Bell Work: Types of Machines Answer with: lever, inclined plane, pulley, or wheel and axel

1. Bell Work: Types of MachinesAnswer with: lever, inclined plane, pulley, or wheel and axel • A pencil sharpener is a _____ and _____. • A crow bar is a ____ and _____. • A can opener is a _____, _____ and _____. • An axe is a _____ and _____. • A block and tackle is a ______.

2. Impulse and Momentum

3. Momentum • Inertia in motion • Combination of mass and velocity • Symbol for momentum is p • v • Momentum units are m/s

4. Big momentum created by big mass, big velocity, or both. • Examples: • cruise ship moving 2 m/s • pebble in orbit around the earth • cruise ship in orbit around the earth

5. NO VELOCITY = NO MOMENTUM

6. Impulse • change in momentum because of a change in speed (acceleration occurs) • it takes force to change the speed (2nd law)

7. to change momentum, exert an impulse • Symbol for impulse is Δp • Impulse units are • Momentum and impulse can be combined: t

8. Δv = change in velocity • To increase momentum: exert more force or exert force for a longer period of time • Ex: follow through in sports – tennis, hockey, etc.

9. To decrease momentum: force must be in opposite direction of motion • Ex: friction, air resistance, tree slowing down a car

10. Changes in momentum caused by cushy objects have less force and more time than those caused by hard items. • Double time, cut force in half (inversely related)

11. Ex: Jump off a 3 meter platform onto concrete – time to slow down your motion is very small once you contact the cement so the force is great.

12. Jump off a 3 meter platform onto a thick mat – time to slow down your motion is extended because the matt gives so the force is lessened.

13. Ex: Car crumple zones increase the time of impact so that less force is exerted causing fewer injuries to passengers.

14. More Examples of increasing time to decrease force: • bending legs when landing • moving away from a punch • stretchy cord when bungee jumping

15. Practice Problem: What is momentum of a 1500 kg car moving at 30 m/s? What is momentum of the car at rest? 0 kg·m/s If the car (going 30 m/s) slams on its brakes and stops in 10 seconds, how much force was exerted by the brakes?

16. An air bag increases the time of impact to 0.9 seconds in a car crash. If the above car crashed when moving 30 m/s, how much force acts on the 75 kg driver with the air bag?

17. Conservation of Momentum • For a closed system (no external forces), total momentum remains the same. • Momentum cannot be created or destroyed, but can be transferred from one object to another.

18. Firing a Cannon

19. Ex: • Cannon with cannon ball ready to fire has 0 kg·m/s of momentum. • Cannon firing  cannon exerts force on cannon ball, cannon ball exerts same force back on the cannon.

20. These forces occur over the same time period. • Cannon ball and cannon have equal and opposite impulses. • Because the cannon ball has less mass, its speed changes more than the cannons. • The acceleration of the cannon is called recoil – it moves backwards because the cannon ball is shot out forward.

21. Collisions • Total momentum before a collision equals the total momentum after a collision. • Momentum transferred from one object to another. • =+

22. Two Kinds of Collisions • Elastic – the objects hit and bounce off one another • The velocities change for each object • The mass will stay the same for each object • Momentum and kinetic energy are conserved.

23. Inelastic – the object collide and stick together • The mass becomes a combination of the two objects • There is only one velocity for the two objects stuck together • The kinetic energy isn’t conserved. • Momentum is conserved.

24. 10 m/s 1.3m/s 0.8m/s ? m/s Ex: Trains: 1st boxcar has a mass of 65000 kg. The 2nd boxcar has a mass of 92000kg After the collision, the boxcars move forward together. Before After =1.01m/s