1. Forces Force – any push or pull acting on an object
Net force – the total of all forces acting on an object
Balanced forces – when the net force = 0 N, forces are considered balanced, and there is no change in motion
Unbalanced forces – when the net force </> 0 N, the forces are unbalanced and can cause acceleration

2. Four Basic Forces Gravitational force – acts on matter & light
Electromagnetic force – attraction & repulsion of electric charges and magnetic fields
Strong nuclear force – binds protons and neutrons together
Weak nuclear force – some interactions between some subatomic particles

3. What forces are interacting?

4. What forces are interacting?

5. What forces are interacting?

6. Same direction? Add forces.

7. Opposite direction? Subtract them.

8. Remember…
Objects can continue moving in a straight line without applying force
Forces act on an object at rest
A force is necessary to change the direction of the force

9. Friction – the force that opposes motion
Internal friction – friction between a substance’s molecules, can create heat
External friction – friction between outer surfaces of two objects
Static – no movement due to balanced force
Kinetic – friction between moving objects
Includes rolling and sliding

10. Friction: Helpful or harmful?
Demo 1 – books on rollers
Demo 2 – lubricated jar lid

11. Factors Surface texture Force pushing the surfaces together
Presence or absence of lubricants

12. Weight affects friction

13. Friction doesn’t change if weight doesn’t change

14. Static Friction = no movement

15. Kinetic friction = movement

16. Gravity Gravitational attraction exists between two objects
How strong the attraction is depends on mass and distance
Because Earth is so “big” relative to everything on it, everything accelerates towards Earth at the same rate, 9.8 m/s2

17. Gravity depends on mass

18. Gravity depends on distance

19. Mass vs Weight