Work, kinetic energy and potential energy: Class 9 practice

What you should know first

• Newton's second law
• Squares

The idea behind work, kinetic energy and potential energy

Work W = F × s (when force is along the motion). Kinetic energy = ½mv²; potential energy near Earth = mgh. Energy and work are measured in joules (J).

Step-by-step method

1. Identify which energy form the question is about.
2. Use SI units (kg, m, m/s).
3. Substitute in the formula.
4. Use energy conservation when an object falls.

Worked example 1 (Basic)

A force of 6 N moves a box 17 m in the direction of the force. How much work is done?

1. W = F × s = 6 × 17
2. W = 102 J
3. Answer: 102 J

Worked example 2 (Standard)

Find the kinetic energy of a 4 kg object moving at 16 m/s.

1. KE = ½ × 4 × 16²
2. KE = 512 J
3. Answer: 512 J

Worked example 3 (Challenge)

A 17 kg stone is dropped from 38 m. Taking g = 10 m/s², find its potential energy at the top and its speed just before hitting the ground (ignore air resistance).

1. PE = mgh = 17 × 10 × 38 = 6460 J
2. All PE → KE: ½mv² = 6460
3. v = √(2gh) = √760 ≈ 27.6 m/s
4. Answer: PE = 6460 J, v ≈ 27.6 m/s

Common mistakes and misconceptions

• Forgetting to square v
• Using g = 9.8 when the question says 10 (follow the question)

Practice questions

1. (Basic) A force of 37 N moves a box 18 m in the direction of the force. How much work is done?
2. (Basic) A force of 48 N moves a box 18 m in the direction of the force. How much work is done?
3. (Standard) Find the kinetic energy of a 36 kg object moving at 8 m/s.
4. (Standard) Find the kinetic energy of a 21 kg object moving at 24 m/s.
5. (Challenge) A 34 kg stone is dropped from 15 m. Taking g = 10 m/s², find its potential energy at the top and its speed just before hitting the ground (ignore air resistance).
6. (Challenge) A 37 kg stone is dropped from 13 m. Taking g = 10 m/s², find its potential energy at the top and its speed just before hitting the ground (ignore air resistance).

Answer key with full working

• Q1: W = F × s = 37 × 18 → W = 666 J → Answer: 666 J
• Q2: W = F × s = 48 × 18 → W = 864 J → Answer: 864 J
• Q3: KE = ½ × 36 × 8² → KE = 1152 J → Answer: 1152 J
• Q4: KE = ½ × 21 × 24² → KE = 6048 J → Answer: 6048 J
• Q5: PE = mgh = 34 × 10 × 15 = 5100 J → All PE → KE: ½mv² = 5100 → v = √(2gh) = √300 ≈ 17.3 m/s → Answer: PE = 5100 J, v ≈ 17.3 m/s
• Q6: PE = mgh = 37 × 10 × 13 = 4810 J → All PE → KE: ½mv² = 4810 → v = √(2gh) = √260 ≈ 16.1 m/s → Answer: PE = 4810 J, v ≈ 16.1 m/s

Syllabus note

This skill is listed under "Work and Energy" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

• Work and Energy chapter notes (Class 9)
• Kinetic and potential energy
• Power and the kilowatt-hour
• Work done by a force at an angle
• Equations of motion numericals
• Newton's second law and momentum
• Pressure and thrust
• Average speed, velocity and km/h ↔ m/s
• All Class 9 practice

Frequently asked questions

Which chapter is work, kinetic energy and potential energy in?

"Work and Energy", Class 9 Physics (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.