Free Work MCQs with Answers

1. Work done by a force is zero when the angle between the force and the displacement is

• A. 0 degrees
• B. 45 degrees
• C. 90 degrees
• D. 180 degrees

Explanation: Work is the scalar product Fd cos theta, and cos 90 degrees is zero, so a force perpendicular to the displacement does no work. This is why the centripetal force in uniform circular motion does no work and the speed stays constant. At 180 degrees the work is negative rather than zero, as with friction.

2. A porter carries a suitcase horizontally across a platform at constant speed. The work done by the porter against gravity is

• A. equal to mgh
• B. zero
• C. equal to the weight times the distance walked
• D. negative

Explanation: The upward force supporting the suitcase is perpendicular to the horizontal displacement, so the work done against gravity is zero. The porter still expends metabolic energy holding the case up, but that is not mechanical work on the suitcase, which is the standard point of this question. Work against gravity appears only when the height changes.

3. Work is done on a body only when

• A. a force acts on it
• B. a force acts and the body moves with a component of displacement along the force
• C. the body is at rest
• D. the body has energy

Explanation: Work is the dot product of force and displacement, so a porter standing still holding a heavy suitcase does no work on it however tiring it feels. Carrying it horizontally also does no work against gravity, since the weight is vertical and the displacement horizontal. The joule is the work done when a force of one newton moves its point of application one metre.

4. A force of 50 N pulls a box 4 m along the floor at 60 degrees to the horizontal. The work done is

• A. 200 J
• B. 100 J
• C. 173 J
• D. 0 J

Explanation: Only the component of force along the displacement counts, so the work is 50 multiplied by 4 multiplied by cos 60, which is 200 multiplied by 0.5, giving 100 J. Ignoring the angle gives the 200 J distractor and using sin instead of cos gives 173 J. Work is a scalar, so the answer carries no direction.

5. The work done by the centripetal force on a body moving in a circle at constant speed is

• A. maximum
• B. zero
• C. equal to the kinetic energy
• D. negative

Explanation: The force points to the centre while the displacement is along the tangent, so the two are perpendicular and the dot product is zero at every instant. This is consistent with the speed and therefore the kinetic energy staying constant. A force can change the direction of motion without doing any work at all.

6. A force acts on a body but does negative work. This means the force

• A. acts in the same direction as the displacement
• B. has a component opposite to the displacement, so it removes energy from the body
• C. does no work at all
• D. acts at right angles to the displacement

Explanation: Friction and air resistance always oppose the motion and therefore always do negative work, reducing the kinetic energy. A force at right angles does zero work rather than negative work, which is the distinction being tested. Gravity does negative work on a body moving upwards and positive work as it comes back down.