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

Gravitational force pulls objects towards the ground, so work must be done to lift an object against it. This work is calculated with the formula mgh, where m is mass, g is 9.8 m/s² and h is height, and it is measured in joules. The lesson also covers balanced and unbalanced forces and frictional force.

Behavioural objectives

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

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

How do you calculate work done against gravity?

Suppose you lift an object of mass 1 kg from the ground to a height of 1 m. The work done in lifting the object is calculated with the formula mgh.

mgh = 1 × 9.8 × 1 = 9.8 joules (g = 9.8 m/s²)

The joule is the unit of work done.

What are balanced and unbalanced forces?

When an object is stationary, the forces acting on it are balanced. The sum of such forces acting on the body is zero and they act in opposite directions.

When an object is accelerating, the forces acting on it are unbalanced. The sum of such forces is not zero. They act either in opposite directions or in the same direction, but they are not equal in value. Unbalanced forces cause a body to accelerate.

What are the general effects of a force?

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.

The SI unit of force is the newton (N). The force acting on an accelerating body of mass m is given by:

F = ma

A force acts in a specific direction. The direction of the action of a force is usually indicated by an arrow.

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.

Uses and 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.

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.

Methods of reducing friction

1. Lubricate surfaces with oil or grease.
2. Use ball bearings.

Evaluation

1. Define frictional force.
2. State two advantages of friction.
3. State two disadvantages of friction.

Assignment

1. State two general effects of a force.
2. State two methods of reducing friction.