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Class 9 Science: Work, Energy, and Simple Machines, 8 practice questions with answers

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These are 8 practice questions on Chapter 7, Work, Energy, and Simple Machines, 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 work done by a force, when work is zero, kinetic and potential energy, conservation of mechanical energy, power, and the mechanical advantage of a ramp and a lever. Try each one on paper before you open the answer. Units and quick estimates are the focus of the Class 9 Science Chapter 1 questions.

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.

In the questions, take g = 10 m s⁻², the value the textbook itself uses in its worked examples (Example 7.7, page 126).

What the chapter says you need

Facts from the NCERT textbook (Exploration: Textbook of Science for Grade 9, Chapter 7, printed pages 116 to 139):

Work

Question 1. A worker pushes a trolley with a steady force of 46 N, and it moves 13.5 m in the direction of the push. How much work does the worker do?

Answer: 621 J. W = F × s = 46 × 13.5 = 621 J. (Eq. 7.2, page 117; the joule, page 118.)

Question 2. A guard holds a heavy box perfectly still, at the same height, for a long time. His arms get tired. How much work does he do on the box?

Answer: zero. The box does not move, so its displacement is zero, and W = F × 0 = 0 J. He feels tired because his muscles use energy, but in the scientific sense he does no work on the box. (When is work done equal to zero, page 118.)

Energy

Question 3. A ball of mass 2.6 kg moves at 17 m s⁻¹. What is its kinetic energy?

Answer: 375.7 J. K = ½mv² = ½ × 2.6 × 17² = ½ × 2.6 × 289 = 375.7 J. (Eq. 7.6, page 122.)

Question 4. A box of mass 3.8 kg is lifted onto a shelf 4.5 m above the floor. How much gravitational potential energy does it gain? Take g = 10 m s⁻².

Answer: 171 J. U = mgh = 3.8 × 10 × 4.5 = 171 J. (Eq. 7.8, page 125.)

Question 5. A stone is dropped from rest from a height of 33.8 m. Ignoring air resistance, how fast is it moving just before it hits the ground? Take g = 10 m s⁻².

Answer: 26 m s⁻¹. Mechanical energy is conserved, so the potential energy lost becomes kinetic energy: mgh = ½mv². The mass cancels, so v² = 2gh = 2 × 10 × 33.8 = 676, and v = √676 = 26 m s⁻¹. (Conservation of mechanical energy, pages 126 and 127; the same method is used for a child on a slide on page 128.)

Power and simple machines

Question 6. A labourer carries a 38 kg sack up a staircase 3.5 m high in 3.8 s. What power does he need just to lift the sack? Take g = 10 m s⁻².

Answer: 350 W. Work done against gravity = mgh = 38 × 10 × 3.5 = 1330 J. Power P = W/t = 1330 ÷ 3.8 = 350 W. (Eq. 7.8, page 125; Eq. 7.11, page 129.)

Question 7. A ramp 5.29 m long is used to raise a load onto a platform 0.92 m high. What is the mechanical advantage of the ramp, if friction can be ignored?

Answer: 5.75. For an inclined plane, mechanical advantage = L/h = 5.29 ÷ 0.92 = 5.75. The effort needed is 5.75 times smaller than the load, but it has to be applied over the longer distance. (Eq. 7.13, page 132.)

Question 8. A crowbar is used as a lever to lift a 630 N rock. The rock is 0.35 m from the fulcrum and the effort is applied 1.47 m from the fulcrum. What effort is needed, and what is the mechanical advantage?

Answer: 150 N; mechanical advantage 4.2. effort × effort arm = load × load arm, so effort = 630 × 0.35 ÷ 1.47 = 220.5 ÷ 1.47 = 150 N. Mechanical advantage = load ÷ effort = 630 ÷ 150 = 4.2, which is also effort arm ÷ load arm = 1.47 ÷ 0.35. (Eq. 7.15 and 7.16, page 134; mechanical advantage, page 130.)

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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 + 115.

The 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.