Class 9 Science · Original EZT practice
Work, Energy, and Simple Machines (Class 9 Science Chapter 7): summary and practice
In science, work has a precise meaning: force times displacement. This chapter builds from there - when work is zero, positive or negative, the forms energy takes, kinetic and potential energy, and how mechanical energy is conserved. 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. Work done by a constant force
Work is done when a force moves an object along its direction - W = F x s, and its SI unit is the joule, where 1 J = 1 N x 1 m.
2. Zero, positive and negative work
Work is zero when the force is perpendicular to the displacement or there is no displacement at all - friction does negative work because it acts opposite to the motion.
3. Kinetic and potential energy
Kinetic energy is the energy of motion, KE = 1/2 m v^2 - potential energy is stored energy due to position, PE = m g h.
4. Conservation of mechanical energy
Mechanical energy (KE + PE) transforms between the two forms but its total stays constant when only gravity does work - a falling object trades potential energy for kinetic energy.
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.
Compute work done
Choose the method: W = F x s along the force's direction.
EZT practice question
A force of 20 N pushes a box 3 m along the floor. Find the work done.
Show the worked solution
- Work done = force x displacement in the direction of the force.
- W = F x s = 20 x 3.
- W = 60 J.
Answer: W = 60 J.
Check: Units: N x m = J, so 20 N over 3 m giving 60 J is consistent.
Watch for this: Do not multiply by time - work depends on displacement, not duration.
Now try it yourself
A 50 N force moves a cart 4 m in the direction of the force. Find the work done.
Check your answer
W = 50 x 4 = 200 J.
Same formula: force times displacement.
Spot zero work
Choose the method: Perpendicular force or no displacement means zero work.
EZT practice question
A coolie carries a load on his head while walking horizontally. How much work does he do against gravity?
Show the worked solution
- The upward force on the load acts vertically.
- The displacement is horizontal.
- Force and displacement are perpendicular, so the work against gravity is zero.
Answer: Zero - the upward force on the load is perpendicular to the horizontal displacement.
Check: A satellite circling Earth is the same idea: gravity is perpendicular to the orbit at every point, so gravity does zero work.
Watch for this: Do not confuse feeling tired with doing work - holding the load still is zero work in physics.
Now try it yourself
Why is the work done by gravity on a satellite in a circular orbit zero?
Check your answer
Gravity is perpendicular to the displacement at every point.
Perpendicular force does no work.
Explain negative work
Choose the method: Force opposing the displacement gives negative work.
EZT practice question
Friction of 15 N acts on a block sliding 4 m. Find the work done by friction.
Show the worked solution
- Friction acts opposite to the block's motion.
- A force opposing the displacement does negative work.
- W = -15 x 4 = -60 J.
Answer: W = -60 J - negative because friction opposes the motion.
Check: Negative work takes energy away from the block, which is why sliding things slow down.
Watch for this: Do not drop the sign - direction matters in work, and friction's work is negative.
Now try it yourself
Define 1 joule.
Check your answer
The work done when a force of 1 N moves an object 1 m in the direction of the force.
That is the SI unit of work and energy.
Compute kinetic energy
Choose the method: KE = 1/2 m v^2.
EZT practice question
A 2 kg ball moves at 3 m/s. Find its kinetic energy.
Show the worked solution
- KE = 1/2 m v^2.
- Substitute m = 2 kg and v = 3 m/s: KE = 1/2 x 2 x 3^2.
- KE = 9 J.
Answer: KE = 9 J.
Check: The velocity is squared, so doubling the speed would quadruple the energy - here 3^2 = 9 does the work.
Watch for this: Do not forget the square on velocity - 1/2 x 2 x 3 = 3 J would be wrong.
Now try it yourself
Find the kinetic energy of a 1200 kg car moving at 20 m/s.
Check your answer
KE = 1/2 x 1200 x 400 = 240000 J = 240 kJ.
20^2 = 400, then halve the 1200 kg.
Compute potential energy
Choose the method: PE = m g h.
EZT practice question
A 0.5 kg book is raised 4 m. Find its potential energy (take g = 10 m/s^2).
Show the worked solution
- PE = m g h.
- Substitute m = 0.5 kg, g = 10 m/s^2, h = 4 m.
- PE = 0.5 x 10 x 4 = 20 J.
Answer: PE = 20 J.
Check: Potential energy grows linearly with height - double the height, double the PE.
Watch for this: Do not use the speed formula here - position, not motion, sets potential energy.
Now try it yourself
A 2 kg bag sits on a shelf 1.5 m high. Find its potential energy (g = 10 m/s^2).
Check your answer
PE = 2 x 10 x 1.5 = 30 J.
Mass times g times height.
Apply conservation of mechanical energy
Choose the method: PE lost equals KE gained; the total stays fixed.
EZT practice question
A ball dropped from a height has 100 J of potential energy at the top. What is its kinetic energy just before hitting the ground (ignore air resistance)?
Show the worked solution
- At the top the energy is all potential: 100 J.
- As the ball falls, potential energy converts into kinetic energy.
- Just before impact, all 100 J is kinetic - the total mechanical energy stays 100 J throughout.
Answer: 100 J - all the potential energy converts to kinetic energy.
Check: Halfway down the fall the split is PE = 50 J and KE = 50 J - the total is still 100 J.
Watch for this: Do not say energy is used up in falling - it changes form; the total is conserved.
Now try it yourself
A pendulum swings from its highest to its lowest point. Describe the energy change.
Check your answer
Potential energy converts into kinetic energy; KE is maximum at the lowest point.
The lowest point has the least height left to trade.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Formula for work done?
Show answer and reason
W = F x s.
Force times displacement in the force's direction, in joules.
2. When is work zero?
Show answer and reason
No displacement, or force perpendicular to displacement.
Carrying a load horizontally does zero work against gravity.
3. KE and PE formulas?
Show answer and reason
KE = 1/2 m v^2 and PE = m g h.
Motion energy and position energy.
4. Conservation of mechanical energy?
Show answer and reason
KE + PE stays constant when only gravity does work.
Energy changes form but is not lost.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- A 50 N force moves a cart 4 m in the direction of the force. Find the work done.
- A block slides 4 m against 15 N of friction. Find the work done by friction.
- A ball with 100 J of potential energy is dropped. What is its kinetic energy just before hitting the ground?
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Questions about this practice page
- What is the formula for work done?
- W = F x s (force times displacement in the direction of the force), measured in joules. 1 J = 1 N x 1 m.
- When is work done zero?
- When there is no displacement, or when the force is perpendicular to the displacement - like carrying a load horizontally against gravity.
- What are the formulas for kinetic and potential energy?
- KE = 1/2 m v^2 and PE = m g h.
- What does conservation of mechanical energy mean?
- The sum of kinetic and potential energy stays constant when only gravity does work - energy changes form but is not lost.
- Are these the NCERT exercise questions for Class 9 Science Chapter 7?
- 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.