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Newton’s 3rd Law Whenever one object exerts a force on a second object the second object exerts a force equal in strength, BUT opposite in direction back on the first object.

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Action/Reaction? Action and Reaction forces act simultaneously. It is often hard to identify the pair of action and reaction forces. Remember that Action: Object A exerts a force on object B. Reaction: Object B exerts a force (same strength, and type) on object A in the opposite direction. Identify the action-reaction pairs in following situations: Driving a car swimming falling boulder.

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**Driving Friction Force from the road pushing on the car**

Friction Force from the car pushing on the road A car’s wheels spin so that they rub the road’s surface creating a friction force pointing to the rear of the car. The road “reacts” by exerting a friction force of equal magnitude on the car wheels in the forward direction.

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**Swimming Contact force of the water on the fish Contact force of**

the fish on the water A fish’s tail moves so that it exerts a contact force on the water, pushing the water backwards. The water “reacts” by pushing on the fish’s tail with a contact force of equal magnitude, but opposite direction making the fish move forward.

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**Falling Boulder The force of gravity from the planet**

pulls downward on the boulder causing it to fall. Fg of planet on boulder The falling boulder reacts by pulling upward on the Earth (trying to “lift it”) with a gravitational force. Fg of boulder on planet

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**Visualizing Newton's 3rd law**

Whenever something places a force on another object, that object being pushed exerts a force back onto the thing that is pushing it. The floor pushes back on the box just as hard The Box exerts a force of gravity pushing down on the floor

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**X X X X The floor pushes back on the person just as hard**

back on the box just as hard X X The block pushes back on the person just as hard A person pushes a block with a force X X Smooth floor The Person exerts a Force of gravity Pushing down on the floor The Box exerts a Force of gravity Pushing down on the floor

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**Newton’s 3rd law and accelerations**

Newton’s third law states that the forces between two objects pushing on each other are the same in size, however it does not state anything about acceleration. The accelerations of the these objects is governed by Newton’s 2nd Law, not the 3rd law.

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**A 50kg person pushes on a 20 kg box with a force of 10 Newtons**

A 50kg person pushes on a 20 kg box with a force of 10 Newtons. If both are standing on a smooth (frictionless) surface. What are the accelerations of the two objects? Step 1: Draw a Free Body Diagram The floor pushes back on the box just as hard Ffloor = +196 N The floor pushes back on the person just as hard Ffloor = +490 N Ffloor = +490 N Ffloor = +196 N The block pushes back on the person just as hard with -10 N A person pushes a block with a force of +10 N +10 N -10 N Smooth floor The Person exerts a Force of gravity Pushing down on the floor Fg = (50kg)*(-9.8m/s2) The Box exerts a Force of gravity Pushing down on the floor Fg = (20kg)*(-9.8m/s2) Fg = -490 N Fg = -196 N

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**A 50kg person pushes on a 20 kg box with a force of 10 Newtons,**

both standing on a smooth (frictionless) surface. Step 2: Look at each object separately Ffloor = +490 N Ffloor = +196 N +10 N -10 N Smooth floor Fg = -490 N Fg = -196 N

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**A 50kg person pushes on a 20 kg box with a force of 10 Newtons,**

both standing on a smooth (frictionless) surface. Step 2: Look at each object separately The forces for the Y axis cancel Ffloor = +490 N FnetY = Ffloor + Fg = maY FnetY = +490 N + (-490 N) = (50 kg)aY FnetY = 0 N= (50 kg)aY aY = 0 m/s2 -10 N Smooth floor This leaves only one force along the X axis. Fg = -490 N FnetX = Fright + FLeft = maX FnetX = 0N + -(10N) = (50 kg)aX aX = -(1/5) m/s2

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**A 50kg person pushes on a 20 kg box with a force of 10 Newtons,**

both standing on a smooth (frictionless) surface. Step 2: Look at each object separately Ffloor = +490 N Ffloor = +196 N +10 N -10 N Smooth floor Fg = -490 N Fg = -196 N

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**A 50kg person pushes on a 20 kg box with a force of 10 Newtons,**

both standing on a smooth (frictionless) surface. Step 2: Look at each object separately The forces for the Y axis cancel Ffloor = +196 N FnetY = Ffloor + Fg = maY FnetY = +196 N + (-196 N) = (20 kg)aY FnetY = 0 N= (20 kg)aY aY = 0 m/s2 +10 N Smooth floor This leaves only one force along the X axis. Fg = -196 N FnetX = Fright + FLeft = maX FnetX = 10N + (0N) = (20 kg)aX aX = (+1/2) m/s2

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aperson = -(1/6) m/s2 aBox = (+1/2) m/s2 60 Kg 20 Kg +10 N -10 N Smooth floor Net force on block = - net force on person Massblock * AccelerationBlock = - Massperson * Accelerationperson

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In Short When dealing with two objects pulling or pushing on each other, and there are no other forces being applied we can always say Mass1 * Acceleration1 = -Mass2 * Acceleration2 This means that the acceleration of the block is based on the ratio of the masses of both the person and the block itself Acceleration1= - (Mass2 / Mass1)* Acceleration2

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More on Newton’s 3 rd Law. Conceptual Example: What exerts the force to move a car? Response: A common answer is that the engine makes the car move forward.

More on Newton’s 3 rd Law. Conceptual Example: What exerts the force to move a car? Response: A common answer is that the engine makes the car move forward.

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