1. Bell Ringer Dec 19, 2012 1. 1.If you push an object twice as far while applying the same force, you do a) a)Four times as much work b) b)The same amount of work c) c)Twice as much work d) d)½ as much work 2. 2.A rifle recoils from firing a bullet. The speed of the rifle’s recoil is small because the a) a)Momentum is mainly concentrated in the bullet b) b)Momentum of the rifle is smaller c) c)Rifle has more mass than the bullet

2. Impulse Day 3 Wednesday Dec 19, 2012

3. Question…… According to the Conservation of Momentum Law, when is momentum not conserved??According to the Conservation of Momentum Law, when is momentum not conserved?? What about the object is changing??What about the object is changing?? –Its mass?? –Its velocity??

4. Change in Momentum When a force is applied, an object’s momentum changes. = Δp = m ∙ Δv = m ∙ (v f – v i ) Δp = m ∙ (v f – v i )

5. Beaker vs. Floor!!! What happens when you drop a glass beaker on the floor?What happens when you drop a glass beaker on the floor? On the carpet??On the carpet??

6. Impulse In order to change an objects momentum, it is necessary to apply a….. – –Force – –Against the objects motion – –For a given time Impulse = F ∙ t Impulse (N ∙ s ) Force (N) Time ( s )

7. What could happen to cause a change in momentum while you are driving?? If you are out of control in a car……If you are out of control in a car…… –Would you rather hit…… a brick wall or a haystack?a brick wall or a haystack? –If the car comes to a stop, which has a greater change in momentum? They have the same change in momentum. Why?They have the same change in momentum. Why? –Because the result in both is a momentum of “0”

8. Haystack vs. Wall By hitting the haystack instead of the wall, youBy hitting the haystack instead of the wall, you –Extend the time of impact The time it takes you to change your momentum to zeroThe time it takes you to change your momentum to zero mVmV FtFt mVmVFtFt

9. Class Discussion 1.Egg toss & Egg into sheet 2.Explain why the goal posts have padding around their base. –The padding will increase the time required to stop and decrease the force. 3.What safety features in cars lengthen the time during a collision in order to decrease the maximum force during that collision. 4.Examine the inside of a football helmet. –How is the helmet constructed so that the player’s head will have more time to stop during a collision? There is space in the interior of the helmet for the head to move, thereby increasing the time.There is space in the interior of the helmet for the head to move, thereby increasing the time.

10. Change in Momentum for an object

11. Change in Momentum #1 A 1800 kg police car at rest increases his velocity to 45 m/s in order to catch up to a speeding car. What is the police car’s change in momentum? A 1800 kg police car at rest increases his velocity to 45 m/s in order to catch up to a speeding car. What is the police car’s change in momentum? Where did the momentum get transferred? GUESS

12. Change in Momentum #2 A 75 kg quarterback running at 11m/s east gets tackled to a stop. What is the quarterback’s change in momentum? A 75 kg quarterback running at 11m/s east gets tackled to a stop. What is the quarterback’s change in momentum? Where did the momentum get transferred? Where did the momentum get transferred? GUESS

13. Impulse Impulse is a change in momentum Impulse = Δp F ∙ t = m ∙ (v f – v i ) F ∙ t = m ∙ (v f – v i ) F ∙ t = m ∙ (v f – v i )

14. Impulse-Momentum Practice Problem A 90 kg football player is moving at 7 m/s. Calculate the force required to stop him in 0.5 seconds. GUESS Find the change in momentum here: Find the force here:

15. Exit Slip List 3 safety features in cars which lengthen the time during a collision in order to decrease the maximum force during that collision.List 3 safety features in cars which lengthen the time during a collision in order to decrease the maximum force during that collision.

16. Change in Momentum #2 A 3 kg tennis ball travels east towards a wall at a speed of 10 m/s. The ball bounces off the wall and is now traveling 5 m/s west. What is the ball’s change in momentum? A 3 kg tennis ball travels east towards a wall at a speed of 10 m/s. The ball bounces off the wall and is now traveling 5 m/s west. What is the ball’s change in momentum? Where did the momentum get transferred? Where did the momentum get transferred? GUESS