1. -Kinetic Energy -Work-Kinetic Energy Theorem -Energy Losses due to Friction -Power
AP Physics C Mrs. Coyle

2. Energy and Work Energy is the ability to do work.
Work is the energy transferred to or from a system by a force that acts on it.

3. Video Link: Kinetic Sculpture

4. Energy Symbol: E Scalar Units: J, Joule cal, calorie
kcal, kilocalorie (Cal)
erg eV
pound -foot

5. Mechanical Energy
Potential Energy
Kinetic Energy

6. Kinetic Energy, K= 1 mv2 2 Energy of Motion
Coyle, Greece, 2005

7. Question
Can Kinetic Energy be Negative?

8. Derivation of K

9. Work-Energy Theorem W=KE W=KEf-KEi
“In the case in which work is done on a system and the only change in the system is in its speed, the work done by the net force equals the change in kinetic energy of the system.”

10. The Work-Kinetic Energy Theorem can be applied to nonisolated systems
A nonisolated system is one that is influenced by its environment (external forces act on the system)

11. Potential Energy, U: stored energy
Examples:
elastic potential energy – stored in a spring
gravitational potential energy
electrical potential energy
Compared to a Reference Point
(base level)

12. Conservation of Energy
Energy can neither be created nor destroyed. It can only change from form to form.
In a closed system energy is conserved
Conservation of Mechanical Energy
U1+ K1 = U2 + K2

13. Energy can be transferred to and from the System by:
Work
Mechanical Waves
Heat transfer
Matter Transfer (across the boundary of the system carrying energy with it)
Electrical Current Transmission
Electromagnetic Radiation

14. Change in Energy of the system equals total energy transferred
DEsystem = ST
T is the energy transferred across the system boundary
Twork = W Theat = Q
The Work-Kinetic Energy theorem is a special case of Conservation of Energy

15. What happens when kinetic friction is present?
When friction is present, the work done by the frictional force W=f·r is transferred to heat energy.

16. Internal Energy
The energy associated with an object’s temperature is called its internal energy, Eint

17. Power
Energy transfer per unit time
Average power :

18. Instantaneous Power

19. Units of Power SI unit of power: Watt 1 watt = 1 J/s = 1 kg . m2 / s2
US Customary unit: horsepower
1 hp = 746 W

20. kilo Watt · hour, kWh kWh is a units of work or energy
1 kWh =(1000 W)(3600 s)=
=3.6 x106 J

21. Example 1 (#26) A 3kg object has a velocity (6i-2j)m/s.
What is the kinetic energy at this time?
Find the total work done on the object if its velocity changes to (8i+4j) m/s .(Note: v2 = v·v) Ans: a)60J, b)60J

22. Pile Driver

23. Example 2 (#27)
A 2,100kg pile driver is used to drive a steel I-beam into the ground. The pile driver falls 5m before coming into contact with the top of the beam and it drives the beam 12cm farther into the ground before coming to rest. Using energy considerations, calculate the average force the beam exerts on the pile driver while the pile driver is brought to rest. Ans: 8.78 x 105 upwards

24. Figure for Example 3

25. Example 3 (#32)
A 2kg block is attached to a spring of a force constant 500N/m on a horizontal table. The block is pulled 5.00cm to the right of equilibrium and released from rest. Find the speed of the block as it passes through equilibrium if a)the horizontal surface is frictionless and b) the coefficient of friction between block and surface is Ans:a)0.791m/s, b) 0.531m/s

26. Example 4 (#40)
A 650kg elevator starts from rest. It moves upward for 3s with a constant acceleration until it reaches its cruising speed of 1.75 m/s. a)What is the average power of the elevator motor during this period? b)How does this power compare with the motor power when the elevator moves at its cruising speed? Ans: a)5.91x103 W= 7.92hp b) 1.11x104 W= 14.9hp