Chapter 7 Momentum

Inertia and Momentum Inertia – the tendency of an object to resist a change in its motion. Inertia is proportional to mass Momentum is inertia in motion, or the mass of an object multiplied by its velocity Momentum = mv

ρ = mv Momentum (ρ) according to our equation a moving object can have a large momentum if it has a large mass, a high speed, or both. If the momentum of an object changes, either the mass or the velocity or both change. Often, the velocity changes rather than mass, producing acceleration.

ρ = mv What produces acceleration? We know it is force (F = ma). The greater the force, the greater the change in velocity, and thus the greater the change in momentum. How long the force acts is important. The quantity of force x time interval is called impulse.

Impulse = F Δ t The greater the impulse, the greater the change in momentum. Impulse = change in momentum Δ ρ = mΔv or: F Δ t = mΔv

Example 1: Tiger woods hits a.05 kg golf ball, giving it a speed of 75 m/s. What impulse does he impart to the ball? Answer: 3.8 kg x m/s

Example 2: Wayne hits a stationary 0.12 kg hockey puck with a force that lasts for.01s and makes the puck shoot across the ice with a speed of 20 m/s, scoring a goal for the team. With what force did Wayne hit the puck? Answer: 240 N

If you catch a baseball with your bare hand you extend your arm forward so there is plenty of room to move it backward after making contact with the ball. By doing this you extend the time of impact and reduce the force of impact. Bouncing increases the impulse on an object since the change in momentum is greater to bring an object to a stop and then “throw it up again” rather than just bring it to a stop.

Net Momentum = 0 mv = 0 Net Momentum is still 0 -mv +mv

Law of Conservation of Momentum “In the absence of an external force, the momentum of a system remains unchanged”

Collisions Two Types: 1.Elastic Collisions m1v1 o + m2v2 o = m1v1 f + m2v2 f 2.Inelastic Collisions m1v1 o + m2v2 o = (m1+m2)v f Net momentum before collision = Net Momentum after collision

Warmup for Oct 26 th Anthony and Sissy are participating in the “Roll-a- Rama” rollerskating dance championship. While 75 kg Anthony rollerskates backwards at 3 m/s, 60 kg Sissy jumps into his arms with a velocity of 5 m/s in the same direction. A)How fast does the pair roll backwards together? B)If Anthony is skating toward Sissy when she jumps, would their combined final velocity be larger or smaller than your answer to part a? Why?