Motion (kinetic) energy, and how mass and speed change it

Answer Key - Grade 8 Science Worksheet - Building Confidence

Circle the best answer, or write your answer on the line.

1. 1.

   Ari's 4 kg skateboard rolls at 3 m/s. Use KE = ½ × m × v². What is its motion energy?

   18 J
2. 2.

   Bo's 8 kg wagon rolls at 3 m/s. Use KE = ½ × m × v². What is its motion energy?

   36 J
3. 3.

   Kira rolls the same ball twice: at 1 m/s, then at 3 m/s. When does it have more motion energy?

   • At 3 m/s
   • The same, since it is one ball
   • At 1 m/s, since it rolls longer
4. 4.

   A bowling ball and a tennis ball roll at the same 4 m/s. Which has more motion energy, Luz?

   • The same, since the speeds match
   • The bowling ball
   • The tennis ball, since it bounces
5. 5.

   Milo's cart stops when its brake rubs the wheel. How much thermal energy is made?

   • 0 J; the energy was used up
   • 20 J; half was lost
   • 40 J

6. Read this first

   Use this

   Energy bar chart (example data), one bar per energy store, height in J. Coasting: motion 600 J; thermal 0 J. Stopped: motion 0 J; thermal ?. A transfer arrow goes from motion to thermal, labeled brake pads.

7. 6.

   Noah brakes a bike to a stop. Where did its 600 J of motion energy go?

   • Nowhere; it ran out
   • Back into the rider's legs
   • Into thermal energy in the brakes

8. Use this

   Energy bar chart (example data), one bar per energy store, height in J. Sliding: motion 90 J; thermal 0 J. Stopped on the snow: motion 0 J; thermal ?. A transfer arrow goes from motion to thermal, labeled friction with snow.

9. 7.

   Pia's sled slides to a stop on flat snow. How much thermal energy is made?

   • 0 J
   • 45 J
   • 90 J

10. Motion energy of a 2 kg cart (example data)

    speed (m/s)	motion energy (J)
    1	1
    2	4
    3	9

11. 8.

    Ren's table shows a 2 kg cart's motion energy. Each value is the speed times itself. What is it at 4 m/s?

    • 16 J
    • 12 J
    • 8 J

12. Motion energy of a 1 kg ball (example data)

    speed (m/s)	motion energy (J)
    2	2
    4	8

13. 9.

    Tala's table shows a ball at 2 m/s and 4 m/s. When the speed doubled, what happened to the energy?

    • It became four times as much
    • It doubled
    • It stayed the same
14. 10.

    Zee's cart has 9 J of motion energy at 3 m/s. Doubling the speed gives four times the energy. What at 6 m/s?

    • 18 J
    • 36 J
    • 12 J