Forces Change Motion

1. Forces Change Motion Force- push or a pull Can speed up or slow down the motion of an object or change its direction without changing its speed.

2. Forces are VECTORS • Have a size and direction

3. Balanced Forces • Ex: Physics book at rest on a table top. • Two forces are acting upon the book. • Gravity pushes down on the book. • The table pushes up on the book • Both forces are equal in size/magnitude/number and opposite in direction

4. Balanced Forces- • The forces are balanced • The book is said to be at equilibrium. • No unbalanced force acting upon the book • When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerated.

5. Balanced Forces • A person standing upon the ground. • Two forces acting upon the person. • Gravity exerts a downward force. • The floor of the floor exerts an upward force.

6. Balanced Forces • These two forces are of equal magnitude/size • In opposite directions • They balance each other. • The person is at equilibrium. • There is no unbalanced force acting upon the person

7. Balanced vs. Unbalanced Forces • Net Force- The overall force acting on a object when ALL the forces are combined • If the NET FORCE is ZERO all of the forces acting on the object are zero.

8. Balanced vs. Unbalanced • BALANCED forces are the same as having no force at all. • The motion of an object doesn’t change

9. Unbalanced Forces • Change the motion of an object

10. Balanced vs. Unbalanced • If one of the basketball players pushes with a grater force than the other player, the ball will move in the direction that player is pushing. • The motion of the ball changes because the forces on the ball are UNBALANCED.

11. Balanced forces • Don’t change an object’s speed or direction • A unbalanced force is needed to change an object’s motion

12. Sir Isaac Newton lived during the 1600s • His laws of motion explain: • Rest • Constant motion • Accelerated motion • Describe how balanced and unbalanced forces act to cause these states of motion.

13. Sir Isaac Newton • Built his ideas on Galileo Galilei’s ideas • Newton’s 3 Laws of Motion

14. Newton’s First Law • Object at rest remains at rest, and objects in motion remain in motion with the same velocity, unless acted upon by an unbalanced force.

15. Newton’s First Law • The ball is in motion with the same velocity, unless acted upon by an unbalanced force.

16. Have you ever been riding in a car when the driver suddenly slammed on the brakes? • How did your body move as the car came to a stop? You probably felt your body move forward..

17. When you felt this happening you experienced Newton's first law of motion. • Newton'sfirst law of motion says that an object in motion will stay in motion and an object at rest will stay at rest unless acted on by an unbalanced force

18. Newton called his first law inertia. • In the car your body was in motion, traveling at the same speed as the car. When the car stopped, your body stayed in motion. If you were not wearing a seatbelt and you were traveling very fast, your body could continue to move forward through the windshield!

19. Try the following activity to demonstrate this law! • Place a 3x5 card on top of a glass. • Put a coin on the center of the card. • Flick the card horizontally with your finger. • What happens to the coin? • Explain what happened to the coin using Newton's first law.

20. Inertia • Resistance of an object to change in the speed or direction of its motion • Newton’s first law • When you measure the mass of an object you are also measuring its inertia • It is easier to push or pull an empty box than the same box when it full of books

21. Main Ideas • Objects tend to stay at rest unless something hits them. • Things keep on doing what they are doing unless something hits them. • Inertia- the name given to an object’s tendency to keep doing what it is doing

22. Force • Mass x Acceleration • F= m x a • Units are Newtons (N) kg x m/s2

23. Newton’s 2nd Law • The acceleration of an object increases with increased force • Decreases with increased mass • Is in the same direction of the force

24. Newton’s 2nd Law- Ties Together- Force, Mass, Acceleration • Acceleration of an object depends on the mass of the object and the size of the force applied • If an object loses mass, it can gain acceleration if the force remains the same

25. Newton’s 2nd Law • Acceleration of an object increases with INCREASED FORCE and decreases with INCREASED MASS. • The direction in which an object ACCELERATES is the same as the direction of the force.

26. Newton’s 2nd Law • Who is exerting the most force behind the cart?

27. Newton’s 2nd Law • Which cart has the largest mass? • Which is the easiest to push?

28. Main Ideas- Newton’s 2nd Law • Small mass leads to LARGE acceleration • Large mass leads to SMALL acceleration

29. Main Ideas • A net force acting on an object causes the object to accelerate. • A net force acting on an object causes an object to accelerate.

30. Centripetal Force • Any force that keeps an object moving in a circle • This force points toward the center of the circle • Without this force object would go flying off in a straight line • Centripetal force keeps the planets in orbit

31. Spinning stuff- Centripetal Force • Spinning a bucket of water over your head without spilling a drop, you are also applying a centripetal force. If you let go of the bucket, it will move in a straight line • Centripetal force is always directed at the centre of an object.

32. Centripetal Force

33. Centripetal Force • Greater acceleration = greater centripetal force • More mass requires more centripetal force to have the same circular speed as a less massive object. • No matter the mass of an object, if it moves in a circle, its force and acceleration are directed to the center of the circle

34. Newton’s 3rd Law • For every action, there is an equal and opposite reaction • Action and Reaction Forces • Rockets going into space • Squid and Jellyfish movement

35. Types of Forces • Contact • Gravity • Friction

36. Types of Forces • Contact Force- when one object pushes or pulls another object by touching it, the first object is applying a contact force to the second.

37. Balanced Forces vs. Action/Reaction Forces • Balanced Forces act on a single object • Action and Reaction Forces act on DIFFERENT objects

38. What are examples of contact forces?

39. Momentum • Found in moving objects • Measure of mass in motion • p= mv • momentum = p • m= mass • v= velocity • Units kg x m/s

40. Momentum • More mass= more momentum • Momentum depends on mass • Momentum takes into account velocity • Is a vector (has size and direction)

41. Collision • Two objects in close contact exchange energy and momentum

42. Conservation of Momentum • Total momentum of a system of objects do not change, as long as no outside forces are acting on that system • To find total momentum of objects moving in the SAME direction, add the momenta of the objects. • Objects traveling in opposite directions, subtract the momentum from one another.

43. Bibliography slide • http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l2a.html • http://cache.daylife.com/imageserve/0c3A5Ce455gen/340x.jpg • http://www.all-creatures.org/hope/img/earth-light.jpg • http://www.autopartsvendor.com/catalog/images/car-engine.jpg • http://images.inmagine.com/img/corbis/crb439/crb439011.jpg • http://www.physorg.com/newman/gfx/news/2005/newton.jpg

44. Types of Forces • The ground produces a CONTACT FORCE on the skater as she pushes against the ground. • GRAVITY pulls the skater down • Friction between the skates and the ground

45. Gravity • Force of attraction between two masses • Earth’ s gravity is pulling on the skater, holding her to the ground • The force of gravity depends on the mass of the objects.

46. FRICTION • Force that resists motion between two surfaces that are pressed together.