# Gravitational Force

## Students’ Learning Outcomes

·         Explain the gravitational force using different examples.
·         Distinguish between mass and weight.

### Information for Teachers:

·         Gravitational force of earth is that force which pulls everything down towards the earth.
·         It is the force of gravity which is responsible for holding objects onto the surface of earth.
·         Gravitational force always acts towards the center of earth.
·         The quantity of matter in an object is called its mass.
·         The gravitational force acting on an object is called its weight.
·         Mass is measured by using a beam balance, comparing a known amount of matter with an unknown amount of matter. Weight is measured by a spring balance.

Material / Resources:

·         A book, eraser, pencil, two wooden blocks of different masses, tennis ball, polystyrene cup, physical balance, spring balance, textbook.

#### Worm up Activity:

·         Ask three students to come in front of the class, one with a book,  other with an eraser and the third one with a pencil.
·         Ask them to drop these objects or throw them upward.
·         Then ask the following questions:
·         In which direction to these things fall? (students’ response: Towards earth ).
·         Why do these things fall down? (students’ response: The earth pulls everything towards it with some force).
·         Inform the students that this force is called the gravitational force of earth.
·         Now show a big and small wooden blocks to the students. Mark “A” on the big block and ‘B’ on the small block.
·         Ask the students following questions:
v  Which of the two blocks has more quantity of matter in it ; A or B ?
v  Which of the two blocks is havier:  A or B ? (students’ response: The quantity of matter block “A” is more than in Block B.)
·         Inform them that the quantity of matter in any objects is called its mass.

#### Activity 1:

·         Give a tennis ball and ask him/her to throw it upwards.
·         What will happen when the ball is thrown upward ? (students’ response: The ball will come down ).
·         Why the ball comes back to ground? (students’ response: The force of attraction of the earth acts on everything which pulls objects towards it. This is called the gravitational force of earth. )
·         Does the gravitational force act on the ball only when it is falling down? (students’ response: No.)
·         Draw out the conclusion that gravitational force of earth pulls the objects downwards it. It acts on the ball not only when it is falling down but also when it is going up.

#### Activity 2:

·         Tell the students that the weight of an object can be measured by a spring balance.
·         Ask one students to hang the wooden block from the hook of the spring balance and read the scale at the pointer.
·         Ask the other students what does scale show?    (scale shows the reading of weight in newton ?)s
·         Then ask the students: If you take the block away from the surface of earth and measure the weight , what will happen to the weight ?
·         After students response: explain to them that : The weight of an object will decrease because the gravitational force acting on an object is called weight There is less gravitational force on the object at a place away from the surface of earth as compared to that on the surface of the earth.
##### Sum Up / Conclusion:
###### ·         Gravitational force of earth is the force with which the earth pulls the object towards it.
·         Mass is a measure of how much matter an object has.
·         Weight is a measure of the gravitational force with which the earth pulls the objects towards it.
##### Assessment:
Ask the following questions to the students:
·         Why do things fall down?
·         What is the gravitational force of earth?
·         In which direction the gravitational force of earth act on the things?
·         What is the difference between mass and weight?
·         What happens to the weight of a body when we take it away from the surface of earth?
·         What happens with the mass of the body when we take it to another place  ?