Work/Energy Review

Which equation below is a correct equation for work? You may select more than one correct answer, if applicable.

• ΔE

• F_IId

• Fdsin(Θ)

• mgh

• Fdcos(Θ)

Polly pushes a crate to the right, speeding it up as she pushes it. Which of the following correctly describes the work that Polly does on the crate system? You may select more than one, if appropriate.

• Polly does positive work because she is exerting a force in the positive direction (to the right)

• Polly does positive work on the crate because she is giving it energy by speeding it up.

• Polly does negative work on the crate because the crate has less energy than it should because she is pushing it.

• Polly does positive work on the crate because she pushes it in the same direction that it moves.

A box is pulled up a rough incline by force F. The unmarked arrow represents the direction of the box's motion. Imagine you are analyzing the box-earth system.

Imagine you are using the equation Fdcos(Θ) to solve for the work done by the following forces. What angle would you use for each?

Force F: ​ ​ (a)  

Frictional force: ​ (b)  

Normal Force: ​ (c)  

• 45 degrees
• 0 degrees
• 15 degrees
• 30 degrees
• 75 degrees
• 180 degrees
• 90 degrees

A box is pulled up a rough incline by force F. The unmarked arrow represents the direction of the box's motion. Imagine you are analyzing the box system.

What kind of work is done by each of the following forces on the box system?

Force F: ​ ​ ​ (a)  

Frictional force: ​ ​ (b)  

Normal Force: ​ ​ (c)  

Gravitational Force: ​ (d)  

• positive work
• negative work
• zero work

Kendall exerts a 30 N force to the right on a box over a 10 m distance. While this is happening, Louis is exerting a 5 N force on the box in the opposite direction for the same distance. What is the net work done on the box?

Mathematical Equivalence

ON

Which of the following energies are considered mechanical energies? You may choose more than one answer, if applicable.

• kinetic energy

• gravitational potential energy

• chemical energy

• elastic potential energy

• thermal energy

Imagine a box sliding down a ramp. What must happen for the box-earth-surface system's total mechanical energy to change? You may sleect more than one answer, if applicable.

• An external force does positive work on the system

• GPE is converted into KE as it slides down.

• Energy in the system is converted into thermal energy.

• An external force does negative work on the system.

Which statement about reference height is true? You may select more than one answer, if applicable.

• Reference height is chosen by the person analyzing the scenario.

• Reference height is always at the ground.

• If an object is at the reference height, it has 0 GPE.

• If an object is below the reference height, it has negative GPE.

• Reference height is always at the location of the final state.

If earth is included in the system...

• The system has GPE if it is above the reference height.

• The earth does work on the system if the system moves in the direction the earth pulls.

• Earth does no work because it is perpendicular to the system's motion.

An system has a kinetic energy of 12 J. What is the system's kinetic energy if the velocity of the system triples?

• 4 J

• 12 J

• 15 J

• 36 J

• 108 J