Class 9 Science · Original EZT practice
How Forces Affect Motion (Class 9 Science Chapter 6): summary and practice
A push, a pull, a slowdown - force is behind every change in motion. This chapter builds the full picture: measuring force in newtons, balanced and unbalanced forces, friction, inertia, and Newton's three laws of motion. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Measuring force, balanced and unbalanced
Force is measured in newtons (N) - forces in the same direction add, forces in opposite directions subtract, and when the net force is zero the forces are balanced and the state of motion does not change.
2. Friction and the first law
Friction is the force that opposes relative motion between surfaces in contact - inertia is the tendency of objects to resist any change in their state of rest or uniform motion, which is the idea behind Newton's first law.
3. Newton's second law - F = m x a
An unbalanced force changes the state of motion, and the acceleration produced is given by F = m x a - force equals mass times acceleration, rearranged as a = F / m or m = F / a when needed.
4. Newton's third law and systems
To every action there is an equal and opposite reaction, and the two forces act on different bodies so they never cancel - forces inside a system cancel out, so only external forces change the system's motion.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Add forces in one direction
Choose the method: Same direction forces add up.
EZT practice question
Two students push a box in the same direction with forces of 30 N and 45 N. What is the net force?
Show the worked solution
- Both pushes point the same way.
- Forces in the same direction add.
- Net force = 30 + 45 = 75 N in that direction.
Answer: 75 N in the direction both students push.
Check: Adding is only for same-direction forces - opposing forces subtract instead.
Watch for this: Do not average the two forces - net force is a sum here, not a mean.
Now try it yourself
Two children push a swing with 20 N and 35 N in the same direction. Net force?
Check your answer
55 N.
Same direction forces add: 20 + 35 = 55 N.
Subtract opposing forces
Choose the method: Opposite directions: subtract, direction of the larger force wins.
EZT practice question
Two teams pull a rope in opposite directions with 200 N and 170 N. Find the net force and its direction.
Show the worked solution
- The pulls point in opposite directions.
- Opposing forces subtract: 200 - 170 = 30 N.
- The net force points towards the stronger pull - the 200 N team.
Answer: 30 N towards the team pulling with 200 N.
Check: Had both teams pulled equally, the net force would be zero - balanced forces, no change in motion.
Watch for this: Do not add 200 + 170 - addition is only for forces pointing the same way.
Now try it yourself
A book rests on a table and does not move. What can you say about the forces on it?
Check your answer
They are balanced - the net force is zero.
No change in motion means no unbalanced force.
Explain friction balancing a push
Choose the method: No motion means some force cancels the push.
EZT practice question
A student pushes a stationary 25 kg block, but it does not move. Which force balances the push?
Show the worked solution
- The block stays at rest, so the net force on it is zero.
- Something must be cancelling the applied push.
- Friction between the block and the floor opposes the push and balances it.
Answer: Friction between the block and the floor balances the applied force, so the net force stays zero and the block remains at rest.
Check: Push harder than the maximum friction and the same block starts to slide - that is the unbalanced case.
Watch for this: Do not say no force acts on the block - forces act, but they cancel.
Now try it yourself
Why does a rolling ball eventually stop on the ground?
Check your answer
Friction between the ball and the ground opposes its motion.
That opposing force slows the ball to rest.
Apply inertia
Choose the method: Bodies resist changes in their state of motion.
EZT practice question
Passengers lurch forward when a bus brakes suddenly. Which property explains this?
Show the worked solution
- The passengers were moving with the bus.
- When the bus slows, their bodies tend to continue in the state of motion.
- That tendency is inertia - the idea behind Newton's first law.
Answer: Inertia - their bodies tend to continue in the state of motion.
Check: The reverse also holds: when the bus starts suddenly, passengers fall backwards as their bodies tend to stay at rest.
Watch for this: Do not call it a push from the bus - no new force throws them forward; it is their own continued motion.
Now try it yourself
Define inertia.
Check your answer
The tendency of an object to resist a change in its state of rest or uniform motion.
It is the content of Newton's first law.
Compute F = m x a
Choose the method: Force equals mass times acceleration.
EZT practice question
A force acts on a 25 kg block and produces an acceleration of 0.4 m/s^2. Find the force.
Show the worked solution
- Newton's second law: F = m x a.
- Substitute m = 25 kg and a = 0.4 m/s^2.
- F = 25 x 0.4 = 10 N.
Answer: F = 10 N.
Check: Units work out: kg x m/s^2 is exactly the newton.
Watch for this: Do not divide - division is for finding acceleration (a = F / m), not force.
Now try it yourself
Find the acceleration when a 600 N net force acts on a 1200 kg car.
Check your answer
a = F / m = 600 / 1200 = 0.5 m/s^2.
Rearrange the same law to solve for acceleration.
Explain action and reaction
Choose the method: The pair acts on two different bodies.
EZT practice question
A swimmer pushes water backwards and moves forward. Explain using Newton's third law.
Show the worked solution
- The swimmer's hands apply a backward force on the water - the action.
- The water applies an equal and opposite forward force on the swimmer - the reaction.
- The reaction acts on the swimmer, so the swimmer moves ahead.
Answer: The hands push water backwards (action); the water pushes the swimmer forward with an equal and opposite force (reaction), so the swimmer moves ahead.
Check: A gun recoiling when fired is the same pattern - bullet forward, gun backward.
Watch for this: Do not say action and reaction cancel - they act on different bodies, so they never cancel on one object.
Now try it yourself
Two students push each other while standing on skates. What happens to each?
Check your answer
Both move - each exerts a force on the other.
Action and reaction act on the two different bodies.
Try a mixed set
Choose a method or explanation before opening each answer.
1. SI unit of force?
Show answer and reason
The newton (N).
1 N gives a 1 kg mass an acceleration of 1 m/s^2.
2. Balanced forces?
Show answer and reason
Net force zero - motion stays unchanged.
An unbalanced force is what changes motion.
3. Inertia?
Show answer and reason
Tendency to resist change in the state of rest or uniform motion.
The idea behind Newton's first law.
4. Newton's second law?
Show answer and reason
F = m x a.
Rearrange to a = F / m or m = F / a as needed.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- Why does a gun recoil when a bullet is fired?
- A 5 kg object accelerates at 3 m/s^2. Find the net force.
- Passengers lurch forward when a bus brakes suddenly. Which property explains this?
Know the rule but lose track of the steps?
Bring your book edition, your school topic list and a question you attempted. Ask a tutor to explain the choice of method and test it with a different problem. Agree the lesson topics, schedule and fee before starting.
For cost context, see home tuition fees in Delhi NCR. Agree the topics, schedule and fee with your chosen tutor.
Questions about this practice page
- What is the SI unit of force?
- The newton (N) - from F = m x a, one newton gives a 1 kg mass an acceleration of 1 m/s^2.
- What are balanced and unbalanced forces?
- Balanced forces cancel to zero net force and motion stays unchanged; an unbalanced (non-zero net) force changes the state of motion.
- What is inertia?
- The tendency of an object to resist change in its state of rest or uniform motion - the idea behind Newton's first law.
- Do action and reaction cancel each other?
- No - they act on different bodies, so they never cancel out on a single object.
- Are these the NCERT exercise questions for Class 9 Science Chapter 6?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.