9 . 1
When we apply a force on an object , what will happens to the object ?

There are two kinds of forces , states them.

Gives some example of the two kinds of force .

In the laboratory , How can we
measure the forces ?

9 . 2
What is friction ?

What is air resistance ?

Briefly explain the heating effect of friction .

Why we need to reduce the friction ?

How can we reduce the friction ?

9 . 3
What is the force of gravity ?

What is mass ?

What is weight ?