Conservation of mechanical energy — KCSE Physics

KCSE Physics · 69 practice questions · 9 syllabus objectives

24 easy22 medium23 hard

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Distinguish between elastic and inelastic collisions; apply conservation of momentum and energy to collision problems

State the law of conservation of energy and apply it to mechanical systems (KE + PE = constant)

Solve problems involving conversion between kinetic and potential energy for free-falling objects and pendulums

Distinguish between elastic and inelastic collisions in terms of kinetic energy conservation

Define energy, state its SI unit, derive KE = ½mv², and calculate kinetic, gravitational potential and elastic potential energy

State the law of conservation of energy; define transducers and describe energy transformations in systems

Apply conservation of energy to falling objects, inclines, pendulums, projectiles and springs

Define efficiency, calculate efficiency and energy wasted; apply the work-energy theorem and calculate power output

Conservation of mechanical energy

Sample Questions

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1
easySHORT ANSWER4 marks

A ball is dropped from a height of 10 m. (a) Identify the form of energy it has just before it is dropped. (1 mark) (b) Identify the form of energy it has just before it hits the ground. (1 mark) (c) Explain how the principle of conservation of mechanical energy applies in this scenario. (2 marks)

View Marking Scheme
Part (a) — 1 mark
Gravitational potential energy (1 mk)
Part (b) — 1 mark
Kinetic energy (1 mk)
Part (c) — 2 marks
The total mechanical energy (potential + kinetic) remains constant (1 mk)
As the ball falls, potential energy is converted to kinetic energy (1 mk)
2
easySHORT ANSWER4 marks

State two key differences between elastic and inelastic collisions in terms of kinetic energy conservation. (4 marks)

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Part (a) — 4 marks
In elastic collisions, total kinetic energy is conserved; in inelastic collisions, total kinetic energy is not conserved (1 mk)
Elastic collisions result in separate final velocities for colliding bodies; inelastic collisions result in combined final velocity or deformation (1 mk)
Elastic collisions can occur with no sound or heat loss; inelastic collisions usually produce sound and thermal energy (1 mk)
Examples of elastic collisions include gas molecules; examples of inelastic collisions include car crashes (1 mk)
3
easySHORT ANSWER4 marks

A ball is dropped from a height of 10 m. Identify the potential energy of the ball at the top and the kinetic energy just before it hits the ground. (4 marks)

View Marking Scheme
Part (a) — 2 marks
Potential energy = mgh (where m is mass, g is 9.81 m/s², h is height) (1 mk)
Assuming mass is 1 kg, potential energy = 1 kg * 9.81 m/s² * 10 m = 98.1 J (1 mk)
Part (b) — 2 marks
Just before hitting the ground, kinetic energy equals initial potential energy due to conservation of energy (1 mk)
Kinetic energy = 98.1 J (1 mk)
4

A roller coaster car is at the highest point of a track. (a) State the type of energy it possesses at this point. (1 mark) (b) As it descends, describe what happens to its potential energy and kinetic energy. (3 marks)

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Why Practise Conservation of mechanical energy?

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