Balanced & Unbalanced Forces — Drills · Grade 8 Science

Chapter: Forces

What these drills covers

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

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

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

Problem 1

Forces on an object are balanced. Its motion
  1. must be zero
  2. does not change
  3. must be speeding up
  4. reverses
Answer and explanation

does not change

Balanced forces mean no change in motion: an object at rest stays at rest and a moving one continues at constant velocity.

Problem 2

A box is pushed right with 30 N and left with 12 N. What is the net force in newtons, to the right?
Answer and explanation

18 N

30 − 12 = 18 N to the right.

Problem 3

Two people pull a rope in opposite directions with 240 N each. What is the net force in newtons?
Answer and explanation

0 N

Equal and opposite forces cancel, giving zero net force.

Problem 4

A car travels at a steady 60 km/h in a straight line. The forces on it are
  1. unbalanced, forward
  2. unbalanced, backward
  3. balanced
  4. zero
Answer and explanation

balanced

Steady velocity means no change in motion, so the driving force and the resisting forces are balanced. They are not zero individually.

Problem 5

A 5 N force acts east and a 9 N force acts west on the same object. What is the magnitude of the net force in newtons?
Answer and explanation

4 N

9 − 5 = 4 N, directed west.

Problem 6

An object's velocity changes. This shows that
  1. the forces are balanced
  2. there is a non-zero net force
  3. no forces act
  4. mass changed
Answer and explanation

there is a non-zero net force

A change in velocity — speeding up, slowing down or turning — requires an unbalanced force.

Problem 7

A parachutist falls at a constant speed. Explain the forces, and why constant speed does not mean no forces act.
Answer and explanation

Weight acts downward and air resistance upward. At constant speed these are equal in size and opposite in direction, so the net force is zero and the velocity no longer changes. Both forces are still large; it is their sum that is zero, which is why the parachutist continues to fall rather than stopping.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Explain how a free-body diagram helps decide whether forces are balanced.
Answer and explanation

It shows every force on one object as an arrow whose length represents size and whose direction shows where it acts, with nothing else in the picture. Adding the arrows along each direction gives the net force: if the opposing arrows match in every direction the forces are balanced, and if any direction is left over there is a net force and the motion will change.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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