Newton's Three Laws of Motion — Practice · Grade 8 Science

Chapter: Motion

What this practice covers

By the end of Newton's Three Laws of Motion you should be able to:

13 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: inertia, Newton's first law, Newton's second law, Newton's third law, newton (N), model, evidence, system boundary, assumption.

Problem 1

A 10 kg object experiences a net force of 50 N. What is its acceleration?
Answer and explanation

5 m/s²

a = F ÷ m = 50 ÷ 10 = 5 m/s².

Problem 2

What net force is needed to accelerate a 2 kg ball at 6 m/s²?
Answer and explanation

12 N

F = ma = 2 × 6 = 12 N.

Problem 3

A 1,500 kg car accelerates at 2 m/s². What net force does the engine provide?
Answer and explanation

3000 N

F = ma = 1,500 × 2 = 3,000 N.

Problem 4

An astronaut floating in deep space, far from any forces, will…
  1. slowly stop on her own
  2. continue moving at constant velocity forever
  3. accelerate away with no cause
  4. spin uncontrollably
Answer and explanation

continue moving at constant velocity forever

With no unbalanced force acting on her, Newton's first law says her velocity won't change.

Problem 5

When a swimmer pushes backward on the water, the water pushes the swimmer forward. This illustrates Newton's…
  1. first law
  2. second law
  3. third law
  4. law of gravity
Answer and explanation

third law

This is an action-reaction pair, which is Newton's third law.

Problem 6

A rocket's engines produce 12,000 N of thrust, accelerating it at 4 m/s². What is the rocket's mass?
Answer and explanation

3000 kg

m = F ÷ a = 12,000 ÷ 4 = 3,000 kg.

Problem 7

A net horizontal force of 12 N acts on a 4 kg cart. Calculate acceleration and identify the reaction to a hand's push. Explain the reasoning in your own words.
Answer and explanation

3 m/s² in the net-force direction; the cart exerts an equal opposite force on the hand.

Acceleration=Fnet/m=12/4=3 m/s².

The reaction to the hand pushing the cart is the cart pushing the hand, on a different object.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

Net force 20 N acts on a 5 kg object. Find acceleration. Show the reasoning, not just the conclusion.
Hints

Use net force on one object in Newton's second law.

Answer and explanation

20/5=4 m/s² in the net-force direction.

Problem 9

A passenger lurches forward when a bus brakes. Explain without inventing a new forward force.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The passenger's body tends to maintain its forward velocity; a restraint or contact must exert a backward force to slow it.

Problem 10

Plan two comparisons from the records: one isolating net force, the other mass. Specify motion measurements, repeats, controls and what outcome would challenge the predicted relation. Explain braking separately from velocity direction.
Hints

Use the illustrated study titled Investigate force and mass separately. Keep the observations, model and conditions separate.

Answer and explanation

Compare the 1 kg cases at 1 and 2 N to isolate net force; compare 1, 2 and 4 kg at 2 N to isolate mass. Match path, resisting conditions, timing, measuring procedure and initial state. Keep all repeated records. The supplied model gives a=Fnet/m; unexpected motion needs a boundary/force/method review rather than automatic rejection of conservation or hidden invented values. A leftward net force can slow rightward motion.

Compare the 1 kg cases at 1 and 2 N to isolate net force; compare 1, 2 and 4 kg at 2 N to isolate mass. Match path, resisting conditions, timing, measuring procedure and initial state. Keep all repeated records. The supplied model gives a=Fnet/m; unexpected motion needs a boundary/force/method review rather than automatic rejection of conservation or hidden invented values. A leftward net force can slow rightward motion.

Response criteria

  • Design two separately controlled comparisons.
  • Specify recorded motion, units and repeats.
  • Explain a challenge result and braking mechanism.

Problem 11

The cart diagram shows 10 N right and 4 N left on a 2 kg cart. A student calculates 5 m/s² using the push alone. Correct the method and say whether the cart must currently move right.
Answer and explanation

Net force is 6 N right; acceleration is 3 m/s² right. The current velocity is not given: the cart could be moving left and slowing.

Subtract resistance from the applied push.

Divide net force by mass.

Keep acceleration distinct from current motion.

Response criteria

  • Calculate 6 N and 3 m/s².
  • Do not count the green net arrow again.
  • Avoid inferring current velocity.

Problem 12

Zero net force allows rest OR constant velocityZero net force allows rest OR constant velocityNet force = 0: cart at rest0 s · 1 s · 2 sSame position each secondNet force = 0: cart at constant rightward velocity0 s1 s2 s
Equal-time snapshots can show no position change or equal changes in position. Both are consistent with zero net force in an inertial reference frame. The moving cart does not need a forward net force to keep its constant velocity. On a real surface, rolling resistance can slow it unless balanced. The rest snapshots overlap intentionally; no extra force called inertia should be drawn.
For this unit’s illustrated case, why is inertia not an extra force on a force diagram?
Answer and explanation

Inertia describes how mass resists acceleration. It is not an interaction from another object and should not be added as a force arrow.

A released toy cart slows due to interactions such as rolling resistance, not because motion uses up a stored force. A passenger’s motion changes when a seat or restraint exerts force; connect the law to interactions rather than calling inertia a pushing force.

Inertia describes how mass resists acceleration. It is not an interaction from another object and should not be added as a force arrow.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 13

A 2.5 kg cart has a 15 N applied push and 5 N resistance. Find acceleration, and explain why its equal force on the pusher does not cancel that acceleration.
Answer and explanation

Net force is 10 N in the push direction and acceleration 10/2.5 = 4 m/s². The cart-on-pusher partner acts on the pusher, so it is not part of the cart’s net force.

Compute net force before dividing by mass.

Name the receiver of each interaction partner.

Net force is 10 N in the push direction and acceleration 10/2.5 = 4 m/s². The cart-on-pusher partner acts on the pusher, so it is not part of the cart’s net force.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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