P5d - MomentumCollisions, crashes and

cannons

P5d - Momentum

Opposite reaction forces

• When one body exerts a force on a second body, the second body exerts an _________ and ____________ force on the first

• In other words: If you push something, it pushes back just as hard

“For every action, there is an equal and opposite reaction.”

Isaac Newton

Spot the action & reaction forces!

Spot the action & reaction forces!

beam

10 kg

Spot the action & reaction forces!

Spot the action & reaction forces!

Opposite reaction forces

Opposite reaction forces

Forces and momentum revision• If forces are balanced…

• If forces are unbalanced…

• Forces and velocities are vectors and must have…

• Momentum =

MomentumQ ~ What would do more damage: a 6 kg

bowling ball traveling at 6 m/s or a 10 g ping-pong ball traveling at 6 m/s?

What’s the momentum?

A 20000 kg elephant charging at 10 m/s

What’s the momentum?

Lewis Hamilton’s 400 kg car traveling at 80 m/s

What’s the momentum?

A 6 kg bowling ball traveling at 6 m/s

What’s the momentum?

A 10 g ping-pong ball traveling at 6 m/s

Collisions• There is no change in momentum during a

collision - momentum is always conserved

Ptotal(before) = Ptotal(after) = 0

Pred(before) + Pblue(before) = Pred(after) + Pblue(after) = 0

Recoil• Many guns and cannons recoil when fired

Ptotal(before) = Ptotal(after) = 0

Pgun(before) + Pbullet(before) = Pgun(after) + Pbullet(after) = 0

Explosions - Total momentum is zero

Preventing injury

Preventing injury in crashes

• Seatbelts, airbags and crumple zones are all safety features of cars

• They reduce the risk of injury in the event of a crash

• They do this by slowing the collision down…

Slowing collisions down

Force = change in momentum

time

Wear your seatbelt!