Newton’s 3 Laws of Motion

2. Change in motionnetF ma= →

1. If 0 No change in motion= →∑F

1 on 2 2 on 13. F F= −

Newton’s First Law (Law of Inertia)

An object will remain at rest or in a constant state of motion unless acted upon by external net forces.

Newton’s 2nd Law

The acceleration of object is directly related to the net forces acting on it and

inversely proportional to its mass.

netFam

=

F ma=∑

Newton’s 3rd Law

To every force there is an equal but opposite reaction force.

hand on wall wall on handF F= −

Newton’s 3rd Law

You can’t TOUCH without being TOUCHED back!!

hand on wall wall on handF F= −

Newton’s 3rd Law

To every force there is an equal but opposite reaction force.

hand on wall wall on handF F= −

Newton’s 3rd Law

This is an INTERACTIVE Universe.

hand on wall wall on handF F= −

Gravity is an Interaction Earth on Rock Rock on EarthF F= −

Gravity is an Interaction The Earth pulls on you, you pull on the Earth. You fall to the Earth, the Earth Falls to you.

You accelerate towards the Earth with g =9.8m/s2. With what acceleration is the Earth falling towards you?

Earth on You You on EarthF F= −

E Emg M a= −

EE

mgaM

=

222 2

24

(65 )(9.8 / ) 1.1 10 /5.98 10Ekg m sa x m s

x kg−= =

This is your weight:

Force is not Acceleration Force is the Same!

Acceleration is NOT!

Earth on You You on EarthF F= −

Earth to You You to Eartha a= −

Gun Pushes Bullet out. Bullet Pushes back on Gun (& Man)

kick

An interaction requires a pair of forces acting on two objects.

Action Reaction Pairs

Gun Pushes Bullet out. Bullet Pushes back on Gun (& Man)

kick

Rocket Thrust

Rocket Pushes Gas Out. Gas Pushes Back on Rocket.

Newton’s 3rd Law Exploding Systems

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The propulsion force on a car is due to

A. Static friction. B. Kinetic friction. C. The car engine. D. Elastic energy.

Reading Question 7.3

Slide 7-14

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The propulsion force on a car is due to

A. Static friction. B. Kinetic friction. C. The car engine. D. Elastic energy.

Reading Question 7.3

Slide 7-15

In order to get an object moving, you must push harder on it than it

pushes back on you.

A) True B) False

Question You push a heavy car by hand. The car, in turn, pushes back with an opposite but equal force on

you. Doesn’t this mean the forces cancel one another, making acceleration impossible?

How is it that the car moves?

Action-Reaction forces act on different objects. For F = ma, the forces must act on ONE object: the system.

The System

Interacting Objects

If object A exerts a force on object B, then object B exerts a force on object A.

The pair of forces, as shown, is called an action/reaction pair.

Slide 7-23

The three blocks are pushed across a rough surface by a 40-N force. If the coefficient of kinetic friction between each of the blocks and the surface is 0.20, determine the magnitude of the force exerted by m2 on m3.

a) 20 N b) 30 N c) 10 N d) 15 N e) 25 N

Problem 1 2 32 , 3 , 5m kg m kg m kg= = =

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A mosquito runs head-on into a truck. Splat! Which is true during the collision?

A. The mosquito exerts more force on the truck than the truck exerts on the mosquito.

B. The truck exerts more force on the mosquito than the mosquito exerts on the truck.

C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito.

D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck.

E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito.

QuickCheck 7.1

Slide 7-39

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A mosquito runs head-on into a truck. Splat! Which is true during the collision?

A. The mosquito exerts more force on the truck than the truck exerts on the mosquito.

B. The truck exerts more force on the mosquito than the mosquito exerts on the truck.

C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito.

D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck.

E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito.

QuickCheck 7.1

Slide 7-40

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A mosquito runs head-on into a truck. Which is true during the collision? A. The magnitude of the mosquito’s acceleration is larger

than that of the truck. B. The magnitude of the truck’s acceleration is larger than that

of the mosquito. C. The magnitude of the mosquito’s acceleration is the same as

that of the truck. D. The truck accelerates but the mosquito does not. E. The mosquito accelerates but the truck does not.

QuickCheck 7.2

Slide 7-41

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A mosquito runs head-on into a truck. Which is true during the collision? A. The magnitude of the mosquito’s acceleration is larger

than that of the truck. B. The magnitude of the truck’s acceleration is larger than that

of the mosquito. C. The magnitude of the mosquito’s acceleration is the same as

that of the truck. D. The truck accelerates but the mosquito does not. E. The mosquito accelerates but the truck does not.

QuickCheck 7.2

Slide 7-42

Newton’s second law:

Don’t confuse cause and effect! The same force can have very different effects.

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Boxes A and B are being pulled to the right on a frictionless surface. Box A has a larger mass than B. How do the two tension forces compare?

QuickCheck 7.5

Slide 7-61

A. T1 > T2 B. T1 = T2 C. T1 < T2 D. Not enough information to tell.

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Boxes A and B are being pulled to the right on a frictionless surface. Box A has a larger mass than B. How do the two tension forces compare?

QuickCheck 7.5

Slide 7-62

A. T1 > T2 B. T1 = T2 C. T1 < T2 D. Not enough information to tell.

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Is the tension in rope 2 greater than, less than, or equal to the tension in rope 1?

A. Greater than rope 2. B. Less than rope 2. C. Equal to rope 2.

Reading Question 7.4

Slide 7-16

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Is the tension in rope 2 greater than, less than, or equal to the tension in rope 1?

A. Greater than rope 2. B. Less than rope 2. C. Equal to rope 2.

Reading Question 7.4

Slide 7-17

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Boxes A and B are sliding to the right on a frictionless surface. Hand H is slowing them. Box A has a larger mass than B. Considering only the horizontal forces:

QuickCheck 7.6

Slide 7-67

A. FB on H = FH on B = FA on B = FB on A

B. FB on H = FH on B > FA on B = FB on A

C. FB on H = FH on B < FA on B = FB on A

D. FH on B = FH on A > FA on B

© 2013 Pearson Education, Inc.

Boxes A and B are sliding to the right on a frictionless surface. Hand H is slowing them. Box A has a larger mass than B. Considering only the horizontal forces:

QuickCheck 7.6

Slide 7-67

A. FB on H = FH on B = FA on B = FB on A

B. FB on H = FH on B > FA on B = FB on A

C. FB on H = FH on B < FA on B = FB on A

D. FH on B = FH on A > FA on B

Tension Forces Tension forces are transmitted undiminished through the rope.

Different T Same T

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Pulleys

Block B drags block A across a frictionless table as it falls.

The string and the pulley are both massless. There is no friction where the pulley turns on its axle. Therefore, TA on S = TB on S.

Slide 7-69

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Pulleys

Since TA on B = TB on A, we can draw the simplified free-body diagram on the right, below.

Forces and act as if they are in an action/reaction pair, even though they are not opposite in direction because the tension force gets “turned” by the pulley.

Slide 7-70

Last Time: Problem A force F = 40 N pulls the two masses. If the table is

frictionless, find the tension in the string. a) 13 N b) 36 N c) 23 N d) 15 N e) 28 N

1 23 , 1.5m kg m kg= =

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The top block is accelerated across a frictionless table by the falling mass m. The string is massless, and the pulley is both massless and frictionless. The tension in the string is

QuickCheck 7.10

Slide 7-78

A. T < mg. B. T = mg. C. T > mg.

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The top block is accelerated across a frictionless table by the falling mass m. The string is massless, and the pulley is both massless and frictionless. The tension in the string is

QuickCheck 7.10

Slide 7-79

A. T < mg. B. T = mg. C. T > mg Tension has to be

less than mg for the block to have a downward acceleration.

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Block A is accelerated across a frictionless table. The string is massless, and the pulley is both massless and frictionless. Which is true?

QuickCheck 7.11

Slide 7-80

A. Block A accelerates faster in case a than in case b. B. Block A has the same acceleration in case a and case b. C. Block A accelerates slower in case a than in case b.

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Block A is accelerated across a frictionless table. The string is massless, and the pulley is both massless and frictionless. Which is true?

QuickCheck 7.11

Slide 7-81

A. Block A accelerates faster in case a than in case b. B. Block A has the same acceleration in case a and case b. C. Block A accelerates slower in case a than in case b.

Problem A constant force F pulls the system as

shown. The pulleys are frictionless. The coefficient of kinetic friction between the block and the table is µ.

a) Draw free body diagrams for both

masses. b) Find an expression for the

acceleration in terms of the given variables.

HW Problem

In the figure shown, the coefficient of kinetic friction between the block and the incline is 0.29. What is the magnitude of the acceleration of the suspended block as it falls? Disregard any pulley mass or friction in the pulley. Draw the free body diagrams for each mass. Derive a general solution for the acceleration in terms of M, and g, box it, then put the numbers in and get a numerical value – then box that too. Then find a numerical value for the tension in the string. Box that. Show all your work and make it pretty! Use 3 significant figures.

2M

M

30

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All three 50-kg blocks are at rest. The tension in rope 2 is

QuickCheck 7.7

Slide 7-71

A. greater than the tension in rope 1. B. equal to the tension in rope 1. C. less than the tension in rope 1.

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A. greater than the tension in rope 1. B. equal to the tension in rope 1. C. less than the tension in rope 1.

All three 50-kg blocks are at rest. The tension in rope 2 is

QuickCheck 7.7

Slide 7-72

Each block is in static equilibrium, with .

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The acceleration constraint here is

QuickCheck 7.9

Slide 7-75

A. aAy = aBy. B. –aAy = –aBy. C. aAy = –aBy. D. aBy = –aAy. E. Either C or D.

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The acceleration constraint here is

QuickCheck 7.9

Slide 7-76

A. aAy = aBy. B. –aAy = –aBy. C. aAy = –aBy. D. aBy = –aAy. E. Either C or D.

Either says that the acceleration vectors point in opposite directions.

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The two masses are at rest. The pulleys are frictionless. The scale is in kg. The scale reads

QuickCheck 7.8

Slide 7-73

A. 0 kg. B. 5 kg. C. 10 kg.

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The two masses are at rest. The pulleys are frictionless. The scale is in kg. The scale reads

QuickCheck 7.8

Slide 7-74

A. 0 kg. B. 5 kg. C. 10 kg.