Download Newton`s Third Law

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Motion & Forces
Lesson 3
• Newton’s Third Law
Newton’s Third Law of Motion
When one object exerts a ___________ on a second object, the second object
exerts an ____________ but _______________ force on the first.
• Newton’s Third Law
Problem: How can a horse pull a cart if the cart is pulling back on the horse with an
____________ but ___________ force?
Aren’t these “______________ forces” resulting in no acceleration
• Newton’s Third Law
forces are equal and opposite but act on ________________ objects
they are _______ “balanced forces”
the __________________ of the horse depends on the ___________________
acting on the horse
• Action and Reaction
When a ________________ is applied in nature, a reaction force occurs at the
same time.
When you jump on a trampoline, for example, you exert a _____________ force on
the trampoline.
Simultaneously, the trampoline exerts an equal force ____________, sending you
high into the air.
• Action and Reaction Forces Don’t Cancel
According to the third law of motion, action and reaction forces act on
______________ objects.
Thus, even though the forces are ____________, they are not balanced because
they act on different objects.
• Action and Reaction Forces Don’t Cancel
For example, a swimmer “_________” on the water, the “___________” of the
water pushes the swimmer forward.
Thus, a net force, or _____________ force, acts on the swimmer so a change in
his or her motion occurs.
Rocket Propulsion
In a rocket engine, burning fuel produces hot gases. The rocket engine exerts a
force on these gases and causes them to ________________ out the back of the
rocket.
By Newton’s third law, the gases exert a force on the rocket and _____________
it forward.
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Newton’s Third Law
Action-Reaction Pairs
The hammer ______________ a force on the nail to the right.
The nail exerts an equal but _______________ force on the hammer to the left.
• Momentum
A moving object has a property called momentum that is related to how much force
is needed to change its _________________.
The _________________ of an object is the product of its mass and velocity
Momentum is given the symbol ____ and can be calculated with the following
equation:
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Momentum
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Momentum
_________________ of motion
p:
________________ (kg·m/s)
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m:
____________ (kg)
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v:
__________________ (m/s)
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Force and Changing Momentum
By ___________________ these two relationships, Newton’s second law can be
written in this way:
_____________________________________________
In this equation __________ is the final momentum and ___________is the initial
momentum
• Law of Conservation of Momentum
The momentum of an object doesn’t change unless its mass, ________________,
or both change.
Momentum, however, can be ____________________ from one object to another.
The law of conservation of momentum states that if a group of objects exerts
forces only on each other, their total momentum _______________ change.
• Law of Conservation of Momentum
The results of a collision depend on the _____________________ of each object.
When the first puck hits the second puck from behind, it gives the second puck
momentum in the same __________________.
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When Objects Collide
If the pucks are speeding toward each other with the same speed, the total
momentum is _________________.
Newton’s Third Law
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Action-Reaction Pairs
Both objects accelerate.
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The amount of acceleration depends on the mass of the object
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• Conservation of Momentum
Law of Conservation of Momentum
The total momentum in a group of objects doesn’t change unless outside
_______________ act on the objects.
• Conservation of Momentum
______________ Collision
KE is conserved
_____________ Collision
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KE is not conserved
• 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
work
answer:
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• Momentum
The momentum of a second bumper car is 675 kg·m/s.
total mass is 300 kg?
given
answer:
work
What is its velocity if its
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• Conservation of Momentum
A 5-kg cart traveling at 1.2 m/s strikes a stationary 2-kg cart and they connect.
Find their speed after the collision.
given
work
answer:
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• Conservation of Momentum
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?
given
work
answer:
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• Conservation of Momentum
So…now we can solve for velocity
given
answer:
work