Calculation of Gravitational Force – JSS1 Basic Science Lesson Note (Third Term)

Gravitational force is the force that pulls objects towards the ground, so work must be done to lift an object against it.

Behavioural objectives

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

1. calculate the work done in lifting an object against gravity;
2. distinguish between balanced and unbalanced forces;
3. state the general effects of a force;
4. define frictional force;
5. state the advantages and disadvantages of friction and two ways of reducing it.

What is gravitational force?

Gravitational force is the force of attraction that pulls objects towards the centre of the Earth.

• Effect: a ball thrown up falls back, and a mango falls from the tree to the ground.
• Weight: the pull of gravity on an object, found with W = mg.
• Mass (m): the amount of matter in an object, measured in kilograms (kg).
• Acceleration due to gravity (g): taken as 9.8 m/s² in this note.
• Height (h): the vertical distance the object is lifted, measured in metres (m).

How do you calculate work done against gravity?

The work done in lifting an object against gravity is calculated with the formula work done = mgh, and it is measured in joules (J).

Meaning of the symbols

Worked example 1

Lift an object of mass 1 kg from the ground to a height of 1 m.

1. Write the formula: work done = mgh.
2. Substitute: mgh = 1 × 9.8 × 1.
3. Answer: 9.8 joules.

Worked example 2

A student lifts a bag of mass 5 kg onto a shelf 2 m high.

1. Write the formula: work done = mgh.
2. Substitute: mgh = 5 × 9.8 × 2.
3. Answer: 98 joules.

What are balanced and unbalanced forces?

Balanced forces are equal forces acting on an object in opposite directions, while unbalanced forces are forces that do not cancel out.

Balanced forces

• Direction: the forces act in opposite directions.
• Size: the forces are equal in value.
• Sum: the sum of the forces on the body is zero.
• Effect: a stationary object stays at rest.
• Example: a book lying on a table, where its weight is balanced by the push of the table.

Unbalanced forces

• Direction: the forces act in opposite directions or in the same direction.
• Size: the forces are not equal in value.
• Sum: the sum of the forces is not zero.
• Effect: the body accelerates.
• Example: a football kicked along the ground.

Comparison

What are the general effects of a force?

A force is a push or a pull that can change the state of motion of an object.

1. A force causes motion of an object.
2. A force causes a moving object to come to rest (stop).
3. A force changes the speed of a body in motion.
4. A force causes changes in the direction of motion.
5. A force can change the shape of an object, such as squeezing a sponge.

Unit and formula of force

• SI unit: the newton (N).
• Formula: the force acting on an accelerating body of mass m is F = ma.
• Direction: a force acts in a specific direction, usually shown by an arrow.

Worked example 3

Find the force that gives a body of mass 2 kg an acceleration of 3 m/s².

1. Write the formula: F = ma.
2. Substitute: F = 2 × 3.
3. Answer: 6 N.

What is frictional force?

Frictional force is the force that acts between the surfaces of solid objects and tends to oppose their motion over each other.

• Direction: it acts opposite to the direction of motion.
• Rough surfaces: they produce more friction.
• Smooth surfaces: they produce less friction.
• Heavier objects: they press harder on the surface and give more friction.

What are the advantages of friction?

1. It helps in walking.
2. It helps in carrying objects.
3. It helps in grinding.
4. It makes nails stay in the wall.
5. Car brakes work by friction.

What are the disadvantages of friction?

1. It causes wear and tear of machine parts.
2. It causes a loss of energy.
3. It produces unpleasant noise.
4. It makes the machine hot and inefficient.

How can friction be reduced?

1. Lubrication: put oil or grease on the moving surfaces.
2. Ball bearings: use them between moving parts, as in bicycle wheels.
3. Smoothing: polish the rubbing surfaces.

Evaluation

1. Define frictional force.
2. State two advantages of friction.
3. State two disadvantages of friction.
4. Calculate the work done in lifting a 4 kg object to a height of 3 m (g = 9.8 m/s²).
5. State two differences between balanced and unbalanced forces.

Assignment

1. (a) State two general effects of a force. (b) State two methods of reducing friction.