Momentum, Force and Impulse CHAPTER 15

Momentum Momentum is a measure of an object’s motion, and how likely it is that the object will remain in motion Objects with a lot of momentum will take more force and longer to stop. You slow momentum by applying friction (2 objects pressing against each other)

momentum = velocity * mass Calculating Momentum momentum = velocity * mass momentum (p) → kg*m/s mass (m) → kg velocity (v) → m/s momentum will increase as the velocity gets bigger or the mass gets bigger

Practice A 60kg girl jogs at a pace of 4m/s. What is her momentum? Formula: Variables

Practice An adult, female fly has a mass of 0.0000012 kg. While flying, her velocity is 5 km/hour (or 1.5 m/s). What is her momentum? Formula: Variables

Bill Nye Time! Complete your questions while you watch.

Changing Momentum is Related to Force 15.2

Force Force is any push or pull on an object. The size of the force required to move an object depends on how long you apply force, the mass of the object, and how fast you want to make the object go.

Force Forces can be large or small They can also act for a short time or a long time. Ex. If you brake gradually for a long time (small force X long time), or if your brake quickly ( large force X short time), the result is the same. The important measurement is the IMPULSE→ the combination of the size of the force and the time the force acts

force = mass *change in velocity Calculating Force force = mass *change in velocity time mass (m) → kg velocity (v) → m/s time (t) → s force (f) → newtons (N) changing speed in a longer time → reduces the force!

Practice What force is involved in stopping a 1200 kg car travelling at 11 km/h (28m/s) in 25 seconds? Formula: Variables

Impulse 15.2

impulse = force *time = change in momentum Calculating Impulse impulse = force *time = change in momentum impulse (I) → kg*m/s time (t) → s force (f) → newtons (N) Impulse will increase as force is applied for a longer time or of the force gets bigger

Increase time → decrease Force! Why Care? In a collision the change in momentum can not be changed therefore the impulse can not be changed. However the idea of impulse can be used to reduce the force upon impact. Increase time → decrease Force!

Practice A car collides with another car. The force of the collision is 2000 kg∙m/s2. The collision lasts for 5 seconds. What is the impulse? Formula: Variables What is the relationship between the force of impact and the impulse number?

Conservation of Momentum 15.2 image from:http://www.physicsclassroom.com/mmedia/momentum/treci.cfm

What happens to the momentum in each of the following collisions: COLLISION ONE (Head- on) m = 20 kg v = 3 m/s p = +60kgm/s m = 20 kg v = 0 m/s p = 0kgm/s m = 20 kg v = 0 m/s p = 0 kgm/s m = 20 kg v = - 3 m/s p = -60kgm/s Total p before = (+60 kgm/s) + (-60 kgm/s) = 0 kgm/s Total p after = 0 kgm/s

What happens to the momentum in each of the following collisions: COLLISION TWO (Rear - end) m = 10 kg v = 5 m/s p = +50kgm/s m = 10 kg v = - 5m/s p = - 50kgm/s m = 20 kg v = 2 m/s p = +40kgm/s m = 20 kg v = + 7 m/s p = + 140kgm/s Total p before = (+50 kgm/s )+ (+40 kgm/s) = +90 kgm/s Total p after = (- 50kgm/s) + (+ 140kgm/s = +90kgm/s