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Newton’s Laws of Motion
A journey into inertia, net force, and
other topics….
Force is central to understanding Newton’s Laws
A Force can be thought of as a
push, like a quarterback throwing a football.
Force is central to understanding Newton’s Laws
A Force can also be
thought of as a pull, like a boat towing a skier.
Another idea central to Newton’s Laws is MASS.
Anything which is made of matter, or atoms, has
mass. Anything you can touch or
see has mass, including
students at Central!!
Along with the idea of
mass is Inertia.
More mass means more
Inertia.
Less mass means less Inertia.
Inertia is the tendency of an object to stay at rest or
move at a constant velocity
• What does that sound like?
Newton’s 1st Law
• Objects tend to either remain at rest or move at constant velocity unless acted on by a Net Force.
• What is a Net Force?
When an object is at rest the forces are all equal or in
equilibrium.
Gravity pulls down on the
rock
But the rock is pushing up
with equal force!
When forces are equal there is NO Net Force!
What if the wind was pushing on the rock with greater force
than the rock could push back?
There would be a Net Force in the direction of the wind!
The rock would fall over in the direction of the Net
Force!
What is the Net Force in this picture?
Contact of the bat on the baseball!!
What other forces are in action in this picture?
Air Resistance
Gravity
Initially the force of the ball coming off of the bat is
stronger!Air
Resistance
Gravity
It is the Net Force!!
Acceleration can also be thought of as a signal that a
force has been applied!!Air
Resistance
Gravity
Even changing directions
requires force!!
Newton’s 2nd Law
Fnet =ma
Net Force
mass
acceleration
Net Force has direction. We call this a vector !
We have already talked about velocity, acceleration, and momentum, which have
direction and are also vectors !
Force is measured in Newtons (N)
1 N =1 kg ∙ ms 2
Free-Body Diagrams
Engine of truck exerting a force forward
Free-Body Diagrams
Why will the truck not go on forever at constant
velocity?
Free-Body Diagrams
Air Resistance, a form of friction, pushes against the
truck.
Free-Body Diagrams
There is also friction between the road and the
tires.
Free-Body Diagrams
Gravity pulls
down on the
sewing machine
Free-Body Diagrams
The desk pushes up
on the sewing
machine
The desk is exerting what is called Normal Force!
Free-Body Diagrams
Is there a Net Force
in this case?
No! The forces are equal and the desk is not moving!
Let’s Look at Types of Forces
• Gravity or Weight• Normal force• Friction• Tension• Thrust
Gravity or Weight
• Can be as simple as an object sitting on a table.
• Formula for Weight is W=mg.
Thrust
• Forces that move
objects from
rockets to cars to people.
Normal Force
• The force exerted by the surface the object is on.
• It is always perpendicular or at 90°.
Normal Force at an Angle
FN
Fg
FNet
FNet is the sum of the vectors. More on that later….
Friction
FN
Fg
FNet
Friction is the resistance of two surfaces sliding past one another
Ff
Tension
• Tension is force being applied to a rope or string.
• It applies when an object is hanging by a string or being pulled by a rope.
FT
Tension
FN
Fg
FNet
When someone is pulling a box up hill by a rope we now have thrust
going up the ramp.
Ff
FT
FThrust
Tension
FN
Fg
FNet
Notice when someone is pulling a box up hill by a rope Net Force and
Friction reverse directions.
Ff
FT
FThrust
Newton’s Third Law
• Whenever one body exerts a
force on another body, the second body exerts an
oppositely directed force of equal magnitude on the first body.
Newton’s Third Law
You probably learned this as
“For every action there is an equal but
opposite reaction.”
The End