Exam 2

Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5.0 Page 2 of 7min longer. Compare the work they do

Neither of them does any work.

Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do.

Neither of them does any work.

An object is moving forward with a constant velocity. Which statement about this object MUST be true?

The net force on the object is zero.

A constant horizontal pull acts on a sled on a horizontal frictionless ice pond. The sled starts from rest. When the pull acts over a distance x, the sled acquires a speed v and a kinetic energy K. If the same pull instead acts over twice this distance:

The sled's speed will be v sqrt 2 and its kinetic energy will be 2K.

In an INELASTIC collision between two objects

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

Consider a plot of the displacement (x) as a function of the applied force (F) for an ideal elastic spring. The slope of the curve would be

the reciprocal of the spring constant

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

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

Two unequal masses M and m (M > m) are connected by a light cord passing over a pulley of negligible mass, as shown in the figure. When released, the system accelerates. Friction is negligible. Which figure below gives the correct free-body force diagrams for the two masses in the moving system?

3.12 m/s2

Two stones, one of mass m and the other of mass 2m, are thrown directly upward with the same velocity at the same time from ground level and feel no air resistance. Which statement about these stones is true?

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

Block 1 and block 2 have the same mass, m, and are released from the top of two inclined planes of the same height making 30° and 60° angles with the horizontal direction, respectively. 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 respective incline?

Block 2 is faster.

A 4.0-kg object is moving with speed 2.0 m/s. A 1.0-kg object is moving with speed 4.0 m/s. Both objects encounter the same constant braking force, and are brought to rest. Which object travels the greater distance before stopping?

Both objects travel the same distance.

A stock person at the local grocery store has a job consisting of the following five segments: (1) picking up boxes of tomatoes from the stockroom floor (2) accelerating to a comfortable speed (3) carrying the boxes to the tomato display at constant speed (4) decelerating to a stop (5) lowering the boxes slowly to the floor. During which of the five segments of the job does the stock person do positive work on the boxes?

(1) and (2)

Two stones, one of mass m and the other of mass 2m, are thrown directly upward with the same velocity at the same time from ground level and feel no air resistance. Which statement about these stones is true?

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

Two identical balls are thrown directly upward, ball A at speed v and ball B at speed 2v, and they feel no air resistance. Which statement about these balls is correct?

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

You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. The force acting on you that causes you to fall forward is

No forces were acting on you to cause you to fall.

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.

A girl throws a stone from a bridge. Consider the following ways she might throw the stone. The speed of the stone as it leaves her hand is the same in each case, and air resistance is negligible. Case A: Thrown straight up. Case B: Thrown straight down. Case C: Thrown out at an angle of 45° above horizontal. Case D: Thrown straight out horizontally. In which case will the speed of the stone be greatest when it hits the water below?

The speed will be the same in all cases.

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

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

Consider a plot of the displacement (x) as a function of the applied force (F) for an ideal elastic spring. The slope of the curve would be

he reciprocal of the acceleration of gravity.

Two bodies P and Q on a smooth horizontal surface are connected by a light cord. The mass of P is greater than that of Q. A horizontal force (of magnitude F) is applied to Q as shown in the figure, accelerating the bodies to the right. The magnitude of the force exerted by the connecting cord on body P will be

less than F but not zero.

On a smooth horizontal floor, an object slides into a spring which is attached to another mass that is initially stationary. When the spring is most compressed, both objects are moving at the same speed. Ignoring friction, what is conserved during this interaction?

momentum and mechanical energy

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

On a smooth horizontal floor, an object slides into a spring which is attached to another mass that is initially stationary. When the spring is most compressed, both objects are moving at the same speed. Ignoring friction, what is conserved during this interaction?

neither momentum nor mechanical energy

Suppose you are playing hockey on a new-age ice surface for which there is no friction between the ice and the hockey puck. You wind up and hit the puck as hard as you can. After the puck loses contact with your stick, the puck will

not slow down or speed up.

When a parachutist jumps from an airplane, he eventually reaches a constant speed, called the terminal speed. Once he has reached terminal speed

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

A 20-ton truck collides with a 1500-lb car and causes a lot of damage to the car. During the collision

the force of on the truck due to the collision is exactly equal to the force on the car.

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 shell explodes into two fragments, one fragment 25 times heavier than the other. If any gas from the explosion has negligible mass, then

the momentum change of the lighter fragment is exactly the same as the momentum change of the heavier fragment.

Two objects having masses m1 and m2 are connected to each other as shown in the figure and are released from rest. There is no friction on the table surface or in the pulley. The masses of the pulley and the string connecting the objects are completely negligible. What must be true about the tension T in the string just after the objects are released?

T < m2g

A box of mass m is pulled with a constant acceleration a along a horizontal frictionless floor by a wire that makes an angle of 15° above the horizontal. If T is the tension in this wire, then

T > ma.

One rock weighs 5 N. The other rock weighs 0.5 N. How much more force will be required to accelerate the first rock at the same rate as the second rock?

Ten times as much

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

True

Consider what happens when you jump up in the air. Which of the following is the most accurate statement?

When you push down on the earth with a force greater than your weight, the earth will push back with the same magnitude force and thus propel you into the air.

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

You cannot tell if it is moving upward or downward.

In an ELASTIC collision between two perfectly rigid objects

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

You 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.