Chapter 11 Energy

1. Chapter 11 Energy Quiz 11

2. Chapter 11 Objectives • Use a model to relate work and energy • Calculate the kinetic energy of a moving object • Determine how to find the gravitational potential energy of a system

3. Chapter 11 Objectives • Identify ways in which elastic potential energy is stored in a system • Solve problems using the law of conservation of energy • Analyze collisions to find the change in kinetic energy

4. Energy • What is energy? What are the different types of energy?

5. Energy • Working definition is the ability to do work • It takes energy to lift a cheeseburger up to your mouth • It takes energy to walk up stairs • Work is the transfer of energy • When work is done on an object, the energy of the object is increased by the amount of work done on it

6. Potential Energy • Energy by virtue of location • Often think of gravitational potential energy

7. Potential Energy • But also exists for electromagnetics

8. Potential Energy • For gravity is described as • PE = mgh • M is mass in kg • G is gravity field strength • H is height in m • A 1 kg object at height 1 m above the ground has 9.8 J of Potential energy • How much work is required to lift a 250 g cheeseburger 0.30 m to your mouth?

9. Potential Energy • Other examples • Chemical • Elastic (spring) • Nuclear

10. Kinetic Energy • When we give an object potential energy (ie lift an object away from earth), when it falls back to earth, it regains the energy it lost as Kinetic Energy • Falling objects convert potential energy into kinetic energy • Kinetic Energy is energy by virtue of its speed

11. Kinetic Energy • Kinetic Energy is described by the equation KE = ½ mv2 • M is mass in kg • V is speed in m/s • An object is dropped from a 10 m high building. What will be the speed of the object right before it hits the ground? Ignore air resistance.

12. Roller Coaster • A roller coaster of mass 1200 kg goes over the first drop. The coaster drops 60 m. What is the speed of the coaster at the following heights (relative to ground) • 45 m • 30 m • 15 m • 0 m

15. Net Work • Work can be + or – (review) • Sometimes there is no equal and opposite • A mountain climber slides down a rope that is 12 m tall. The climber started at rest and hits the ground at a speed of 2.0 m/s. Discuss and calculate changes in energy.

16. Work • When you lift an object, you do positive work on the object • You give the object energy • Positive work = work in the direction of motion • Gravity does negative work on the object (against the motion) when lifted against gravity • But then does positive work on the object when it is released

17. Work • Friction always does negative work, because it is always opposing motion • A man pushes a piano up a U-Haul ramp. Describe the signs of work from the 3 forces.

18. Work (review) • Work as a transfer of energy can give objects kinetic energy • Gravity doing positive work on a falling object increases its speed • Your car engine does positive work on the car • Your car brakes do negative work on the car • Work is described by the equation W = FD • Force is Newtons, D is meters

19. Work Questions • If an object is moving it possesses • a)  kinetic energy. • b)  potential energy. • c)   work. • d)  power

20. Work Questions • Which of the following statements is true for a pine cone falling with acceleration less than g? • a)     It is converting potential and kinetic energy into heat. • b)      It is converting potential energy into kinetic energy and heat. • c)      It is converting potential energy into heat only. • d)     It is converting potential energy into kinetic energy only.

21. Work Questions • A light car and a heavy truck have the same kinetic energy. • a)     They both have the same speed. • b)      The car is going faster than the truck. • c)      The car is going slower than the truck.

22. Work Questions • A car is traveling twice as fast as a truck. The car has half the mass of the truck. • a)     Both have the same kinetic energy. • b)      The car has more kinetic energy than the truck. • c)      The truck has more kinetic energy than the car.

23. Work Questions • Car A has twice the mass of car B. Both are initially at rest. Equal forces act on each car for equal distances. At the end: • a) Car A has more kinetic energy than car B. • b) Car A has less kinetic energy than car B. • c) Car A is traveling faster than car B. • d) Car A is traveling slower than car B. • e) Cars A and B are traveling at the same speed.

24. Work Questions • The form of energy produced by sliding friction is • a)     kinetic. • b)      potential. • c)      gravitational. • d)     thermal.

25. Work Questions • An object’s kinetic energy change is equal to • a)     its loss of potential energy. • b)      the total work done to it. • c)      the heat produced by its motion.

26. Work Questions • A car of mass 1,400 kg is traveling at 20 m/s. If the engine pumping out 8000 J per second, what is the drag force (air resistance) on the car?

27. Archery • An arrow of mass 0.23 kg is fired with a velocity of 45 m/s. The arrow penetrates a target to a depth of 5.0 cm. • A) How much kinetic energy did the arrow have in flight? • B) How much work did the target do on the arrow? • C) What force did the target apply?