Work & Energy Review

1. The work-energy theorem states Mass is equal to weight Net work equals change in mechanical energy Net work equals change in KE Net work equals change in PE

2. How much work is done by a 500 N force pushing a 10 kg crate along the floor 2 m? 500 J 20 J 250 J 1000 J

3. A 300 N block is raised 2 m from the floor in 3 seconds 3. A 300 N block is raised 2 m from the floor in 3 seconds. What is the power? 300 W 600 W 200 W 1800 W

horizontal parallel perpendicular normal 4. The only force that is capable of doing work on an object is one that is __ to the distance traveled. horizontal parallel perpendicular normal

5. A 1 kg wagon is pulled up a ramp to a height of 1 m 5. A 1 kg wagon is pulled up a ramp to a height of 1 m. When released, the total energy at the bottom of the ramp is 7 J. How much energy was lost to friction? 2.8 J 7 J 9.8 J 0 J

6. The velocity of an car doubles. What factor does it change its KE? 2 4 1/2 1/4

7. Do you think the Skater will make it over the first hump 7. Do you think the Skater will make it over the first hump? (No friction on the track) No, because his potential energy will be converted to thermal energy No, because he doesn’t have enough potential energy Yes, because all of his potential energy will be converted to kinetic energy Yes, because some of his energy will be potential and some kinetic

8. Do you think the Skater will make it over the first hump 8. Do you think the Skater will make it over the first hump? (lots of track friction) No, because his potential energy will be converted to thermal energy No, because he doesn’t have enough potential energy Yes, because all of his potential energy will be converted to kinetic energy Yes, because some of his energy will be potential and some kinetic

9. Do you think the Skater will make it over the first hump 9. Do you think the Skater will make it over the first hump? (No friction on the track) No, because his potential energy will be converted to thermal energy No, because he doesn’t have enough potential energy Yes, because all of his potential energy will be converted to kinetic energy Yes, because some of his energy will be potential and some kinetic

10. Do you think the Skater will make it over the first hump 10. Do you think the Skater will make it over the first hump? (lots of track friction) No, because his potential energy will be converted to thermal energy Yes, if not too much energy is converted to thermal Yes, because all of his potential energy will be converted to kinetic energy Yes, because some of his energy will be potential and some kinetic

11. In the next moment, the KE piece of the pie gets larger, then B. The Skater is going up hill (left) The Skater is going down hill (right) There is no way to tell

12. In the next moment, the KE piece of the pie gets larger, then The PE part stays the same The PE part gets larger too The PE part gets smaller There is no way to tell C

13. In the next moment, the KE piece of the pie gets larger, then The Skater will be going faster The Skater will be going slower There is no way to tell

14. The bar graph shows the energy of the Skater, where could she be on the track?

15. The pie graph shows the energy of the Skater, where could she be on the track? KE B PE

16. If the ball is at point 4, which chart could represent the ball’s energy? KE PE A. B. C. D. 2 1 3 4 The track is saved, but you have to select the ball and give it maximum mass to see the pie well. I used the loop track with the ball skater and made it more massive. The answer is C A is at 1 or 3 B is at the lowest point on the track

17. If a heavier ball is at point 4, how would the pie chart change? KE No changes The pie would be larger The PE part would be larger The KE part would be larger PE 2 1 3 4 B Change the ball’s mass to show this

18. As the ball rolls from point 4, the KE bar gets taller 18. As the ball rolls from point 4, the KE bar gets taller. Which way is the ball rolling? At 4 Next step 2 1 3 4 You will need to Zoom out on the bar graph window to see the top of the bars Up Down not enough info

Emech at 1 + W = Emech at 2 mgh1 = mgh2 19. How high will pendulum rise? (Ignore air resistance) A) Less than h B) h C) More than h h From Tanner/Dubson CU Boulder Assuming no energy lost to anything else B Emech at 1 + W = Emech at 2 mgh1 = mgh2 Reference level (h = 0) Pendulum height

20. A 5000 kg coaster is released 20 meters above the ground on a frictionless track. What is the approximate speed at ground level? (point A) 7 m/s 10 m/s 14 m/s 20 m/s none of the above   Adapted from Franklin/Beale CU Boulder From rest D KEf+PEf=KE0+PE0 so 1/2 m v2+0=0+mgh v=sqrt(2gh)=20m/s Velocity from PE

22. What is its approximate speed at 10 meters high (point B )? A)       7 m/s B)       10 m/s C)       14 m/s D)      20 m/s E)        none of the above Adapted from Franklin/Beale CU Boulder C)       14 m/s KEf+PEf=KE0+PE0 so 1/2 m v2+mghf=0+mgh0 v=sqrt(2g(h0-hf))=14m/s  

23. What will the speed of the 75kg Skater be at 2 seconds? Total =2918 J KE=509 J PE=2408 J PE = 0 at dotted line D This is the default track with the PE line moved up to the track A. 14m/s B. 8.8m/s C. 8.0m/s D. 3.7m/s

24. At what height is the 60kg Skater at 2 seconds? Total =3829 J KE=2429 J PE=1365 J B I used the Double well roller coaster track with the Skater changed to the girl and I moved the PE line to the bottom of the first well. Then I started from the “Return Skater” position A. 6.5m B. 4.2m C. 2.3m D. 1.9m