Momentum and Impulse

What is Momentum? The word momentum is used a lot in sports. A team with momentum is on the move and will be hard to stop. A team with a lot of momentum is REALLY on the move and will be REALLY hard to stop. For once, the English language uses it right!

Momentum Defined Momentum is mass in motion. Momentum is a vector. Momentum is directly proportional to mass and directly proportional to velocity. Momentum = mass x velocity p = m • v The unit of momentum is kg • m/s.

Practice Determine the momentum of a ... a. 60-kg halfback moving eastward at 9 m/s. b. 1000-kg car moving northward at 20 m/s. c. 40-kg freshman moving southward at 2 m/s.

Impulse How would you stop something with a lot of momentum? You would have to apply a force! Impulse = Force x time J = F • ∆ t Impulse = change in momentum J = ∆p F • ∆ t = m • ∆v

Practice Determine the impulse when... a. a 20 N force is applied for 0.2 seconds b. a 100 N force is applied for 3 seconds c. a 200 N force is applied for 8 seconds

Conservation of Momentum Momentum is a conserved quantity. The total momentum of a system is conserved. The total momentum of a system is only changed by external forces. When a moving object collides with another object, momentum is conserved.

Practice A hockey player applies an average force of 80.0 N to a 0.25 kg hockey puck for a time of 0.10 seconds. Determine the impulse experienced by the hockey puck.

Practice If a 5-kg object experiences a 10-N force for a duration of 0.10-second, then what is the momentum change of the object?

Impulse in real life This idea of impulse is what car companies use when designing air bags. Because the impulse applied to you is going to be equal to your change in momentum, there are two quantities that can be manipulated: force and time. When you increase the amount of time in a collision, the force experienced decreases. Airbags use this idea to lessen the force you feel during a crash. If the time of the collision is shortened, the force experienced increases.

Practice Jennifer, who has a mass of 50.0 kg, is riding at 35.0 m/s in her red sports car when she must suddenly slam on the brakes to avoid hitting a deer crossing the road. She strikes the air bag, that brings her body to a stop in 0.500 s. What average force does the seat belt exert on her?

Practice If Jennifer had not been wearing her seat belt and not had an air bag, then the windshield would have stopped her head in 0.002 s. What average force would the windshield have exerted on her?

Impulse-Momentum A force is applied to a 2200 kilogram car initially at rest for 10 seconds. If resulting velocity of the car is 3 m/s, what was the force that was applied?

Notes in brief Momentum: p = m • v Impulse: J = F • t Impulse-momentum: F • t = m • ∆v Conservation of momentum: momentuminitial=momentumfinal