summary of ch. 3-5 for 9th grade physics

1. inertia is the ability of an object to stay in its state.
2. the greater the inertian of an object, the more difficult it gets influenced by force.
3. there are 2 kinds of fraction. they are dynamic friction and static friction.
4. dynamic friction is a force that cause an object move while static friction is a force tha cause an object stopped.
5. mass is the fixed content of an object while weight is mass influenced by force.
6. density = mass / volume.
7. moment of force means the way of force produced temporarily and in particular time.
8. moment of force  = f x d
9. there are 3 kinds of equlibrium. they are stable equlibrium, unstable equlibrium and neutral equlibrium.