Kinetic and potential energy: Class 9 practice

What you should know first

• Squares
• Units

The idea behind kinetic and potential energy

Kinetic energy = ½mv²; potential energy = mgh. Doubling speed makes KE four times. When a body falls, PE turns into KE (ignoring air resistance).

Step-by-step method

1. Write m, v or h in SI units.
2. KE = ½mv², PE = mgh.
3. Use energy conservation for falling bodies.

Worked example 1 (Basic)

Find the kinetic energy of a 15 kg object moving at 2 m/s.

1. KE = ½ × 15 × 2²
2. = 30 J
3. Answer: 30 J

Worked example 2 (Standard)

A 13 kg object is lifted 15 m. Find the gain in potential energy (g = 10 m/s²).

1. PE = mgh = 13 × 10 × 15
2. = 1950 J
3. Answer: 1950 J

Worked example 3 (Challenge)

By what factor does KE change if speed becomes 2 times?

1. KE ∝ v²
2. Factor = 2² = 4
3. Answer: 4 times

Common mistakes and misconceptions

• Forgetting to square v
• Using grams

Practice questions

1. (Basic) Find the kinetic energy of a 20 kg object moving at 9 m/s.
2. (Basic) Find the kinetic energy of a 16 kg object moving at 10 m/s.
3. (Standard) A 17 kg object is lifted 3 m. Find the gain in potential energy (g = 10 m/s²).
4. (Standard) A 4 kg object is lifted 5 m. Find the gain in potential energy (g = 10 m/s²).
5. (Challenge) By what factor does KE change if speed becomes 4 times?
6. (Challenge) By what factor does KE change if speed becomes 3 times?

Answer key with full working

• Q1: KE = ½ × 20 × 9² → = 810 J → Answer: 810 J
• Q2: KE = ½ × 16 × 10² → = 800 J → Answer: 800 J
• Q3: PE = mgh = 17 × 10 × 3 → = 510 J → Answer: 510 J
• Q4: PE = mgh = 4 × 10 × 5 → = 200 J → Answer: 200 J
• Q5: KE ∝ v² → Factor = 4² = 16 → Answer: 16 times
• Q6: KE ∝ v² → Factor = 3² = 9 → Answer: 9 times

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)
• Work, kinetic energy 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 kinetic 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.