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Physical World Forces Middle School

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Physical World Forces Middle School

1. 1. The Physical World: Forces Ms. Novak 2008-2009
2. 2. Objectives <ul><li>Define force. </li></ul><ul><li>Describe how forces combine. </li></ul><ul><li>Identify contact forces and non contact forces. </li></ul>
3. 3. Force <ul><li>A push or pull that one object exerts on another object. </li></ul><ul><li>Examples: Earth (pulls us downward), chairs, pencils. </li></ul><ul><li>Example: pulling a drawer open; you exert force on the drawer. </li></ul>
4. 4. Force <ul><li>Has direction and size! </li></ul><ul><li>Push a book from the side the book moves/slides. </li></ul><ul><li>Push a book downward, same amt of force, it doesn’t move. </li></ul><ul><li>Motion depends on direction and size of force. </li></ul>
5. 5. Direction of Force <ul><li>The direction of the push or pull on an object. </li></ul><ul><li>Size of force or amount is measured in N or newtons. </li></ul>Force 1 (pulling) Force 2 (Pushing)
6. 6. Combining Forces <ul><li>Net Force: combination of forces acting on an object. </li></ul><ul><li>Same Direction: Amt of forces for both are added together for the Net Force </li></ul>Force 1 Force 2 Force 1 + Force 2 = Net Force
7. 7. Combining Forces: Same Direction <ul><li>Example: Force 1 = 15 N </li></ul><ul><li>Force 2 = 20 N </li></ul><ul><li>Force 1 + Force 2 = Net Force </li></ul><ul><li>15N + 20 N = 25 N = Net Force </li></ul>
8. 8. Combining Forces: Opposite Direction <ul><li>The Net Force is the difference between the two forces and in the direction of the larger force. </li></ul><ul><li>Force 1 – Force 2 = Net Force (in direction of larger force) </li></ul>Force 1 Force 2
9. 9. Opposite Direction cont. <ul><li>Example Force 1 = 20 N </li></ul><ul><li>Force 2 = 15 N </li></ul><ul><li>Force 1 – Force 2 = Net Force </li></ul><ul><li>20 N – 15 N = 5N (in direction of Force 1) </li></ul>
10. 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. 11. Balanced/Unbalanced Forces <ul><li>Forces are in balance if the net force equals zero. </li></ul><ul><li>Forces are unbalanced when the net force is not zero. </li></ul><ul><li>Motion changes an objects direction/speed etc ONLY if the forces are unbalanced. </li></ul>
12. 12. Contact/Non Contact Forces <ul><li>Forces exerted on objects when objects touch are contact forces. </li></ul><ul><li>Forces that are exerted without touching are non contact forces; example: electric forces, magnetic forces, and gravity. </li></ul>
13. 13. Gravity <ul><li>Non Contact force exerted on EVERY object. </li></ul><ul><li>An attractive force. </li></ul><ul><li>Size of force depends on masses of objects. </li></ul><ul><li>Gravitational Force increases with the increase of mass or when two objects move closer together. Why? </li></ul>
14. 14. Friction <ul><li>Contact Force </li></ul><ul><li>Resists the sliding motion of two surfaces. </li></ul>
15. 15. Static Friction <ul><li>Net forces equal zero. </li></ul><ul><li>Object does not move. (static means not moving) </li></ul>Applied Force Static Friction Object Does NOT MOVE
16. 16. Sliding Friction <ul><li>Push on an object and it slides. </li></ul><ul><li>Stop pushing and object slows down. </li></ul><ul><li>Force that caused object to slow down is sliding friction. </li></ul>
17. 17. Causes of Friction <ul><li>Microscopic dips and bumps </li></ul><ul><li>In all surfaces, including polished ones! </li></ul><ul><li>Static Friction occurs when force is not large enough to overcome force applied. </li></ul>