Force and Motion

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Development of Ideas of Motion and Inertia. Aristotle's IdeasMotion was in two formsNatural-- straight up or down on earth, circular in heavensViolent--result of forces making things moveThe natural state of things was to be at rest Motion only happens if a continual force makes it

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Force and Motion

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1. Force and Motion Newton’s Laws

2. Development of Ideas of Motion and Inertia Aristotle’s Ideas Motion was in two forms Natural-- straight up or down on earth, circular in heavens Violent--result of forces making things move The natural state of things was to be at rest Motion only happens if a continual force makes it happen Basis of the Geocentric theory

3. Copernicus Stated that earth was in motion Based on observations from astronomy Galileo Showed that friction causes slowing and stop of motion Friction is caused by surfaces in contact Without friction, moving object would not stop Force is needed only to overcome friction

4. Galileo (cont.) Developed idea of inertia Every material object has a resistance to change in its state of motion At rest, stays at rest In motion, keeps its velocity Earth demonstrates inertia in revolution, no force pushes it around the sun and none is needed since there is no friction

5. The Principia Based on work done by Galileo and Kepler Principle of Inertia - Galileo Principle of Inverse Square Force - Kepler Attraction of sun on planet seemed to be inversely proportional to the distance squared Newton expanded on both of these ideas

6. Force Generally, a push or pull Can cause motion, but may not Vector quantity, with direction Total sum of forces on a system is Net Force Two types: Contact forces and Fundamental forces

7. Fundamental Forces Gravity - attraction between bodies due to mass Electromagnetic - attraction/repulsion in electricity and magnetism Strong Nuclear Force - hold atomic nucleus together Weak Nuclear Force - interactions of subnuclear particles

8. Inertia - The 1st Law Every object continues in its state of rest or of motion in a straight line at constant speed unless it is compelled to change that state by forces exerted upon it. Examples Magician pulls tablecloth from under dishes Air hockey games at arcades Voyager and Pioneer spacecraft

9. Mass-a Measure of Inertia What is mass? Not volume Volume is space taken up by object--cm3 or L—m3 in Phyz Mass is the amount of matter contained by object-- kg or g Compare pillow with car battery

10. Mass-a Measure of Inertia Not weight Weight depends on gravity--more gravity more weight Mass is in object no matter the gravity-bowling ball in space Weight and mass are proportional--more mass makes more weight

11. Inertia and the Moving Earth Bird catching worm—should miss by 30 km Inertia says bird, tree, worm all moving at 30 km/s--no relative motion Ancients had no fast motion, thus did not see inertia

12. Newton’s Second Law of Motion Force and Acceleration Force Causes Acceleration Single force causes a start or change in motion-- acceleration Multiple forces may act together which add or subtract to make net force Amount of acceleration is proportional to net force

13. Fnet=ma Acceleration depends both on mass and force jointly “The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the body” With consistent units Fnet--> Newtons m--> kg a--> m/s2 the equation becomes exact acceleration = net force or a = F net mass m

14. Example A prospector pushes a 2030 kg cart with a horizontal net force of 700N for 5.0sec. If the cart starts from rest, how far will it go during the time the force is applied? F = 700N m = 2030 kg t = 5.0 sec v0 = 0 x = ? Find a a = F/m = 700/2030 m/s2 x = v0t +1/2 at2 = 0 + (0.5) (700/2030) (25) x = 4.3m

15. Homework #1 p. 104 ff 1, 3, 5, 7, 9 (F = ma)

16. Weight The pull of gravity on an object is a force w = mg Weight is offset by the surface a body rests upon If the surface is parallel to the ground, the offset force balances the weight force If the surface is at an angle, only (w cosq) of the weight force is offset, and the object will have acceleration.

17. Example A furniture van has a smooth ramp for making deliveries. The ramp makes an angle q with the horizontal. A large crate of mass m is on the ramp. What is its acceleration if the surface is frictionless? w= mg

18. Action and Reaction - The 3rd Law For every action there is an equal and opposite reaction Every object which touches another has an equal touch upon itself.

19. Interactions Interactions Produce Forces Force applied causes motion - hammer to nail Opposing force causes slowing - nail to hammer Law of Action and Reaction Whenever one object exerts a force on a second object, the second objects exerts an equal and opposite force on the first

20. Force Denomination Are called action and reaction arbitrarily Identifying the forces may be complex Gravity - What is action and reaction for a falling rock? If we identify action force by relating objects, A and B, action can be discovered Earth (A) pulls rock (B) & Rock (B) pulls earth (A)

21. Action and Reaction on Different Masses Earth does accelerate to meet falling rock, but since its mass is so large, its acceleration is very tiny

22. Action and Reaction on Different Masses Firing rifle, bullet causes reaction at rifle Acceleration is different because of mass difference Bullet acceleration large, mass small a= F m Rifle acceleration small, mass large a = F m

23. Action and Reaction on Different Masses Rocket propulsion is similar - gases forced out cause motion of rocket in opposite direction

24. Example of Use A 68 kg passenger rides in an elevator which is accelerating upward at 1.0m/s2. What force is exerted by the passenger on the floor? a = -1.0 m/s2 g = -9.8 m/s2 m = 68 kg Passenger is accelerating downward when elevator goes up. Fnet = F - mg - ma F = ma + mg = 68 (-1) + 68 (-9.8) = -730 N Passenger’s actual weight force = 68 (-9.8) = -670 N

25. Weightlessness If the elevator was accelerating downward, the passenger’s weight force would be reduced, the opposite of going upward. If acceleration = g, then weight is cancelled. In a spacecraft, the value of g toward earth for both the vehicle and occupants are the same, so weightlessness also occurs.

26. Applications of Action and Reaction Action and Reaction Don’t Cancel Action is on one object, reaction on another Forces only cancel if on same object

27. Horse - Cart Problem Horse pulls on cart, cart pulls on horse - Why no cancel? The horse is not just acting on the cart, but the ground as well Action of horse on ground produces reaction of ground on horse, pushing horse forward with cart

28. External vs. Internal Forces between horse and cart are internal which doesn’t affect motion Forces between horse + cart and ground are external and do produce motion Pushing on dashboard of your car from inside is useless - internal Pushing on car from outside interacts with ground – external

29. Applications of Newton’s Laws

30. Example 4.8

31. Example 4.9

32. Example 4.10

33. Homework #2 p 105 ff 11, 13, 20, 21, 22

34. Opposing Forces and Application of Forces Friction Friction is a force which acts opposite to motion Caused by irregularities in surfaces in contact Amount depends on the materials involved Occurs in fluids as well as in solids Viscosity in liquids Air resistance in air (similar in other gases) Friction causes the need for applied force to keep constant velocity

35. Friction (cont) Value of friction can be calculated using the coefficient of friction, m , which depends on materials of object and surface Ff = m FN where FN is the “normal force.” Discussed earlier, and is equal to (w cos q) Values of m are tabulated in books.

36. Example 4.11

37. Homework #3 p107ff 25, 26, 28, 30, 33

38. Equilibrium Static - no velocity of the object Dynamic - constant velocity of the object Both have Fnet = 0 and a = 0

39. Force in Statics Forces can be balanced and thus produce no motion Book on a table has two forces acting upon it Support force (of the table) upward (FN) Gravity downward Hanging from ropes --gravity downward, support upward divided by number of ropes

40. Example 4.12

41. Example 4.13

42. Example 4.14

43. Homework #4 p. 108 ff 41, 44, 45, 48, 52 Homework #5 p. 110ff 54, 56, 60, 69, 70

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