Newtons Third Law of Motion 1 FORCES AND

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Newton's Third Law of Motion

Newton's Third Law of Motion

1. FORCES AND INTERACTIONS n Force is an interaction between one thing and another.

1. FORCES AND INTERACTIONS n Force is an interaction between one thing and another. n Ex: 2 people pushing each other on skate boards, driving a nail, punching a bag n There is no single isolated force

2. NEWTON'S THIRD LAW OF MOTION n Law 3 - Whenever one object exerts

2. NEWTON'S THIRD LAW OF MOTION n Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. n Action-reaction pairs never act on same object.

Newton’s Third Law deals with two forces acting on two different objects. Newton’s Third

Newton’s Third Law deals with two forces acting on two different objects. Newton’s Third Law pairs of forces always, sometimes, never cancel each other out. The only way for two forces to cancel each other out is for them to beequal and opposite and act on thesame object!

Action & Reaction on Different Masses Consider you and the earth Action: earth pulls

Action & Reaction on Different Masses Consider you and the earth Action: earth pulls you Reaction: you pull earth

You F m =a F=a m Earth

You F m =a F=a m Earth

Action: tire pushes on road Reaction: road pushes on tire

Action: tire pushes on road Reaction: road pushes on tire

Reaction: gases push on rocket Action: rocket pushes on gases

Reaction: gases push on rocket Action: rocket pushes on gases

n n Hwk read Review book 82 -86 do pg 100 #4, 5, 8

n n Hwk read Review book 82 -86 do pg 100 #4, 5, 8 – 14, 24 - 27, 31 -35. Separate sheet.

rd Newton’s 3 Law n Six minutes http: //www. youtube. com/watch? v=c. P 0

rd Newton’s 3 Law n Six minutes http: //www. youtube. com/watch? v=c. P 0 B b 3 WXJ_k&feature=relmfu

The subscript notation: n Convention: Fa, b means “the force acting on a due

The subscript notation: n Convention: Fa, b means “the force acting on a due to b”. n So Fhead, thumb means “the force on the head due to the thumb”. Fhead, thumb

Newton's Third Law says. . . FA , B = - FB , A.

Newton's Third Law says. . . FA , B = - FB , A. Fm, w Fw, m Ff, m Fm, f

Hmmm…. The man leans on the box, the box moves. n Since Fm, b

Hmmm…. The man leans on the box, the box moves. n Since Fm, b = -Fb, m, why isn’t Fnet = 0 and a = 0 ? Fm, b Fb, m a ? ? ice

Need free body diagram of box only. n Consider only the box as the

Need free body diagram of box only. n Consider only the box as the system! – Fon box = mabox = Fb, m Fm, b Fb, m abox ice

The forces acting on the box are: 1. The force of the man 2.

The forces acting on the box are: 1. The force of the man 2. The force of the ice (neg). abox

3. SUMMARY OF NEWTON'S THREE LAWS n Law 1 (Law of Inertia) Every object

3. SUMMARY OF NEWTON'S THREE LAWS n Law 1 (Law of Inertia) Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. n Law 2 n Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

rd Newton’s 3 law … 6 minutes n http: //www. youtube. com/watch? v=c. P

rd Newton’s 3 law … 6 minutes n http: //www. youtube. com/watch? v=c. P 0 Bb 3 WXJ_k &feature=relmfu n Newton’s 3 Laws 4 min http: //www. youtube. com/watch? v=i. H 48 Lc 7 wq 0 U n n http: //www. youtube. com/watch? v=8 b. Td. Mm. NZm 2 M&list=PLEA 0 D 7 C 501 E 484 B 31&index=8

Review Questions

Review Questions

A bug and a car collide. Which experiences the greater force? (a) bug (b)

A bug and a car collide. Which experiences the greater force? (a) bug (b) car (c) neither, they both experience the same magnitude of force

Consider hitting a baseball with a bat. If we call the force applied to

Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force. (a) the force applied to the bat by the hands (b) the force applied to the bat by the ball (c) the force the ball carries with it in flight (d) the centrifugal force in the swing

n Two blocks are stacked on the ground. n How many action-reaction pairs of

n Two blocks are stacked on the ground. n How many action-reaction pairs of forces are present in this system? a (a) 2 (b) 3 b (c) 4

Solution: Fa, E a b Fb, E Fb, a Fa, b a a b

Solution: Fa, E a b Fb, E Fb, a Fa, b a a b b Fb, g Fg, b FE, a FE, b (c) 4