Third Law Notes Handout - Houston Independent School

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Motion & Forces

Action and Reaction !  Newton�s Third Law !  Momentum !  Conservation of Momentum

Dynamic Carts

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Dynamic Carts

" With carts only # Describe the motion of the carts # What evidence do you have that

there is a force exerted on each of the carts?

# Are the accelerations of the carts equal? Is the same amount of force exerted on each cart?

Dynamic Carts

" With additional weights on 1 cart # Describe the motion of the carts # What evidence do you have that

there is a force exerted on each of the carts?

# Are the accelerations of the carts equal? Is the same amount of force exerted on each cart?

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Demo with Balloon Helicopter

http://videos.howstuffworks.com/nasa/2139-how-helicopters-work-video.htm

A. Newton�s Third Law

" Newton�s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

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" �For every action there is an equal and opposite reaction.�


" Action-Reaction Pair

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" Action-Reaction Pair


# Action-Reaction Pairs

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" Action-Reaction Pairs

" The rocket exerts a downward force on the exhaust gases.

" The gases exert an equal but opposite upward force on the rocket.

FG

FR


" Physics of walking

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" Propulsion of fish through water # A fish uses its fins

to push water backwards. In turn, the water reacts by pushing the fish forwards.


" Action-Reaction Pairs

"  The hammer exerts a force on the nail to the right.

"  The nail exerts an equal but opposite force on the hammer to the left.

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" Consider the interaction between a baseball bat and baseball. # Action: the baseball forces the

bat to the right. # Reaction: ?


" Using a diving board to spring into the air before a dive is a good example of Newton�s third law of motion. Explain.

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Newton�s Third Law

Newton vs. Elephant Who will move fastest?


" Action-Reaction Pairs " Both objects accelerate. " The amount of acceleration

depends on the mass of the object.

=a F m

" Small mass ⇒ more acceleration " Large mass ⇒ less acceleration

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" http://esamultimedia.esa.int/docs/issedukit/en/activities/flash/start_toolbar.html#ex03_gm01.swf

A.  Newton�s Third Law

"  Collision between club head and ball #  Experience equal but

opposite forces #  Ball experiences greater

acceleration due to its smaller mass.

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"  Collision between moving 7-ball and an 8-ball at rest #  Each ball experiences an equal force

directed in opposite directions. #  Each ball experiences the same

acceleration.


"  Pair figure skaters – what happens when he gracefully throws her through the air onto the ice? #  She experiences a forward

force which causes her to accelerate (speed up)

#  He experiences a backward force which causes him to decelerate (slow down)

#  Her acceleration is greater than his deceleration due to her smaller mass.

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"  While driving, you observe a bug striking the windshield of your car.

Obviously, a case of Newton�s third law! The bug hits the windshield and the windshield hits the bug. Is the force on the bug or the force on the windshield greater?

A. Newtons�s Third Law

"  A gun recoils when fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is…

a.  Greater than the acceleration of the bullet. b.  Smaller than the acceleration of the bullet. c.  The same size as the acceleration of the

bullet.

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B. Momentum

" Momentum #  �mass in motion� # Depends on object�s mass and

velocity. #  The more momentum an object

has, the harder it is to stop. #  It would require a greater amount

of force or a longer amount of time or both to bring an object with more momentum to a stop.

B. Momentum

" Momentum

p = mv

p: momentum (kg ·m/s) m: mass (kg) v: velocity (m/s) m

p v

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B. Momentum and Impulse

" Newton�s 2nd Law

Impulse = Change of momentum

B. Momentum and Impulse

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B. Momentum

" Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s.

GIVEN: m = 280 kg v = 3.2 m/s p = ?

WORK: p = mv

p = (280 kg)(3.2 m/s)

p = 896 kg·m/s

m p

v

B. Momentum

" The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg?

GIVEN: p = 675 kg·m/s m = 300 kg v = ?

WORK: v = p ÷ m

v = (675 kg·m/s)÷(300 kg)

v = 2.25 m/s

m p

v

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C. Conservation of Momentum

" Law of Conservation of Momentum #  The total momentum in a group of

objects doesn�t change unless outside forces act on the objects.

pbefore = pafter


" If momentum is lost by one object, it must be gained by another object in the system so that the total momentum of the system is constant.

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" Collision between 1-kg cart and 2-kg dropped brick

" Momentum of the loaded cart-dropped brick system is conserved


" Big fish in motion catches little fish

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C. Conservation of Motion

" Little fish in motion is caught by big fish


" Elastic Collision (KE conserved)

"  Inelastic Collision (KE not conserved)

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" A 5-kg cart traveling at 4.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision.

BEFORE Cart 1: m = 5 kg v = 4.2 m/s Cart 2 : m = 2 kg v = 0 m/s

AFTER Cart 1 + 2: m = 7 kg v = ?

p = 21 kg·m/s

p = 0

pbefore = 21 kg·m/s pafter = 21 kg·m/s

m p

v v = p ÷ m v = (21 kg·m/s) ÷ (7 kg) v = 3 m/s


" A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon?

BEFORE Clown: m = 50 kg v = 0 m/s Cannon: m = 250 kg v = 0 m/s

AFTER Clown: m = 50 kg v = 20 m/s Cannon: m = 250 kg v = ? m/s

p = 0

p = 0

pbefore = 0

p = 1000 kg·m/s

pafter = 0

p = -1000 kg·m/s

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" So…now we can solve for velocity.

GIVEN: p = -1000 kg·m/s m = 250 kg v = ?

WORK: v = p ÷ m

v = (-1000 kg·m/s)÷(250 kg)

v = - 4 m/s (4 m/s backwards)

m p

v

Documents

Third Law Notes Handout - Houston Independent School