Impulse, Momentum and Conservation of Momentum

Newton Again!

Newton observed objects colliding and realized that two things dictate what it takes to change the motion of an object.

Mass (how much matter)Velocity (how fast it’s going

in a given direction)

He Called This Momentum

p = m.v

Impulse By rewriting his own 2nd law, Newton

defined Impulse

F=m.a=m.

F.t=m.Δv = IMPULSE “J” J=F.t= m.Δv=Δp=change in momentum

t

v

If a 5 N force pushes a 2 kg object for 3 seconds, how much will the momentum change?

How much is the impulse?

How much will the speed change?

How much is the acceleration?

Conservation of Momentum

Let’s look at what happens to momentum before, during and

after a collision/explosion.

If pGun/Emily = pPotato Then

Mgun/Emily.vgun/Emily=mpotatoe.vpotatoIf the mass on the left of the equation is

large compared to the right, then the velocity on the right must be large compared to the left if they are to be equal.

Action/Reaction

Conservation of momentum is just a

consequence of Newton’s third Law

Conservation of Momentum

Let’s look at what happens to momentum before, during and

after a collision/explosion.

Momentum is Conserved

TZ student defends school with potato gun

Conservation of Momentum

Newton’s Third Law:The potato goes one way and the gun

“recoils” in the opposite direction.The gun exerts a force on the potato

and the potato exerts an equal but opposite force on the gun, recoil.

pGun/Girl = pPotatoThese forces produce equal but opposite

changes in momentum.Since the girl is “attached” to the gun,

the combination of the girl /gun mass is much greater than the mass of the potato.

The girl and the gun recoil at a velocity much smaller than the potato.

If pGun&Person = pPotato Then

Mgun/person.vgun/person=mpotatoe.vpotato If the mass on the left of the equation is large

compared to the right, then the velocity on the right must be large compared to the left if they are to be equal.

Before collision

After collision

The system has the same total momentum.

M=2000 kg, v = 5 m/s M = 500 kg, v = 0

Impulse “J”By rewriting his own 2nd law, Newton defined

impulse

F= m . a =m .

F . t = m . ΔvJ = F . t= mΔv = Δp =change in momentum

t

v

So the safest way to stop an object is to stretch out the time it takes to stop things so you could use the smallest possible force. That’s why if something is cushioned and soft it is less likely to break.

Action/Reaction

Conservation of momentum is just a

consequence of Newton’s third Law