Name: _______________________________ Class: ____ 12.1 Forces

1. Name: _______________________________ Class: ____12.1 Forces Chapter 12 Forces and Motion

2. What are we going to learn?

4. A soccer ball sits at rest. You come along and kick it, sending it flying across the field. It eventually slows to a stop. You applied a force to start it moving, and then it stopped all on its own, right?

5. No! • Forces cause all changes in motion. Just as you applied a force to the ball to speed it up from rest, the ground applied a force to slow it down to a stop. If the ground didn’t apply a force to the ball, it would keep rolling forever without slowing down or stopping.

6. Forced to Change

7. What’s the BIG Idea?

10. She jumps with enough force to stay in the air long enough to turn before she lands

12. We will find out the answer as we go along...

13. What is a Force?

14. How are Forces Described? • Force – a push or pull • When one object pushes or pulls another object, the first object exerts a force on the second object • Ex: You exert a force on a computer key when you push it • Ex: You exert a force on a chair when you pull it away from a table

15. Force is described by its strength and by the direction in which it acts • Pushing to the left is a different force from pushing to the right • The direction and strength can be represented by an arrow

16. The arrow points in the direction of the force • The arrow length tells you the strength (ex: the longer the arrow, the greater the force)

17. The strength of the force is measured in the SI unit called a newton (N), after the scientist Sir Isaac Newton • 1 newtonis the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2)

18. A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction

22. We will find out the answer as we go along...

23. How do Forces Affect Motion? • Often more than one force acts on an object at the same time • The combination of all the forces on an object is called the net force • The net force determines if and how an object will accelerate

24. When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion

25. When an unbalanced force acts on an object, the object accelerates

26. You can find the net force on an object by adding together the strengths of all the individual forces acting on the object • Look at the figure below

27. The big dog pushes on the box with a force of 16 N to the right • The small dog pushes on the box with a force of 10 N to the right • The net force on the box is the sum of these forces

28. In the situation, there was a nonzero net force • A nonzero net force causes a change in the object’s motion

29. What if the Forces on an Object are not acting in the same Direction? • The big dog pushes with a force of 20 N • The small dog pushes with a force of 10 N • The dogs are pushing against each other

30. When the forces on an object act in opposite directions, the strength of the net force is found by subtracting the strength of the smaller force (think negative) from the strength of the larger force (think positive) • When forces act in opposite directions, the net force is in the same direction as the larger force

31. In the example, the box will accelerate to the right

32. Let’s try a few examples...

34. Use what you know about net force to describe the motion of the box in the figure. Assume the box starts at rest.

36. 1. You pull on your dog’s leash to the right with a 12 N force. Your dog pulls to the left with a 6 N force. Sketch this situation, including labeled force arrows.

37. 1.

38. 2. What is the net force on the leash? Calculate it. Draw and label it.

39. 2. 6 N to the right

40. Tug of War Clip • http://techtv.mit.edu/videos/850-mit-physics-demo----no-win-tug-of-war

41. What's the definition of balanced forces?

42. What's the definition of balanced forces? • Equal but opposite forces in strength and direction resulting in a zero net force

43. What's the definition of unbalanced forces?

44. What's the definition of unbalanced forces? • Direction and strength of forces acting on an object result in a non-zero net force which causes the object to accelerate

45. When forces are unbalanced and in opposite directions, what is the combined force equal to? What direction is it?

46. When forces are unbalanced and in opposite directions, what is the combined force equal to? What direction is it? • The combined force is equal to the difference between the 2 forces • It will be in the direction of the larger force

47. Use your answers above to explain what happens in a tug-of-war stand-off.

48. Use your answers above to explain what happens in a tug-of-war stand-off. • The 2 sides each exert an opposite force. The side that exerts the most force will cause a movement of the other side toward them.

49. Use your answers above to explain what happens in a arm-wrestling stand-off.

50. Use your answers above to explain what happens in a arm-wrestling stand-off. • Each arm wrestler is exerting a force opposite the other wrestler. The person with the most force will cause the other’s arm to move in the direction of the greater force.