PHY031-OQuiz-M01-W02-01

Which one of the following choices represents the largest kinetic energy?

• Mars is moving in its orbit around the Sun.

• A cyclist is racing in the annual Tour de France bicycle race.

• A leaf falls from a tree.

• A cheetah runs at its maximum speed to catch a fleeing zebra.

• An oil tanker sails through the Panama Canal.

A high school baseball pitcher can typically throw a ball at 22 m/s. Professional baseball pitchers can throw the ball with twice that speed, but few others can. To see why this is the case, determine the difference in the kinetic energy of a baseball thrown at v m/s and one thrown at 2v m/s and express the difference as a percentage.

• 50 %

• 100 %

• 200 %

• 300 %

• 400 %

Ignoring friction effects, the amount of energy required to accelerate a car from rest to a speed v is E. The energy is delivered to the car by burning gasoline. What additional amount of energy is required to accelerate the car to a speed 2v?

• 0.5E

• E

• 2E

• 3E

• 4E

Some children are practicing catching baseballs. The coach tosses the ball into the air and hits the ball with his bat. The ball travels nearly horizontally, directly at the short stop who manages to catch the line drive. Did the coach, via the bat, do any work on the ball as it was hit?

• No, it travels nearly horizontally, and no work is done.

• Yes, work was done on the ball because during the time the force acted on the ball, the bat and ball moved through some distance.

• No, there was a force acting on the ball, but there was no displacement while the force was acting.

• Yes, work was done on the ball because the force of gravity was acting on the ball while it was being hit.

• No work was done on the ball because the ball flew even though the force was no longer acting on it.

Two wooden blocks are sliding with the same kinetic energy across a horizontal frictionless surface. Block A has a mass m and block B has a mass 2m. At time t = 0 s, the blocks both slide onto a horizontal surface where the kinetic coefficient of friction between the blocks and surface is µk. Let xA represent the distance that block A slides before coming to a stop; and xB the distance that block B slides before it stops. Which one of the following expressions concerning these distances is correct?

• x_A = x_B

• x_A = 2x_B

• x_A = 0.5x_B

• x_A = 0.25x_B

• x_A = 4x_B

An elevator supported by a single cable descends a shaft at a constant speed. The only forces acting on the elevator are the tension in the cable and the gravitational force. Which one of the following statements is true?

• The work done by the tension force is zero joules.

• The net work done by the two forces is zero joules.

• The work done by the gravitational force is zero joules.

• The magnitude of the work done by the gravitational force is larger than that done by the tension force.

• The magnitude of the work done by the tension force is larger than that done by the gravitational force.

A mountain climber pulls a supply pack up the side of a mountain at constant speed. Which one of the following statements concerning this situation is false?

• The net work done by all the forces acting on the pack is zero joules.

• The work done on the pack by the normal force of the mountain is zero joules.

• The work done on the pack by gravity is zero joules.

• The gravitational potential energy of the pack is increasing.

• The climber does "positive" work in pulling the pack up the mountain.

Two balls of equal size are dropped from the same height from the roof of a building. One ball has twice the mass of the other. When the balls reach the ground, how do the kinetic energies of the two balls compare?

• The lighter one has one fourth as much kinetic energy as the other does.

• The lighter one has one half as much kinetic energy as the other does.

• The lighter one has the same kinetic energy as the other does.

• The lighter one has twice as much kinetic energy as the other does.

• The lighter one has four times as much kinetic energy as the other does.

Block A has a mass m and block B has a mass 2m. Block A is pressed against a spring to compress the spring by a distance x. It is then released such that the block eventually separates from the spring and it slides across a surface where the friction coefficient is µk. The same process is applied to block B. Which one of the following statements concerning the distance that each block slides before stopping is correct?

• Block A slides one-fourth the distance that block B slides.

• Block A slides one-half the distance that block B slides.

• Block A slides the same distance that block B slides.

• Block A slides twice the distance that block B slides.

• Block A slides four times the distance that block B slides.

An SUV is accelerated from rest to a speed v in a time interval t. Neglecting air resistance effects and assuming the engine is operating at its maximum power rating when accelerating, determine the time interval for the SUV to accelerate from rest to a speed 2v.

• 2t

• 4t

• 2.5t

• 3t

• 3.5t