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Grade 8 · unit review

Review: Net force, acceleration, and work

Use signed quantities to connect net forces to velocity changes and energy.

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Unit review questions
1. Right is positive. Forces are +11 N and −7 N with no other horizontal forces. What is the net?
2. A 5 kg cart has a net force of −10 N. What is its acceleration?
3. A cart starts at +8 m/s with constant a = −3 m/s². What is v after 3 s?
4. A 4 kg cart at rest receives 18 J of net work. What final speed satisfies K = 1/2 mv²?

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Use the skills in a new task

Assess a fictional transport cart on paper. A 2 kg cart initially moves right at 5 m/s. All horizontal forces are 3 N right and 5 N left, constant for 2 s. Vertical forces balance. Find net force, acceleration, and final velocity. Its supplied displacement is 8 m right during this interval; compare net work with its kinetic-energy change.

Work on paper or in your own document. Explain the choices you made.

  • Use right positive and find −2 N net and −1 m/s² acceleration.
  • Calculate final velocity +3 m/s after 2 s.
  • Find net work (−2)(8) = −16 J.
  • Compare initial K = 25 J and final K = 9 J, giving the same −16 J change.
Compare with a model response

Right positive gives Fnet = 3 − 5 = −2 N and a = −2/2 = −1 m/s². Final v = 5 − 2 = +3 m/s. Initial K = 25 J and final K = 9 J, so ΔK = −16 J. For the supplied 8 m rightward displacement, net work is −2×8 = −16 J, matching ΔK. The leftward net force slows the right-moving cart over this interval; net work transfers energy out of its motion.

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