Exam 6: Work and Energy

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?

A warehouse worker uses a forklift to raise a crate of pickles on a platform to a height 2.75 m above the floor. The combined mass of the platform and the crate is 207 kg. If the power expended by the forklift is 1440 W, how long does it take to lift the crate?

A 10.0-g bullet traveling horizontally at 755 m/s strikes a stationary target and stops after penetrating 14.5 cm into the target. What is the average force of the target on the bullet?

A care package is dropped from rest from a helicopter hovering 25 m above the ground.What is the speed of the package just before it reaches the ground? Neglect air resistance.

A block of mass m is released from rest at a height R above a horizontal surface. The acceleration due to gravity is g. The block slides along the inside of a frictionless circular hoop of radius R. -Which one of the following expressions gives the speed of the mass at the bottom of the hoop?

A 12.5-kg crate slides along a horizontal frictionless surface at a constant speed of 4.0 m/s. The crate then slides down a frictionless incline and across a second horizontal surface as shown in the figure. -While the crate slides along the upper surface, how much gravitational potential energy does it have compared to what it would have on the lower surface?

A 12-kg crate is pushed up an incline from point A to point B as shown in the figure. What is the change in the gravitational potential energy of the crate?

A 0.50-kg ball on the end of a rope is moving in a vertical circle of radius 3.0 m near the surface of the Earth where the acceleration due to gravity, g, is 9.8 m/s2. Point A is at the top of the circle; C is at the bottom. Points B and D are exactly halfway between A and C. -The ball moves on the circle from A to C under the influence of gravity alone. If the kinetic energy of the ball is 35 J at A, what is its kinetic energy at C?

A woman stands on the edge of a cliff and throws a stone vertically downward with an initial speed of 10 m/s. The instant before the stone hits the ground below, it has 450 J of kinetic energy. If she were to throw the stone horizontally outward from the cliff with the same initial speed of 10 m/s, how much kinetic energy would it have just before it hits the ground?

A donkey pulls a crate up a rough, inclined plane at constant speed. Which one of the following statements concerning this situation is false?

A 3.0-kg cylinder falls vertically from rest in a very tall, evacuated tube near the surface of the earth. What is its speed after the cylinder has fallen 6.0 m?

A dam is used to block the passage of a river and to generate electricity. Approximately 5.73 * 10⁴ kg of water fall each second through a height of 19.6 m. If 85 % of the gravitational potential energy of the water were converted to electrical energy, how much power would be generated?

An automobile approaches a barrier at a speed of 20 m/s along a level road. The driver locks the brakes at a distance of 50 m from the barrier. What minimum coefficient of kinetic friction is required to stop the automobile before it hits the barrier?

In which one of the following situations is zero net work done?

A 2.0-kg projectile is fired with initial velocity components vox = 30 m/s and voy = 40 m/s from a point on the earth's surface. Neglect any effects due to air resistance. -What is the kinetic energy of the projectile when it reaches the highest point in its trajectory?

Larry's gravitational potential energy is 1870 J as he sits 2.20 m above the ground in a sky diving airplane before it takes off. What is Larry's gravitational potential energy when be begins to jump from the airplane at an altitude of 923 m?

A block is dropped from a high tower and is falling freely under the influence of gravity. Which one of the following statements is true concerning this situation? Neglect air resistance.

A 9.0-kg box of oranges slides from rest down a frictionless incline from a height of 5.0 m. A constant frictional force, introduced at point A, brings the block to rest at point B, 19 m to the right of point A. -What is the coefficient of kinetic friction, $\mu$ _k, of the surface from A to B?

A skier leaves the top of a slope with an initial speed of 5.0 m/s. Her speed at the bottom of the slope is 13 m/s. What is the height of the slope?

A roller-coaster car is moving at 20 m/s along a straight horizontal track. What will its speed be after climbing the 15-m hill shown in the figure, if friction is ignored?