Work and Energy

You push a box up an inclined plane. Match each force to the best description of work it does.

• Zero

• Normal Force

• Positive

• Applied Force

• Negative, depends on the angle

• Force of Gravity

• Purely Negative

• Force of Friction

You store 200 J of potential energy in a spring with spring constant 100 N/m. How far will it compress? (Two decimal places)

What would the force vs displacement graph (y-axis is N, x-axis is meters) look like for a spring with a spring constant of 7 N/m?

Sort the types of energy into the appropriate categories

Groups:

(a) Kinetic Energy

,

(b) Potential Energy

,

(c) Internal Energy

• Gravitational

• Chemical

• Thermal

• Electric

• Kinetic (hmm)

• Spring

• Radiant (Light)

• Nuclear

A child is on a swingset. Their parent pushes them at the swing's lowest point.​ This ​ (a)   adds ​ (b)   ​ to the child, so they begin to move. The swing raises, so the ​ (c)   increases and the ​ (d)   decreases. Without additional pushing, the energy will eventually be converted into ​ (e)   and the swing will stop.

• work
• kinetic energy
• gravitational potential energy
• internal energy
• chemical energy
• spring potential energy

Assume the moon moves in a circle around the Earth. Gravity points straight in from the moon towards the Earth. During its orbit, Earth's gravity on the moon:

• Always does positive work

• Always does negative work

• Never does work

• Alternately does positive and negative work

Answer explanation

Consider: the velocity of the moon is always tangent to the circle, and the force of gravity is always along a radius; they're perpendicular, so they do no work!

OR: If the moon is always the same distance away, delta h is zero so delta PEg is zero - gravitational potential energy never changes, so gravity does no work!

A squirrel drops a 0.2 kg walnut shell out of a tree. The shell falls 9 meters, and hits the ground with a speed of 10 m/s. How much energy was lost to air resistance during the fall? (Use g=9.8 m/s^2, 1 decimal place).

Answer explanation

Calculate delta PEg (23.5 J), calculate KEf (10 J) - the difference between the two is how much total energy was lost.

You must push with a force of 35 N to keep a box sliding across the floor with a constant speed. You move the box 12 m in 7 seconds while pushing it. How much power are you using?

Order from LEAST energy to MOST energy

• The energy consumed by a 60 W light bulb in 10 seconds.

• A 600 N/m spring compressed 0.04 m.

• A 6 kg object moving at 2 m/s

• A 6 kg object 12 meters above the ground (on Earth)

A force of 25 N is applied at an angle of 20 degrees above the horizontal to pull a box forward 12 meters across level ground. How much work does the force do? (1 decimal place)