimmer reaching the bottom of slide 2.

V1=V2

A stock person does the following at their job: 1) Picks up boxes from the stockroom floor. 2) Accelerates to a comfortable speed 3) Carries boxes to a display at constant speed 4) Decelerates to a stop 5) Lowers the boxes slowly to the floor. During which of these is positive work done on the boxes?

1) and 2) only.

Which of the following statements concerning work done by a conservative force is true or not true? -When the starting and ending points are the same, the total work is zero. -It can always be expressed as the difference between the initial and final values of a potential energy function. -It is independent of the path of the body and depends only on the starting and ending points.

All of the statements are true.

A box mass m is pressed against a spring with force constant k, spring is compressed a distance of x. After its release the box slides up a frictionless incline and eventually stops. If we repeat this experiment with a force constant of 2k:

All of these are true: The box will go up the incline twice as high as before. Just as it moves free of the spring, the speed of the box will be √2 times as great as before. Just as it moves free of the spring, the kinetic energy of the box will be twice as great as before.

Two balls, one mass m, and the other mass 2m, are thrown directly upward with the same velocity, at the same time and feel no air resistance:

At its highest point, the heavier ball will have twice as much gravitational potential energy as the lighter ball because it is twice as heavy.

Work.

Work can be positive, negative or zero.

Two identical balls are thrown directly upward, ball A at speed v and ball B at speed 2v, they feel no air resistance:

Ball B will go four times as high as ball A because it had four times the initial kinetic energy.

Block 1 and Block 2 have same mass m, and are released from the top of two inclined planes of the same height. Plane 1 is at a 30° and plane 2 is at a 60° angle with the horizontal. If the coefficient of friction is the same in both cases, which of the blocks is going faster when it reaches the bottom of its incline?

Block 2, going down the plane with a 60° angle is faster.

Is it possible for a system to have negative potential energy?

Yes, since the choice of the zero of potential energy is arbitrary.

In a collision between two objects having unequal masses, how does magnitude of the impulse imparted to the lighter object by the heavier one compare with the magnitude of the impulse imparted to the heavier object by the lighter one?

Both objects receive the same impulse.

Seated in a bus, a hand strap hanging from the ceiling hangs away from the vertical in the backward direction. From this observation you can conclude:

You cannot conclude anything about the direction of the velocity of the bus.

A 4-kg object is moving with speed 2 m/s. A 1-kg object is moving with speed 4 m/s. Both objects encounter same braking force and are brought to rest. Which object travels the greater distance before stopping?

Both objects travel the same distance because their kinetic energy is equal using: 1/2mv^2

You are on a skateboard, at rest and a friend throws a heavy ball at you. You can catch or deflect the ball (so that it moves away from you with the same speed as it was initially thrown). What should you do in order to MINIMIZE your speed on the skateboard?

Catch the ball.

Swing a bat and hit a heavy box with a force of 1500 N. The force the box exerts on the bat is:

Exactly 1500 N whether or not the box moves.

If you swing a bucket of water fast enough in a vertical circle, at the highest point the water does not spill out because an outward force balances the pull of gravity on the water, True or False.

FALSE.

A ball drops some distance and gains 30 J of kinetic energy. Do NOT ignore air resistance. How much gravitational potential energy did the ball lose?

More than 30 J.

Two men push against a wall. One stops after 10 min, while the other continues to push for 5 min longer. Compare the work they do.

Neither of them does any work (the wall does not move).

On ice with no friction, you hit a hockey puck as hard as you can. After the puck loses contact with your stick, the puck will:

Not slow down or speed up.

A woman strains to lift a heavy crate without success. P, is upward force the woman exerts, C is the vertical contact force exerted on the crate by the floor and W is the weight of the crate. How are these forces related while the woman unsuccessfully attempts to lift the crate?

P+C=W

Suppose the force of air drag on an object is proportional to the speed of the object and in the direction opposite the object's velocity. If you throw an object upward, the magnitude of its acceleration is greatest:

Right after the object is released.

Box, mass m is pulled with constant acceleration along a horizontal friction-less floor by a wire that makes an angle of 15-degrees above the horizontal. If T is the tension in the wire, then:

T > ma.

If a force always acts perpendicular to an object's direction of motion, that force cannot change the object's kinetic energy, True or False.

TRUE.

When an object is just under the influence of conservative forces (gravity and springs) the sum of its kinetic and potential energies does not change, true or false?

TRUE.

A small car has a head on collision with a large truck. Which of the following statements concerning the magnitude of the average force due to the collision is correct?

The small car and the truck experience the same average force.

In a perfectly ELASTIC collision between two perfectly rigid objects:

Both the momentum and the kinetic energy of the system are conserved.

When a parachutist jumps from a plane, he eventually reaches constant speed (terminal speed. At this speed:

The force of air drag on him is equal to his weight.

A car going 30 mph crashes into another car going 30 mph. Second scenario: a car going 30 mph crashes head on into a stationary brick wall. In neither case does the car bounce off the thing it hits, and collision time is the same in both cases. Which of these scenarios would result in the greatest impact force?

The force would be the same in both cases.

Two objects, one of mass m and the other of mass 2m, are dropped from the top of a building. When they hit the ground:

The heavier one will have twice the kinetic energy of the lighter one.

A 1-kg block and a 2-kg block are pressed together on a horizontal frictionless surface with a compressed spring between them with negligible mass. After they are released and move free from the spring:

The lighter block will have more kinetic energy than the heavier block.

A box of mass m is pressed against a compressed spring, after it is released the box travels up a frictionless incline and eventually stops. If we repeat this with a box of mass 2m:

The lighter box will go twice as high up the incline as the heavier box.

A shell initially at rest explodes into 2 fragments, one fragment is 25 times heavier than the other.

The momentum change of the lighter fragment is exactly the same as that of the heavier fragment. The kinetic energy change of the lighter fragment is 25 times as great as that of the heavier fragment.

In an INELASTIC collision between two objects:

The momentum of the system is conserved but the kinetic energy of the system is not conserved.

Object is moving forward, with a constant velocity. What is true?

The net force on the object is zero.

Two objects have masses m and 5m. They are both placed side by side on a friction-less inclined plane and allowed to slide from rest.

The two objects reach the bottom of the incline at the same time.

A ball is tossed vertically upward. When it reaches its highest point (before falling back down:

The velocity is zero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward.

A 3-kg ball swings rapidly in a completely vertical circle of radius 2 m by a light string fixed at one end. The string is always taut and perpendicular to the velocity of the ball. As the ball swings from its lowest point to its highest point:

The work done on it by gravity is -118 J and the work done on it by the tension in the string is zero. Uf-Ui = (0-2mgh).

Three cars (F,G,H) move with the same velocity when they all suddenly brake and come to a stop. The biggest car is F, the smallest car is H, all cars have identical tires. Which car travels the longest distance before coming to a stop?

They all travel the same distance in stopping.