1. Springs and other stretchy things Hooke’s Law: Demo: springs and k F x

2. Springs store energy (conservative)

3. Spring potential energy F x P1. If a spring is compressed by 20 cm, the work done in the first 10 cm is A.less than B.the same as C.more than the work done in the last 10 cm. Hint: think of work as the average force * distance in each part Potential energy stored in spring:

5. Spring potential energy F x A rubber slingshot with spring constant k is stretched a distance d. A rock of mass m is shot up at a balloon floating a height h above the rock’s lowest point, and a horizontal distance L away. How fast is the rock going just before it hits the balloon?

6. Conservation of total energy F x What energy goes into his change of height? Nonconservative work is done on the system by muscles, motors, external power, which add to or take from the mechanical energy (KE+PE) of the system.

7. Conservation of total energy F x

8. A 1200 kg car is moving at 12 m/s down the slope. What braking force can stop it in 100m?

9. 40 m 20 0 Fred goes ice-blocking racing on the grass. Starting from rest he rides down a hill with some friction. On the way down, Fred pushes with his hands on the ground to make himself go faster. For each force decide how it will affect the mechanical energy. A. (+)W nonconservative …mechanical energy gained B. (-)W nonconservative …mechanical energy lost C. it will change only the PE P2. Gravity _________ P3. sliding friction_________ P4. Fred’s pushing on the ground (and hence the ground pushing him) _________

10. 40 m 20 0 Fred, 50 kg (including ice), goes ice-blocking racing on the grass. Starting from rest he rides 40 m down a hill which has a 20  slope.  k = 0.2 between the ice and grass. On the way down, Fred pushes with his hands with an average force of 30 N forward along the slope. 1.Find the work done by Fred’s pushing (in Joules) 2. Find the work done by friction 3. What is his speed at the bottom?

11. Power! Empire State building: Height: 1,250 feet, 443 meters Stories: 102 There are 1,575 steps from the building's lobby to the 86th floor (374 m). Paul Crake holds the record for racing these steps in 10 minutes, 15 seconds. 1Watt = 1 Horse-power=746 W What average power did he expend to lift himself? (Assume m=100 kg) From work: From velocity:

12. Power! A car weighing 3000 N moves at a constant speed of 30 m/s on level ground. To do this, it pushes backwards on the road with a 5000 N force. a) What is the power output of the car engine? b) Where does this power go?