momentum  inertia in motion  inertia the tendency an object has to resist a change in motion  inertia is measured by mass the more mass an object has, the greater it resists a change in motion

momentum = mass velocity p = mv

units for momentum  mass is measured in kg  velocity is measured in m/s  momentum is therefore measured in kgm/s

find the momentum of  a 1.5 kg runaway skateboard rolling down the street at 4 m/s  a.5 kg fish swimming away from a shark at 18 m/s  a 20,500 g horse galloping at 12 m/s in a meadow

Which has greater momentum?  a.5 kg shooting star moving in space at 8,000 m/s or  a 1,000 kg space shuttle cruising at 4 m/s

 If the momentum of an object changes either the mass, the velocity, or both have to change.  If the mass remains unchanged, the velocity must change—which produces an acceleration.  What produces an acceleration?

a force! a force produces an acceleration when exerted on a mass Newton’s Second Law of Motion

impulse  the change in momentum impulse = F∆t  The greater the impulse exerted on an object, the greater will be the change in momentum ∆(mv) = F∆t

average force  the force exerted on an object changes from instant to instant, so when we talk about the force used to change the momentum of an object we are actually mentioning the average force

Increasing Momentum  to gain the greatest change in momentum, it makes sense to apply the greatest force for the longest period of time  Example: following through while shooting a basketball or swinging a baseball bat

 If you are driving a car that’s out of control and you had to decide to crash into a concrete wall or stack of hay, which would you choose?

the stack of hay! ….but why?

 by extending the time during which your momentum is brought to zero, the force of impact is reduced  Example: catching a baseball with your bare hands ∆(mv) = F∆t

 What advantage do bumpers on a car provide in a collision?  What about air bags?

∆(mv) = F∆t  if the time of impact in a collision is increased by 4 times, how much does the force of impact change?  if the time of impact in a collision is decreased by 100 times, how much does the force of impact change?

 by shortening the amount of time in an impulse given to an object, the force of impact is greatly increased  Example: breaking wood slats in karate class quick jabs in a fight