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Class 9 Science: How Forces Affect Motion, 8 practice questions with answers

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These are 8 practice questions on Chapter 6, How Forces Affect Motion, of the new NCERT Class 9 Science textbook, Exploration (First Edition, April 2026). Each has a worked answer and the textbook page it comes from. They cover the net force from two forces, balanced forces and the first law, the second law F = ma both ways, weight with g = 9.8 m s⁻², why a third-law pair does not cancel, two people pushing apart, and two boxes pulled together as one system. Try each one on paper before you open the answer. A short chapter on how science works, with questions on laws, models and units, is Class 9 Science Chapter 1: Exploration.

These questions are our own, drafted with AI help. Every number was re-computed by a test script and none of the numbers is taken from the chapter, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Exploration: Textbook of Science for Grade 9, Chapter 6, printed pages 94 to 115):

Net force and the first law

Question 1. Two people push a crate in opposite directions: one with 23 N to the east, the other with 14.5 N to the west. What is the net force on the crate?

Answer: 8.5 N towards the east. The forces are opposite and unequal, so the net force is the difference, 23 − 14.5 = 8.5 N, in the direction of the larger force, east. (Net force, page 96.)

Question 2. A box slides across a floor. You push it forward with 37 N while friction acts on it backwards with 37 N. What happens to its motion?

Answer: it keeps moving at the same constant velocity. The two forces are equal and opposite, so they are balanced and the net force is zero. By Newton's first law, an object in motion keeps moving with a constant velocity when no net force acts on it. (Balanced forces, page 96; Newton's first law, page 100.)

The second law, F = ma

Question 3. A net force of 33.3 N acts on a trolley of mass 4.5 kg. What is its acceleration?

Answer: 7.4 m s⁻². a = F/m = 33.3 ÷ 4.5 = 7.4 m s⁻², in the direction of the force. (Eq. 6.1, page 104.)

Question 4. What net force is needed to give a car of mass 760 kg an acceleration of 3.5 m s⁻²?

Answer: 2660 N. F = ma = 760 × 3.5 = 2660 N. (Eq. 6.2, page 104.)

Question 5. What is the gravitational force of the Earth on a student of mass 42 kg? Take g = 9.8 m s⁻².

Answer: 411.6 N. F = mg = 42 × 9.8 = 411.6 N, acting downwards. (Eq. 6.3, page 104.)

The third law and systems

Question 6. A book rests on a table. The Earth pulls the book down and the table pushes the book up, with forces of the same size. Is this pair of forces a pair of Newton's third law?

Answer: no. Both forces act on the same object, the book, so they are two balanced forces on the book. A third-law pair always acts on two different objects: for example, the book pushes down on the table and the table pushes up on the book. (Third law, page 108; the Note on page 109.)

Question 7. A boy of mass 70 kg standing on smooth ice pushes a trolley of mass 35 kg with a force of 154 N. What are the accelerations of the trolley and of the boy at that moment?

Answer: trolley 4.4 m s⁻², boy 2.2 m s⁻², in opposite directions. The boy pushes the trolley with 154 N, so by the third law the trolley pushes the boy with 154 N the other way. Trolley: a = 154 ÷ 35 = 4.4 m s⁻². Boy: a = 154 ÷ 70 = 2.2 m s⁻². The same force gives the smaller mass the larger acceleration. (Third law, page 108; F = ma, page 104.)

Question 8. Two boxes of mass 9.5 kg and 3.7 kg are tied together with a string on a frictionless floor, and the front box is pulled with a force of 47.52 N. Find the acceleration of the two boxes.

Answer: 3.6 m s⁻². Treat the two boxes and the string as one system of mass 9.5 + 3.7 = 13.2 kg; the string's pull is an internal force. a = F/(m₁ + m₂) = 47.52 ÷ 13.2 = 3.6 m s⁻². (Eq. 6.4, page 111.)

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Sources

All opened and read on 21 October 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 93.

The laws and equations are from the textbook pages above. The questions, worked answers and checks are our own, and every number was re-computed by script. This guide is general information; follow your teacher and the textbook.