Work/Energy

1. Work/Energy

2. The student will… • Investigate and calculate quantities using the work-energy theorem in various situations • Investigate examples of kinetic and potential energy and their transformations • Calculate the mechanical energy of, power generated within, impulse applied to, and momentum of a physical system • Demonstrate and apply the laws of conservation of energy and conservation of momentum in one dimension

3. Work W = Fd W – work (J) F – Force (N) d – distance/displacement (m) For work to be done, the object must move Displacement (distance) and force must be parallel for work to be done, NOT perpendicular

4. Potential Energy (PE) • Stored energy • Gravitational (PEg) PEg = mgh • Elastic (PEe) PEe = ½kx² mass Gravity (-9.8) Height (neg) Distance stretched or compressed Spring constant

5. Kinetic Energy (KE) • Energy of moving objects KE = ½ mv² mass velocity

6. Conservation of Energy • Energy can’t be created or destroyed • Mechanical Energy (total energy) stays the same ME = KE + PE ME = ½ mv² + mgh

7. Total energy at any point must remain the same, so: KEi + PEi = KEf + PEf ½ mvi² + mghi = ½ mvf² + mghf

8. Work-Energy Theorem • Work done is equal to the change in the kinetic energy of an object W = ∆KE W = KEf – KEi W = ½ mvf² - ½ mvi²

9. Power • Power is the rate at which work is done • The faster you get work done, the more powerful you are • Power is measured in Watts P = W / t P = Fd / t P = (ma)d / t

10. Momentum • Symbol is p • Measured in kg·m/s p = mv velocity momentum mass

11. Conservation of Momentum • Total momentum before and after two objects collide is conserved, so momentum before = momentum after m1v1i + m2v2i = m1v1f + m2v2f Object 1 mass Object 2 Final velocity Object 2 mass Object 1 mass Object 2 mass Object 1 Initial velocity Object 2 initial velocity Object 1 Final velocity

12. Impulse The force applied over time, or the change in momentum of an object Symbol is J J = F ∆t = m∆v Impulse Change in velocity (vf – vi) Mass Force Change In time

13. Which of the following is the best example of kinetic energy being transformed into potential energy? A

14. If a powerlifter raises a 500 N weight a distance of 2.0 meters in 0.5 seconds, what is his power output in watts? A

15. A 15.00 kg crate is accelerated from 3.000 m/s to 8.000 m/s. What is the amount of work needed to accelerate the crate? a. 147 J b. 412.5 J c. 67.5 J d. 480 J B

16. If you double the mass of a moving object and the velocity stays constant, how is the momentum of the object affected? a. p b. 4p c. 2p d. ½ p C

17. What is the momentum of .03 kg bullet traveling at 250 m/s? 95.7 kgm/s b. 937.5 kgm/s c. 8333 kgm/s d. 7.5 kgm/s D

18. A force does work on an object if a component of the force B

19. The more powerful the motor is, B

20. A 3.00 kg toy falls from a height of 10.0 m. Just before hitting the ground, what will be its kinetic energy? (Disregard air resistance. g=9.81m/s2.) D

21. What is the kinetic energy of a 0.135 kg baseball thrown at 40.0 m/s? C

22. Which of the following energy forms is associated with an object in motion? D

23. A 1000 kg truck moving at 10 m/s runs into a concrete wall. It takes 1.0 second for the truck to completely stop. What is the magnitude of force exerted on the truck during the collision? D

24. A crane lifts two 100 kg loads of building materials to the top of a building under construction. The first load is lifted in 10 s and the second load is lifted in 13 s. Which of the following statements best compares the work and power used by the crane lifting the two loads? C

25. The diagram below shows the path of a student on a roller coaster starting from rest at point W. The student rides in a frictionless cart past point Z, whichis at ground level. Which of the following statements best describes the energy of the student and the cart from point W to point Z? H

26. A force of 500 N is exerted on a baseball by the bat for .001 s. What is the change in momentum of the baseball? A