Q: A 510 kg bobsled starts from rest at the top of a frictionless icy track 12 m above the bottom. What is its speed at the bottom? (Use $g = 9.8 \mathrm{\ m/s^2}$)

$v = \sqrt{2gh} = \sqrt{2 \times 9.8 \times 12} \approx 15.3 \mathrm{\ m/s}$

Q: A skateboarder of mass 70 kg starts from rest at the top of a frictionless ramp 4 m high. What is the speed at the bottom?

$v = \sqrt{2gh} = \sqrt{2 \times 9.8 \times 4} \approx 8.85 \mathrm{\ m/s}$

Question 1

A 2 kg object is dropped from a height of 5 m above the ground. What is its gravitational potential energy relative to the ground just before it is dropped? (Use $g = 9.8 \mathrm{\ m/s^2}$)

Accepted Answer

$U_g = 49 \mathrm{\ J}$

Question 2

Which of the following statements about the work done by the normal force on a block sliding down a frictionless ramp is correct?

Accepted Answer

The work done by the normal force is zero.

Question 3

A woman pulls a crate up a rough inclined plane at constant speed. Which statement is NOT true?

Accepted Answer

The work done on the object by gravity is zero.

Question 4

If you do positive work on an object, its kinetic energy:

Accepted Answer

Increases.

Question 5

Which equation correctly expresses the work-kinetic energy theorem?

Accepted Answer

$W_{	ext{total}} = \Delta K$

Question 6

A ball is dropped straight down and another slides down a frictionless ramp from the same height. Which reaches the bottom with the highest speed?

Accepted Answer

The same speed for both.

Question 7

Which of the following is the correct expression for gravitational potential energy?

Accepted Answer

$U_g = mgy$

Question 8

A 510 kg bobsled starts from rest at the top of a frictionless icy track 12 m above the bottom. What is its speed at the bottom? (Use $g = 9.8 \mathrm{\ m/s^2}$)

Accepted Answer

$v = \sqrt{2gh} = \sqrt{2 	imes 9.8 	imes 12} \approx 15.3 \mathrm{\ m/s}$

Question 9

Which of the following is true about the choice of reference height $y=0$ for gravitational potential energy?

Accepted Answer

It is arbitrary and can be chosen for convenience.

Question 10

A spring with spring constant $k$ is compressed by a distance $x$. What is the elastic potential energy stored in the spring?

Accepted Answer

$U_s = \frac{1}{2}kx^2$

Question 11

When only conservative forces act on a system, the total mechanical energy:

Accepted Answer

Is conserved.

Question 12

A 2000 kg elevator is brought to rest by compressing a spring by 3.00 m. If the elevator was moving at 2.00 m/s just before contacting the spring, what must be the value of the spring constant $k$? (Ignore friction, use $g = 9.8 \mathrm{\ m/s^2}$)

Accepted Answer

$k = \frac{mgx + \frac{1}{2}mv^2}{\frac{1}{2}x^2}$

Question 13

Which of the following forces does zero work on an object moving along a surface?

Accepted Answer

Normal force.

Question 14

A skateboarder of mass 70 kg starts from rest at the top of a frictionless ramp 4 m high. What is the speed at the bottom?

Accepted Answer

$v = \sqrt{2gh} = \sqrt{2 	imes 9.8 	imes 4} \approx 8.85 \mathrm{\ m/s}$

Question 15

Which of the following is the correct generalized work-energy principle for a system including kinetic and potential energy?

Accepted Answer

$K_i + U_{g,i} + W_{other} = K_f + U_{g,f}$

Question 16

A skier rides a ski lift to the top of a hill. What happens to the total energy of the skier-plus-Earth system?

Accepted Answer

Increases in value.

Question 17

A skier makes their way down an icy, frictionless slope. What happens to the total energy of the skier-plus-Earth system?

Accepted Answer

Remains the same (is conserved).

Question 18

Which of the following is true about the work done by tension in a pendulum string as the bob swings?

Accepted Answer

The work done by tension is zero.

Question 19

If a spring is stretched from its natural length by an external agent, what happens to the work done by the agent?

Accepted Answer

It is stored as elastic potential energy in the spring.

Potential Energy & Energy Conservation: Work, Kinetic Energy, and Systems

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