Force is a push or pull acting on an object. When you push a door to open it or pull a cart, you are applying a force.

Effects of Force on Matter

Force can cause different changes in an object:

1. Change in motion — Force can make a stationary object move, stop a moving object, or change its direction.
2. Change in speed or acceleration — A stronger force makes an object speed up faster.
3. Change in shape or size — Force can stretch, compress, or bend a material. For example, pulling a rubber band stretches it, and pushing a spring compresses it.
4. Change in direction — Force can turn a moving object onto a new path.

These changes show how force interacts with the mechanical properties of matter.

Measuring Force

A spring balance is used to measure the magnitude of a force. It has a coiled spring inside. When you attach an object to the hook, the spring stretches. The more force you apply, the more the spring stretches. The scale on the balance shows the force in newtons (N).

Work is done when a force makes an object move in the direction of the force. No work is done if the object does not move, even if a force is applied.

Work Formula

$W = F \times d$

Where:

• $W$ = work done (in joules, J)
• $F$ = force applied (in newtons, N)
• $d$ = distance moved in the direction of the force (in metres, m)

Worked Example 1

A student pushes a school desk with a force of 50 N through a distance of 3 m. How much work is done?

Solution: $W = F \times d = 50 \, \text{N} \times 3 \, \text{m} = 150 \, \text{J}$

The work done is 150 joules.

Energy is the ability to do work. When work is done, energy is transferred from one form to another. Two important forms of mechanical energy are kinetic energy and potential energy.

Kinetic Energy

Kinetic energy is the energy of motion. Any moving object has kinetic energy.

$E_k = \frac{1}{2} m v^2$

Where:

• $E_k$ = kinetic energy (joules, J)
• $m$ = mass of the object (kilograms, kg)
• $v$ = speed of the object (metres per second, m/s)

Gravitational Potential Energy

Gravitational potential energy is stored energy due to an object's position above the ground.

$E_p = m \times g \times h$

Where:

• $E_p$ = potential energy (joules, J)
• $m$ = mass (kilograms, kg)
• $g$ = gravitational field strength (approximately 10 N/kg on Earth)
• $h$ = height above the ground (metres, m)

Worked Example 2

A bag of maize weighing 5 kg is lifted onto a shelf 2 metres high. Calculate the gravitational potential energy gained by the bag.

Solution: $E_p = m \times g \times h = 5 \, \text{kg} \times 10 \, \text{N/kg} \times 2 \, \text{m} = 100 \, \text{J}$

The bag gains 100 joules of potential energy.

Worked Example 3

A bicycle of mass 20 kg is moving at 5 m/s. Calculate its kinetic energy.

Solution: $E_k = \frac{1}{2} m v^2 = \frac{1}{2} \times 20 \, \text{kg} \times (5 \, \text{m/s})^2 = \frac{1}{2} \times 20 \times 25 = 250 \, \text{J}$

The bicycle has 250 joules of kinetic energy.

Power tells us how fast work is done or how fast energy is transferred.

$P = \frac{W}{t}$

Where:

• $P$ = power (watts, W)
• $W$ = work done (joules, J)
• $t$ = time taken (seconds, s)

Worked Example 4

A student lifts a bucket of water (mass 10 kg) to a height of 1.5 m in 3 seconds. Calculate the power used. (Take g = 10 N/kg)

Solution:

First, calculate the work done: $W = F \times d = (m \times g) \times h = (10 \times 10) \times 1.5 = 100 \times 1.5 = 150 \, \text{J}$

Now calculate power: $P = \frac{W}{t} = \frac{150 \, \text{J}}{3 \, \text{s}} = 50 \, \text{W}$

The student uses 50 watts of power.

• Force is a push or pull that can change the shape, motion, or direction of an object.
• Work is done when a force causes movement: $W = F \times d$
• Kinetic energy is energy of motion: $E_k = \frac{1}{2}mv^2$
• Potential energy is stored energy due to height: $E_p = mgh$
• Power is the rate of doing work: $P = \frac{W}{t}$

These concepts are connected — when a force does work on an object, energy is transferred to or from the object, changing its mechanical properties.

In Tanzania, understanding mechanical properties of matter, force, and energy is useful in many everyday activities. For example, when pumping water from a shallow well using a hand pump, the force you apply does work to lift the water. The energy you use to pump is transferred to the water as gravitational potential energy. Knowing how force and energy work together helps farmers and traders lift heavy sacks of maize or beans more efficiently, reducing tiredness and increasing productivity at the market.