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# Forces and newton’s laws

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Force Notes

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### Forces and newton’s laws

1. 1. Forces and Newton’s Laws Ch 6, 7.1
2. 2. Force <ul><li>a push or a pull </li></ul><ul><li>Measured in Newtons (N) </li></ul>
3. 3. Types of Forces
4. 4. Gravitational Force <ul><li>Attractive force that exists between all objects </li></ul><ul><li>Weakest of the 4 forces, but acts over the longest distance </li></ul>
5. 5. Electromagnetic <ul><li>Gives materials their strength, ability to bend, squeeze, stretch, etc. </li></ul>
6. 6. Strong Nuclear Force <ul><li>Holds the particles in a nucleus together </li></ul><ul><li>strongest of the 4 forces, but acts over the smallest distance. </li></ul>
7. 7. Weak force <ul><li>Involved in the radioactive decay of some nuclei </li></ul><ul><li>form of the electromagnetic force </li></ul>
8. 8. Newton’s Laws of Motion
9. 9. 1 st Law - Inertia <ul><li>Any object will continue at constant velocity until acted on by some outside force. </li></ul>
10. 10. 2 nd Law - F=ma <ul><li>The acceleration of an object is directly proportional to the net force on it and inversely proportional to its mass. </li></ul>
11. 11. 3 rd Law - Action-Reaction pairs <ul><li>For every action there is an equal and opposite reaction. </li></ul>
12. 12. Question <ul><li>The horse knows if he pulls with 100N of force and the carriage pulls back with 100 N of force then the carriage isn’t going to move. Is his logic right? </li></ul>
13. 13. Net Force vs Action-Reaction Pairs <ul><li>Action-reaction pairs work on different objects and they are always equal to opposite; EX : I pull on the cart and the cart pulls on me. </li></ul><ul><li>Net force is the sum of all forces and it causes acceleration; the forces work on the same object; EX: I pull on the cart and friction pulls on the cart. </li></ul>
14. 14. Mass vs Weight <ul><li>Mass - amount of matter or inertia an object has. </li></ul><ul><li>Weight - gravitational force exerted by the earth on an objects mass. </li></ul><ul><li>W=mg </li></ul><ul><li>Normal Force - force that is perpendicular to the contact surface. </li></ul>
15. 15. Friction <ul><li>Force that opposes motion between 2 surfaces that are in contact. </li></ul>
16. 16. Types of Friction
17. 17. Static Friction <ul><li>Force that opposes the start of motion. </li></ul>
18. 18. Sliding (Kinetic) Friction <ul><li>Force that opposes motion once the object is moving. </li></ul><ul><li>Note: Static friction is greater than sliding friction because of Newton’s First Law </li></ul><ul><li>F f = μ N </li></ul>
19. 19. Example Problems <ul><li>A 12 kg box that is pulled across the floor at 2 m/s 2 with a force of 40 Newtons. Find the coefficient of friction. </li></ul><ul><li>A 400 kg crate is accelerated upward at 3 m/s 2 by a crane. Find the tension in the cable. </li></ul>
20. 20. Equilibrant <ul><li>Force equal and opposite to the resultant. </li></ul><ul><li>Ex: Find the equilibrant for 20 N @ 125 degrees. </li></ul><ul><li>Answer: 20 N @ 305 degrees </li></ul>
21. 21. Example Problems <ul><li>A 30 kg box is on a 35 degree incline plane. Find: </li></ul><ul><li>A. The force that causes the box to move down the incline. </li></ul><ul><li>B. The normal force. </li></ul><ul><li>C. The acceleration if μ = 0.25 </li></ul>
22. 22. <ul><li>A 400 kg sign is hung by cables that are 110 degrees apart. Find the tension in each cable. </li></ul>
23. 23. <ul><li>A 50 kg box is pulled across the ground with a force of 300 Newtons @ 35 degrees and μ = 0.3. Find the acceleration. </li></ul>