Work energy and power

When to use W=Fxcosθ vs W=Fx

• W=Fx = Force and distance are in the same direction. So the distance moved in the direction of the force is used. Cos 0 = 1

• W=Fxcos⁡θ = Force acts at an angle to the distance only the component of the force parallel to the distance travelled is taken into account. The angle ⁡θ is the angle between the force and the direction of motion.

Where does cosθ come from in W=Fxcosθ ? Give two different ways it can be used.

1. Resolve force into parallel & perpendicular components

2. Resolve displacement into components along & perpendicular to force

3. Choose the component parallel always.

• Both give W=Fxcos⁡θ

Why does having motion not always mean work is done?

Work is only done when a force acts in the same direction as the displacement.

If the force is perpendicular to the motion, it does no work even if the object moves. cos 90 = 0.

This is because only the component of force along the direction of motion contributes to work

Definition of a Joule.

The energy transferred by a 1N force moving an object 1m in the force’s direction.

What does the law of conservation of energy tell us ?

Energy cannot be created or destroyed, only transferred to other forms.

How is the work done on an object the same as the energy transferred?

When a force does work by moving an object over a distance, it transfers energy to that object.
Work done is the process by which energy is transferred.