Unit 8 Impulse and Momentum

Outcomes

Part 1 – Forces Change Momentum Part 2 – Momentum is Conserved Unit 8 - Momentum Part 1 – Forces Change Momentum Part 2 – Momentum is Conserved

Momentum “mass in motion” All objects have mass; so if an object is moving, then it has momentum- it has mass in motion. The amount of momentum an object has depends on two things How much stuff is moving How fast the stuff is moving

Momentum = Mass x Velocity Symbol of momentum is “p” Formula p = m v Units kg x m/s = kg m /s A 500 kg car traveling at 2 m/s p = mv = 500kg x 2 m/s = 1000 kgm/s

Momentum = Mass x Velocity If the velocity is zero (if the object is not moving) then the momentum is zero. 500kg x 0 m/s = 0 kgm/s Inertia is a property of moving and non-moving objects. Momentum is a property of moving objects only.

If Velocity Changes, Momentum Changes The formula for change in momentum is similar to that for momentum: Δp = m Δ v (Sometimes mass changes, but not very often, so we only consider those cases where velocity is changing.)

500 kg car accelerates from 2 m/s to 10 m/s. Δp = m Δ v Δp = 500 (10-2) = 500(8) = 4000 kgm/s

What causes velocity to change? A change is velocity is called an acceleration. Accelerations are caused by a net FORCE. If you apply a net force to an object, the velocity of the object will change and therefore its momentum will change.

F = m a a = Δv / t The first equation tells us what “causes” acceleration. The second equation tells us what acceleration “is”. Together they state F = m Δv / t

F = m Δv / t is messy because it has a fraction on one side. It is more often stated in its easier form: F t = m Δv

F t = m Δv F t (the left side of the equation) is called the impulse. It is a force applied for a specific amount of time. m Δv (the right side of the equation) already has a name – it is called change in momentum. impulse = change in momentum. Impulse = Force x time

A +1000 N force is applied for 3 s to a 500 kg car that is traveling 2 m/s. Find the impulse. Impulse = force x time Impulse = +1000 N (3 s) = +3000 Ns

A +1000 N force is applied for 3 s to a 500 kg car that is traveling 2 m/s. Find the change in momentum. Impulse = change in momentum Impulse = +3000 Ns Change in Momentum = +3000 kgm/s

A +1000 N force is applied for 3 s to a 500 kg car that is traveling 2 m/s. vi Find the new velocity (or vf). Ft=mΔv Δv = Ft/m Δv = +1000(3)/500 Δv = +6 m/s Δv = +6 m/s vi = 2 m/s vf=2+6 = 8 m/s Δv = vf - vi

Change in velocity Remember Δv = vf – vi If a car speeds up from 2 m/s to 10 m/s Δv = vf – vi = 10 – 2 = +8 m/s If a car slow from 10 m/s to 2 m/s Δv = vf – vi = 2 – 10 = -8 m/s

Bouncing – change in velocity is large. Vi = +10 m/s Vf = - 6 m/s Δv = (-6)-(10) Δv = (-16) m/s

Assignment 7a Due_____