Download Newtons 3rd Law pdf - Mayfield City Schools

The Questions
What is Newton’s 3rd Law?
Newton’s 3rd Law
Newton’s 3rd Law says that
for every action force there
must be an equal and
opposite reaction force.
The reaction force must be
equal in strength and
opposite in direction.
There can never be a single
force, alone!!
Forces always occur in
pairs.
Newton’s 3rd Law
 Newton’s Third Law (action-reaction)
applies when a force is pushed (or pulled)
on any object, like a door.
 _______ exerted a force on the door.
And
 The door exerted a force on _______.
Newton’s 3rd Law
 When figuring out
action and
reaction forces it
is helpful to draw
a picture.
The action force (thing that is doing something) is the
________________.
The reaction force (force that happens because of the
thing that is doing something) is the
_______________.
Newton’s 3rd Law
 http://science.discovery.com/inte
ractives/literacy/newton/newton.
html
Solving Problems
A woman with a
weight of 500N is
sitting on a chair.
Describe one actionreaction pair of
forces in this
situation.
Give the magnitude
of each force and
what object the force
acts on.
Solving Problems
Looking for:
…pair of action-reaction forces
Given
…girl’s weight which is the force of…
FG = -500 N (down)
Relationships:
Fc = 500 N
Fw = -500 N
Action-reaction forces are equal and opposite and act on
different objects.
Solution
Draw a free body diagram
The downward force of 500N exerted by the woman on the
chair is the action force.
Therefore, the chair acting on the woman provides an
upward force of 500 N and is the reaction.
Solving Problems
A car drives along the highway at constant
velocity. Find the car’s weight and the friction
force if the engine produces a force of 2,000N
between the tires and the road and the normal
force on the car is 12,000 N.
Describe one action-reaction pair of forces in
this situation.
Give the magnitude of each force and what
object the force acts on.
Solving Problems
Looking for:
Reaction force Pairs.
Given:
…ForceG = 12,000N (up);
…ForceE = 2,000N (forward)
Relationships:
Newton’s 3rd Law: For every action there is an
equal and opposite reaction.
Solving Problems
Looking for:
Reaction force Pairs.
Given:
…ForceG = 12,000N (up);
…ForceE = 2,000N (forward)
Relationships:
Newton’s 3rd Law:
Car pushes down on road with 12000N Force.
Road pushes up on car with 12000N Force
Tires push on road with 2000N Force.
Road pushes on tires with a 2000N Force.
Solving Problems
A car has a mass of 1,000 kilograms. If
a net force of 2,000 N is exerted on the
car, what is its acceleration?
Looking for:
…car’s acceleration
Given
…mass = 1,000 kg; net force = 2,000 N
Relationships:
Newton’s Seconde Law:
Solution:
a=F/m
2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s2
The Answers
What is Newton’s 3rd Law?
1. For every action force there is
an equal and opposite
reaction force that occurs on
the other object.
2. Forces occur in pairs.
Solving Problems
A car drives along the highway at constant
velocity. Find the car’s weight and the
friction force if the engine produces a
force of 2,000N between the tires and the
road and the normal force on the car is
12,000 N.
4. Solution
FE = 200 N
FN = 12,000N
 Draw a free body
diagram.
 There is no net force
upward, so the weight of
the car is an equal
downward force of
FF = -200 N
−12,000 N.
 The forward engine force
balances the friction
force so the friction force
is −2,000 N.
FW = -12,000N
Solving Problems
Newton’s 3rd Law
Solving Problems
 Calculate the weight of a 60-kilogram
person (in newtons) on Earth and on
Mars.
1. Looking for:




…weight of person in newtons on both planets
Given:
…mass = 60 kg; g = 3.7 N/kg on Mars;
…implied g = 9.8 N/kg on Earth
2. Relationships:
 W=mxg
3. Solution:
 60 kg x 9.8 N/kg = 588 N
 60 kg x 3.7 N/kg = 222 N
Sig. fig. = 600 N
Sig. fig. = 200 N
Solving Problems
1. Looking for:
 … the velocity of the astronaut (backward)
2. Given
 …velocity1 = 10 m/s; mass1= 2 kg;
 ...mass2 = 100 kg;
3. Relationships:
 m1v1 = m2v2
4. Solution
 Draw a free body diagram.
The Answers
What is Newton’s 3rd Law?
For every action there is an equal
and opposite reaction.
Simple English:
Forces always occur in pairs and act
on different objects.