Impulse, Momentum and Conservation of Momentum

1. Impulse, Momentum and Conservation of Momentum

2. contact forces appear when things touch ………… When contact disappears, contact forces disappear. What remains unchanged? What is changed or left behind after forces interact? Where do they go?

3. What is changed or left behind after forces interact? Impetus? Speed? Mass? “Forces transfer something…….. But what? Momentum Ch 6 100 years after Newton Energy Ch 5 Angular Momentum Ch 8 & 9 “What remains unchanged”….today we say “What is conserved? Total momentum Total Energy Total Angular Momentum These things can be transferred between objects but never can be created or destroyed. Forces cause the transfer. In the absence of forces, they are conserved.

4. Newton figured out momentum, but not energy • He 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)

5. He Called This Momentum p = m.v Find the momentum of a 1000 kg car moving at 5 m/s. p = mv = 1000 kg * 5 m/s = 1000 kg m/s

6. Impulse is the change in momentum created when a force acts • 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

7. 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?

8. Conservation of Momentum Let’s look at what happens to momentum before, during and after a collision/explosion.

9. Momentum is Conserved Student defends school with potato gun

10. If pGun&girl= pPotato Then • Mgun&girl.vgun&girl=mpotato.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.

11. Action/Reaction Conservation of momentum is just a consequence of Newton’s Third Law

12. Conservation of Momentum Let’s look at what happens to momentum before, during and after a collision/explosion.

13. 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.

14. pGun/Girl =pPotato • These 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.

15. If pGun&Person = pPotato Then • Mgun/person.vgun/person=mpotato.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.

16. Before collision M=2000 kg, v = 5 m/s M = 500 kg, v = 0 After collision The system has the same total momentum.

17. Impulse “J” • By rewriting his own 2nd law, Newton defined impulse F= m . a =m . • F . t = m . Δv • J = F . t= mΔv = Δp =change in momentum

18. 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.