Mass in motion

1. Mass in motion

2. A hockey player hits a puck across the ice. • What “physics stuff” could you use to describe the motion of the puck • Kinematic equations • How do you explain why the motion of the puck changed with the hit, in physics terms. • Newton’s Laws & Force • We now want to look at how the force & duration of the hit affect the motion of the puck

3. After back-to-back jacks, the Cubs have gained momentum. After the interception on the 4 yard line, the offense lost all its momentum. (NU v NU ) Scoring 10/10 on the Quest gave the student a boost in momentum for the next Physics unit. What do these have in common?

4. Momentum! • “Mass in motion” • Linear momentum of an object of mass m moving with a velocity v. • p = mv • In other words, how much stuff is moving and how fast the stuff is moving • Units = kgm/s • Vector quantity – direction matches the velocity.

5. Momentum is directly proportional to an object's mass and to the object's velocity. • Greater mass = greater momentum • Greater velocity = greater momentum • An object has a large momentum if it has a large mass or a large velocity, and both variables are of equal importance in determining the momentum of an object.

6. Consider a Mack truck and a roller skate moving down the street at the same speed. • Which will have a greater momentum? • The considerably greater mass of the Mack truck gives it a considerably greater momentum • Consider that the same Mack truck is at rest, but the roller skate is moving. • Which will have a greater momentum? • The roller skate. • Anything at rest has a momentum of “0”. • It is not an “mass in motion.”

7. Impulse-Momentum Theorem • To stop an object, it is necessary to apply a… • FORCE AGAINST • To stop such an object, it is necessary to apply a forceagainst its motion for a given period of time = IMPULSE • i

8. Why is follow-through important?

9. 4 ways to • Change mass • Mass has direct affect on p • Increase mass – increase p/Decrease mass – decrease p • Change velocity • Velocity has direct effect on p • Increase velocity – increase p • Decrease velocity – decrease p • Change force • Change force applied has direct effect on p • Change time • Change how long force is applied = direct effect on p

10. Force or Time • If you are out of control in a car, would you rather hit a brick wall or a haystack? • A haystack right? • Why? • Which has a greater impulse? • They have the same impulse. Why? • Because the result in both is a momentum of “0” • However, stopping is a product, it doesn’t mean that the time or force was the same

11. Ft mV • By hitting the haystack instead of the wall, you • Extend the time of impact • The time it takes you to change your momentum to zero

12. Large Force/Short Time mV Ft For short impact times, the impact forces are large! Remember: for an object brought to rest, the impulse is the same no matter how it is stopped. But, if the time is short, the force will be large

13. The “human cannonball” has long been a popular – and extremely dangerous- circus stunt. In order for a 45 kg person to leave the cannon with the fastest speed yet achieved by a human cannonball, a 1.6 x 103 N force must be exerted on that person for 0.68 s. What is the record speed at which a person has been shot from a circus cannon?

14. Four Cases of p Illustrate four different ways to change momentum. Be sure to identify how F, t, m, or v are changing. Cannot use examples we have discussed. Explain what/why/how momentum was changed.

15. Amazing Spider-Man #121 Gwen’s mass = 50.0 kg How fast was she going before she hits the water? George Washington Bridge, NYC = 604 ft 1 ft = 0.305 m H = 184. 2 m Mgh = ½ mv2 V = = 60 m/s What is her momentum? p = mv P = (50kg)(60 m/s) = 3000 kg m/s

16. If Spidey stopped her in 0.5 s, what was the magnitude of the force he needed? Ft = ∆p = 6000 N F = 30 g’s How did they change the movie version to keep the MJ alive after the fall?