NMAT Physics | Work & Energy

Work

A force (N) that causes an object to move

3 things needed

• Force

• Displacement

• Direction

Unit: Joules (J)

Energy

Ability to do work

• If something has energy, it can make something happen

Kinetic Energy

Energy of motion

• If something is moving, then there’s KE

• 2 double the speed = 4x the energy

Potential Energy (PE)

Potential to move

• Higher the object = more PE

Total Mechanical Energy

Stored energy + moving energy

Conservation of Energy

Energy cannot be created nor destroyed

What happens to KE, PE, and total energy when an object falls?

PE decreases, KE increases, total E stays the same

Work-energy Theorem

Doing work changes energy

Power

Answers how fast work is done; speed of doing work

Unit: Watts

Efficiency

How much useful energy we have

Momentum

How hard is something to stop?

Impulse

Factor that changes momentum

Collisions

Momentum is conserved where the energy before the action is equivalent to the energy after

ex: Pool balls where momentum transfers

Elastic collision

Momentum and KE is conserved

ex: Billiard balls

Inelastic Collision

Momentum is conserved but KE is not conserved

ex: Car crash; some of the energy becomes heat, sound, and deformation

Perfectly Inelastic Collision

Objects stick together after collision

ex: Football tackle

Conservative Forces

Mechanical energy is conserved

ex: Gravity, spring force

Non-conservative Forces

Some mechanical energy is converted into heat or sound

ex: Friction, air resistance

Hooke’s Law

Force needed to stretch or compress a spring is directly proportional to the distance it moves

If it’s about the PE: