Module 2 Dynamics (Part Two)

Kinetic Energy Formula

½mv²

Gravitational Potential Energy Formula

mgh

Mechanical Energy Formula

KE + GPE

What to remember for KE, GPE and ME?

• Measured in joules

• When height is zero, GPE = 0

• When velocity is zero, KE = 0

• Use the law of conservation to your advantage in questions

Change in KE or change in GPE

1/2mv² - 1/2mu²

Power Definition

Amount of energy consumed per unit time.

Power is change in energy over time.

Power is measured in watts.

Power Formula

Work/Time

or

(Force x Displacement)/Time

Therefore meaning Power = Force x Velocity

What does work equal when lifting an object.

W = mgh

Power is change in energy over time, therefore…

½mv² / t

or

mgh / t

or

KE + GPE / t

If not given time use…

P = FV

You can assume what due to gravity?

Acceleration

What is ‘p’?

Momentum

p = ?

mv

What units is momentum measured in?

Momentum in (kg m/s ) units = mass (kg) units x velocity (m/s ) units.

In a closed system, mv before an interaction such as a collision = ?

mv after the interaction.

What is also conserved?

In an elastic collision, kinetic energy is also conserved, so 1/2mv² before =1/2mv² after.

If momentum = 0 before…

It will equal zero after. For e.g two ice skaters start off still.

For objects that collide and combine…

m1v1 (before) + m2v2 (before) = mv (after)

What must you always do?

Consider direction.

For objects that collide and separate…

m1​u1 ​+ m2​u2​ = m1​v1 ​+ m2​v2​

For explosions…

mv (before) = m1v1 (after) + m2v2 (after)

When do you need to resolve vectors in collisions?

When collision occurs in 2 dimensions. (Not just in a straight line.)

Impulse = ?

Force x Time

Unit for Impulse?

Newton Seconds (Ns).

Impulse is also…

Change in momentum.

Change in momentum formula?

m(v-u)

In force time graph…

Area under curve = impulse.

This means…

Change in momentum = Force x Time

and

Time = Change In Momentum / Force

and

Force = Change In Momentum / Time

In terms of Impulse…

Force = Impulse / Time

and

Time = Impulse / Force

In inelastic collisions?

Objects colliding have less kinetic energy after the collision. You can easily tell if a collision is elastic or inelastic by seeing whether kinetic energy has been conserved. If it has, it is elastic and if it has not, then it is inelastic.