Force – JSS1 Basic Science Lesson Note (Third Term)

Force is a push or a pull that makes or tries to move, stop or change the speed of a body. Its unit is the newton (N).

Behavioural objectives

By the end of the lesson, students should be able to:

1. define force and state its unit;
2. write the formula for force and use it in a simple calculation;
3. name the two main types of force and give examples of each;
4. describe how magnets behave and state what gravitational force is;
5. state how like and unlike electric charges act on each other.

What is force?

Force is any action that makes or tries to move, stop or change the speed of a body in a given direction.

• Simple meaning: force is a push or a pull.
• Unit: the newton, written N.
• Instrument: force is measured with a spring balance (newton meter).
• Example: kicking a football pushes it forward. Pulling a bucket from a well pulls it upward.

What does a force do to a body?

A force can change the state of motion, direction or shape of a body.

• Starts movement: a force can make a resting body move, as when you push a stalled bicycle.
• Stops movement: a goalkeeper’s hands stop a moving ball.
• Changes speed: pedalling harder makes a bicycle go faster.
• Changes direction: a hockey stick turns the path of a ball.
• Changes shape or size: pressing clay or stretching a rubber band changes its shape.

What is the formula for force?

Force is the product of mass and acceleration.

• Formula: Force (F) = Mass (m) × Acceleration (a), or F = ma.
• F: force, measured in newtons (N).
• m: mass of the body, measured in kilograms (kg).
• a: acceleration, measured in metres per second squared (m/s²).
• One newton: the force that gives a mass of 1 kg an acceleration of 1 m/s².

Worked example

Find the force that gives a body of mass 5 kg an acceleration of 2 m/s².

1. Write the formula: F = ma.
2. Substitute the values: F = 5 × 2.
3. Answer: F = 10 N.

What are the types of force?

There are two main types of force: contact force and non-contact force.

• Contact force: the source touches the body it acts on.
• Non-contact force: the source acts on the body without touching it.

What are contact forces?

Contact forces are forces in which the source is in contact with the body to which they are applied.

• Push: a force that moves a body away from you, as in pushing a door open.
• Pull: a force that moves a body towards you, as in pulling a drawer.
• Tension: the pulling force in a stretched rope, string or wire, as in a tug of war.
• Friction: the force that opposes motion between two surfaces in contact, as when you rub your palms together.
• Upthrust: the upward force of a liquid or gas on a body in it, which helps a boat to float.

Contact forces may be direct or indirect.

What are non-contact forces?

Non-contact forces are forces that act on a body without physical contact.

• Field: the region in space through which these forces act.
• Other name: they are also called force fields.
• Magnetic force: the force of a magnet on magnetic materials or other magnets.
• Gravitational force: the force with which the Earth pulls objects towards itself.
• Electric force: the force between electric charges.

What is the difference between contact and non-contact forces?

A contact force needs touching. A non-contact force acts across a gap through a field.

What are magnetic, gravitational and electric forces?

These are the three non-contact forces taught at this level.

What is magnetic force?

Magnetic force is the force a magnet exerts on magnetic materials and on other magnets.

• Poles: a bar magnet has two poles, the North Pole and the South Pole.
• Like poles repel: North repels North, and South repels South.
• Unlike poles attract: North and South pull towards each other.
• Magnetic materials: magnets attract iron filings, iron nails, steel, nickel and cobalt.
• Magnetic field: the region around a magnet where its force acts.
• Field force: magnetic force acts over an area, so it is a field force.

What is gravitational force?

Gravitational force is the force with which the Earth attracts all objects towards itself.

• Effect: objects above the ground fall down when released.
• Example: a ripe mango falls from the tree to the ground.
• Weight: the gravitational force on an object, measured in newtons.

What is electric force?

Electric force is the force that exists between two electric charges.

• Unlike charges attract: a positive charge and a negative charge pull together.
• Like charges repel: two positive charges push apart, and so do two negative charges.
• Example: a comb rubbed on dry hair picks up small bits of paper.

How do magnetic poles and electric charges compare?

Evaluation

1. What is force? Give its SI unit.
2. State two examples of contact forces.
3. State two examples of non-contact forces.
4. What happens when two like poles of magnets are brought close together?
5. Calculate the force that gives a mass of 4 kg an acceleration of 3 m/s².

Assignment

Explain the difference between contact forces and non-contact forces, with two examples of each. Then write the formula for force and say what each letter stands for.