Class 9 Science Exploration Chapter 7 MCQ – Work, Energy and Simple Machines

Work done by a constant force equals the force multiplied by displacement in its direction.

For force and displacement in the same direction, W = Fs.

Use W = Fs because force and displacement have the same direction.

Thus, W = 10 × 3 = 30 J.

Work transfers energy, so both quantities have the same SI unit, the joule.

One joule equals one newton metre.

Work requires displacement in the direction of the force.

The wall has no displacement, so the work done on it is zero even if the student feels tired.

The supporting force is vertical while the displacement is horizontal.

These directions are perpendicular, so there is no displacement along the supporting force and its work is zero.

A force does negative work when it acts opposite to the displacement.

For example, a goalkeeper’s stopping force does negative work on a moving ball.

The stopping force and displacement are opposite, so the work is negative.

W = −Fs = −200 × 0.15 = −30 J.

An object possesses energy when it has the capacity to do work.

For example, a moving ball can transfer energy and make another object move.

The net work done on an object equals the change in its kinetic energy.

Positive net work of 40 J therefore increases kinetic energy by 40 J.

Kinetic energy is the energy associated with motion.

Its value depends on the object’s mass and the square of its speed.

Kinetic energy is K = ½mv².

Substituting gives K = ½ × 2 × 3² = 9 J.

Kinetic energy is proportional to speed squared: K = ½mv².

Doubling the speed multiplies kinetic energy by 2² = 4.

For equal kinetic energy, m × v₁² = 4m × v₂².

Thus, v₁² = 4v₂² and v₁ = 2v₂, giving a speed ratio of 2:1.

Stretching changes the rubber band’s shape and stores potential energy.

When released, the band can transfer this energy to an object and set it moving.

Raising a mass m through height h against gravity requires work mgh.

This work increases the gravitational potential energy by mgh.

Use U = mgh.

Therefore, U = 0.2 × 10 × 10 = 20 J relative to the ground.

The gain in gravitational potential energy is mgΔh near Earth’s surface.

For the same mass and height increase, this gain does not depend on the path taken.

As the object loses height, gravitational potential energy decreases while kinetic energy increases.

The gain in kinetic energy equals the loss of potential energy, so mechanical energy stays constant.

Mechanical energy includes both the energy of motion and stored potential energy.

Therefore, mechanical energy = kinetic energy + potential energy.

At the lowest point, gravitational potential energy is smallest and the pendulum’s speed is greatest.

The converted potential energy appears as maximum kinetic energy.

Conservation of energy gives mgh = ½mv², so v = √(2gh).

Mass cancels. For the same vertical drop, both children reach the same speed under these ideal conditions.

Power tells us how quickly work is done.

Average power equals work divided by time, P = W/t.

Average power = work ÷ time.

Thus, P = 1500 ÷ 5 = 300 W, meaning an average of 300 J of work each second.

Average power is P = W/t.

For the same work, reducing the time to half doubles the power.

Mechanical advantage compares the force overcome with the force applied.

It equals load divided by effort and has no unit because both are forces.

An ideal fixed pulley lets a downward pull raise a load upwards.

It changes force direction but has mechanical advantage 1, so the required effort equals the load.

For an ideal inclined plane, mechanical advantage = length ÷ height.

Here, 4 ÷ 1 = 4, so the effort is one-quarter of the load.

The work required to raise the load remains mgh.

A longer ramp increases the distance over which the effort acts, allowing a smaller force to do the same work.

A lever is a rigid bar that can turn about a fixed point.

That point is the fulcrum; effort and load act at distances measured from it.

For balance, effort × effort arm = load × load arm.

Effort = (90 × 20) ÷ 60 = 30 N.