Balanced & Unbalanced Forces — Practice · Grade 8 Science

Chapter: Forces

What this practice covers

By the end of Balanced & Unbalanced Forces you should be able to:

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

Vocabulary used: net force, balanced forces, unbalanced forces, model, evidence, system boundary, assumption.

Problem 1

A rope pulls a sled forward with 60 N while friction pulls back with 20 N. What is the net force?
Answer and explanation

40 N

Net force = 60 − 20 = 40 N (forward).

Problem 2

If the net force on an object is zero, the object…
  1. must be at rest
  2. is either at rest or moving at constant velocity
  3. must be accelerating
  4. must be speeding up
Answer and explanation

is either at rest or moving at constant velocity

A zero net force means no change in motion — the object could be still, or cruising at a steady velocity.

Problem 3

Two tug-of-war teams pull with 500 N and 470 N in opposite directions. Find the net force.
Answer and explanation

30 N

Net force = 500 − 470 = 30 N, in the direction of the stronger team.

Problem 4

A car moving at a constant 60 mph on a straight highway has forces that are…
  1. unbalanced, speeding it up
  2. balanced, since velocity isn't changing
  3. balanced, since it's still moving
  4. unbalanced, since it's heavy
Answer and explanation

balanced, since velocity isn't changing

Constant velocity means no acceleration, which means the net force is zero — the forces are balanced.

Problem 5

Three forces act on a ring: 30 N right, 10 N left, and 5 N left. What is the net force?
Answer and explanation

15 N

Net force = 30 − 10 − 5 = 15 N to the right.

Problem 6

Explain why a hockey puck sliding on frictionless ice would keep moving at constant velocity forever.
Answer and explanation

With no friction or other horizontal forces acting on it, the net force is zero, so by Newton's first law the puck's velocity never changes — it keeps gliding at the same speed and direction indefinitely.

Problem 7

A cart experiences 14 N right and 9 N left. Find net force and describe the change in velocity. Explain the reasoning in your own words.
Answer and explanation

5 N right; velocity changes toward the right.

Choose right as positive; add 14+(−9)=5 N.

The acceleration is toward the right, regardless of the cart's initial motion.

Response criteria

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

Problem 8

Forces of 11 N left and 11 N right act on a moving cart. Find net force and predict motion if they remain the only forces. Show the reasoning, not just the conclusion.
Hints

Opposite directions require subtraction, not addition.

Answer and explanation

Net force is 0 N; the cart continues at constant velocity in an inertial-frame model.

Problem 9

A cart moves left but net force is right. Must it instantly move right?
Hints

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

Answer and explanation

No. Its leftward speed initially decreases. Its velocity can become rightward later if the force continues.

Problem 10

Add forces with a chosen directionAdd forces with a chosen directionreceiver6 N left10 N rightRight positive: +10 + (−6) = +4 NBoth red arrows act ON this cart. The green arrow is their net summary.
The two horizontal forces on this one cart have a common stated scale. The net is 10−6 = 4 N right, shown as a separate summary. Do not count the net arrow as another interaction, and do not add a partner force acting on a different object. Vertical weight and support are omitted from this horizontal comparison.
For this unit’s illustrated case, if the left force becomes 12 N while the right stays 10 N, what is the new net force and direction?
Answer and explanation

10−12 = −2 N with right positive: 2 N left. That predicts leftward acceleration, not necessarily leftward current velocity.

Choose the receiver before adding arrows. A sign records direction along the chosen line. Net force is the combined result of the real forces, not an extra push that exists in addition to them.

10−12 = −2 N with right positive: 2 N left. That predicts leftward acceleration, not necessarily leftward current velocity.

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 11

A 3 kg cart receives 14 N right and 5 N left while initially moving left. Find net force and acceleration and describe the initial speed change.
Answer and explanation

Net 9 N right; acceleration 9/3 = 3 m/s² right. Its initial leftward motion slows because acceleration opposes velocity.

Subtract opposing forces on the same receiver.

Compare acceleration direction with the supplied velocity.

Net 9 N right; acceleration 9/3 = 3 m/s² right. Its initial leftward motion slows because acceleration opposes velocity.

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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