This document defines key concepts about forces including: - A force is a push or pull exerted by one object on another. Forces have both direction and magnitude. - Forces can combine in the same or opposite directions. Forces adding in the same direction give the net force, while opposing forces subtract. - Balanced forces result in no motion, while unbalanced forces cause changes in motion. - There are contact forces like friction and gravity, as well as non-contact forces like magnetism and electricity. Friction opposes motion between surfaces.

1. The Physical World: Forces Ms. Novak 2008-2009

2. Objectives Define force. Describe how forces combine. Identify contact forces and non contact forces.

3. Force A push or pull that one object exerts on another object. Examples: Earth (pulls us downward), chairs, pencils. Example: pulling a drawer open; you exert force on the drawer.

4. Force Has direction and size! Push a book from the side the book moves/slides. Push a book downward, same amt of force, it doesn’t move. Motion depends on direction and size of force.

5. Direction of Force The direction of the push or pull on an object. Size of force or amount is measured in N or newtons. Force 1 (pulling) Force 2 (Pushing)

6. Combining Forces Net Force: combination of forces acting on an object. Same Direction: Amt of forces for both are added together for the Net Force Force 1 Force 2 Force 1 + Force 2 = Net Force

7. Combining Forces: Same Direction Example: Force 1 = 15 N Force 2 = 20 N Force 1 + Force 2 = Net Force 15N + 20 N = 25 N = Net Force

8. Combining Forces: Opposite Direction The Net Force is the difference between the two forces and in the direction of the larger force. Force 1 – Force 2 = Net Force (in direction of larger force) Force 1 Force 2

9. Opposite Direction cont. Example Force 1 = 20 N Force 2 = 15 N Force 1 – Force 2 = Net Force 20 N – 15 N = 5N (in direction of Force 1)

10. Equal Sized Forces Force 1 Force 2 15 N 15 N Force 1 – Force 2 = Net Force 15 N – 15 N = O N

11. Balanced/Unbalanced Forces Forces are in balance if the net force equals zero. Forces are unbalanced when the net force is not zero. Motion changes an objects direction/speed etc ONLY if the forces are unbalanced.

12. Contact/Non Contact Forces Forces exerted on objects when objects touch are contact forces. Forces that are exerted without touching are non contact forces; example: electric forces, magnetic forces, and gravity.

13. Gravity Non Contact force exerted on EVERY object. An attractive force. Size of force depends on masses of objects. Gravitational Force increases with the increase of mass or when two objects move closer together. Why?

14. Friction Contact Force Resists the sliding motion of two surfaces.

15. Static Friction Net forces equal zero. Object does not move. (static means not moving) Applied Force Static Friction Object Does NOT MOVE

16. Sliding Friction Push on an object and it slides. Stop pushing and object slows down. Force that caused object to slow down is sliding friction.

17. Causes of Friction Microscopic dips and bumps In all surfaces, including polished ones! Static Friction occurs when force is not large enough to overcome force applied.