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# Physics: Mechanics

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PowerPoint on Newton's Laws, gravity, forces, vectors, momentum, impulse and moments

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### Physics: Mechanics

1. 1. Newton’s Laws And forces, vectors, gravity, momentum, impulse and moments
2. 2. Newton’s First Law <ul><li>Objects at rest remain at rest </li></ul><ul><li>Objects in motion remain in motion </li></ul><ul><li>UNTIL YOU APPLY A FORCE </li></ul>
3. 3. Newton’s Second Law <ul><li>F = ma </li></ul><ul><li>What forces are important in sports? </li></ul>
4. 4. Newton’s Third Law <ul><li>For every action there is an equal and opposite reaction. </li></ul><ul><li>But do they act on the same body? </li></ul>
5. 5. Vector <ul><li>Force is a vector - ? </li></ul><ul><li>It has a magnitude and a direction. </li></ul><ul><li>This allows forces to be cancelled out. </li></ul><ul><li>Force brings about acceleration </li></ul><ul><li>Which law relates to that? </li></ul><ul><li>Newton’s 2 nd Law </li></ul><ul><li>F=ma </li></ul>
6. 6. Non-parallel forces <ul><li>Can add vectors in different directions: </li></ul><ul><li>c 2 = a 2 + b 2 ; angle x = tan -1 a/b </li></ul>x Resultant force a b c
7. 7. Vectors <ul><li>Remember: the resultant force is not a new force </li></ul><ul><li>It is the sum of the 2 components </li></ul><ul><li>Use either the resultant or component in any calculation, not both! </li></ul>
8. 8. Newton <ul><li>What is a Newton? </li></ul><ul><li>Unit </li></ul><ul><li>1N = force that accelerates 1kg mass by 1ms -2 </li></ul>
9. 9. weight <ul><li>What’s the weight of 1 litre of water? </li></ul><ul><li>1kg is the mass </li></ul><ul><li>The weight = mass x g </li></ul><ul><li>g is the gravitational field strength </li></ul><ul><li>g = 10 N kg -1 </li></ul><ul><li>or 9.81 N kg -1 (more accurately) </li></ul><ul><li>Weight of 1 litre of water is 10N or 9.81N </li></ul>
10. 10. Quiz <ul><li>What is my acceleration due to weight? </li></ul><ul><li>9.81 ms -2 </li></ul><ul><li>What is your acceleration due to weight? </li></ul><ul><li>9.81 ms -2 </li></ul><ul><li>What about the marker? </li></ul><ul><li>9.81 ms -2 </li></ul><ul><li>Acceleration due to weight is always g </li></ul>
11. 11. Mass <ul><li>Amount of matter </li></ul><ul><li>Measure of inertia </li></ul><ul><li>Inertia is the resistance to acceleration </li></ul><ul><li>Mass attracts gravity </li></ul><ul><li>Gravitational force is directly proportional to mass </li></ul><ul><li>Note: Gravitational force is inversely proportional to distance 2 between 2 points </li></ul>
12. 12. Quiz <ul><li>As one moves away from the centre of the earth, his/her weight decreases. </li></ul><ul><li>At 3000m, g = 9.8009 N kg -1 </li></ul><ul><li>What if we throw shot put at 3000m? </li></ul><ul><li>What if we run marathon at 3000m? </li></ul><ul><li>At 3000m, oxygen pressure is reduced by more than 10% </li></ul><ul><li>Always consider other factors! </li></ul>
13. 13. Impulse <ul><li>Momentum = mass x velocity </li></ul><ul><li>Since F = ma and a = ∆ v / t , F = m ∆ v / t </li></ul><ul><li>Therefore F t = m ∆v, or </li></ul><ul><li>Impulse = change of momentum </li></ul><ul><li>The larger the impulse, the greater is the change in velocity. </li></ul><ul><li>Application? </li></ul>
14. 14. Impulse <ul><li>Hence, to increase velocity, we either increase F or t (time of contact). </li></ul><ul><li>Remember, F = m ∆ v / t </li></ul><ul><li>To reduce impact, encourage soft landing </li></ul><ul><li>Use cushioning, bend joints, etc </li></ul>
15. 15. Conservation of energy <ul><li>Energy is always conserved </li></ul><ul><li>Regardless of whether you can observe </li></ul><ul><li>Mechanical energy is not always conserved </li></ul><ul><li>Mechanical energy is only conserved in a closed system </li></ul><ul><li>Initial (total kinetic + potential energy) = final (total kinetic + potential energy) </li></ul>
16. 16. Conservation of momentum <ul><li>Momentum is conserved when there is no external forces </li></ul><ul><li>E.g. snooker, assuming no friction </li></ul><ul><li>Application? </li></ul><ul><li>Momentum can be transferred in a collision </li></ul><ul><li>Application? </li></ul>
17. 17. Conservation of Momentum If m 1 = m 2 , then v 1 = v 2 http://www.phys.washington.edu/~young
18. 18. Moments <ul><li>Turning effect of a force </li></ul><ul><li>Moment = force x perpendicular distance </li></ul><ul><li>Also called torque </li></ul>