Energy and work AP physics

What are Joules

Joules are the metric

What is mechanical energy?

An Object’s mechanical energy is the sum between a objects potential and kinetic energy.

How do you know if you are doing positive, negative or no work?

• Positive work: The force aids the motion, adding energy to the system. An example of this is pushing a sled forward.

• Negative work: The force opposes the motion, removing energy from a system. An example of this is a box being slowed due to friction.

• Zero work: If the force is perpendicular to the displacement or if there is force, no displacement would happen.

If a person is holding an object without the object moving, what equation can we use to represent the work being done?

F = w = mg

• m = mass

• g = gravity

What equation can be used to represent the work to lift an object a certain amount of distance?

W = PE_e = mgd

• PE_e: Potential energy

• m: mass

• g: gravity

• d: distance

What is the equation for kinetic energy?

K = mv²

What is the work-energy theorem?

The work-energy theorem states that the net work done on an object, equals the change in its kinetic energy.

W_net = ΔKE = ½ mv_f² - ½ mv_i²

W_{net =} ½ m(v_f² - v_i²) ( simplified version )

The work-energy theorem is essentially Newton's 2nd law repackaged in terms of energy rather than force and acceleration.

Important nuances:

• It only work when the force is parallel to the motion

• Work depends on displacement not distance

• A force can do work even if the object doesn’t move in the direction you’d expect.

What is power

Power is the rate in which work is being done

• P = W/t

Power is expressed through the unit Watt

How do you find the speed of an object based off the work-energy theorem?

You find the energy by doing basic algebra

Step 1: switch the position of W net and ½ mv².

• $\frac12mv^2=Wnet-\frac12mvi^2$

Step 2: get the amount of joules.

Step 3: use basic algebra to single out the speed:

• $v=\sqrt{\frac{2\left(joules\right)}{m}}$

What does $d^{\prime}$ mean?

It means the the distance the object stopped after the force was exerted on the object.

• $d^{\prime}=\frac{Wfr}{F}$

• Wfr