Transcript Newton’s Three Laws

Newton’s Three Laws
Forces and Motion
What Is a Force?
• Any influence that may
change the motion of an
object
• For example: a push, a pull,
an attraction, a repulsion
The
pull of
gravity
Newton’s Laws of Motion
1. Law of Inertia
2. F = ma
3. Action/reaction
Law of Inertia
• All objects resist change.
– The more mass an object has, the more it resists
• An object in motion will remain in motion
unless acted upon by force
• An object at rest will remain at rest unless
I don’t want
acted upon by a force
to either, but I
Change!
Move!
I don’t
want to!
Change!
Move!
am more
willing to
change than
that big guy.
An object in motion
•
•
•
•
Car and Driver start in motion
Car stops by force from wall
No force on unbelted driver
Driver remains in motion due to
inertia
Because of Inertia. . .
• If there is no force the object does not
change what it is doing Nothing is even
Trying to make
Me move, I’ll just
Keep sitting here!
But what if the forces acting are equal?
If the forces are equal, the motion
does not change!
Moving
6m/s
5N
Still moving
6m/s
5N
• The equal and opposite forces cancel out!
• The object continues to do what it was
doing before the balanced forces.
An UNBALANCED net force
causes acceleration
Moving
6m/s
5N
Accelerating!
More than 6m/s
• The 5N force is unbalanced so the object
must accelerate (go faster and faster)
Newton’s Second Law
F = ma
force
mass
times
acceleration
The force here is the NET force which
means the total vector force acting on the
object.
Net Force
• All of the forces added together
Net F = 0 no forces
Net F = 5 N
5N
5N
5N
Net F = 0 (5N – 5N=0N)
5N
2N
Net F = 3 N (5N – 2N = 3N)
Because F=ma
If Net Force Is Zero
• Acceleration is zero
• Object does not accelerate
(but can still move)
If Acceleration Is Zero
•The net force must be zero
0 = ma
Therefore a = 0
F=mx0
Therefore F = 0
•The forces have to be balanced
•Forward force equals backward force, etc.
F = ma
• Big F gives big a
• Small F gives small a
• Big a comes from big F
Small Acceleration
Big
A
BIG
• Small a comes from small F
small
Small a
Solve
• A mass of 6kg accelerates at
3m/s/s
• What force was applied?
• Use F=ma
F = (6kg) x (3m/s/s)
F = 18kg m/s/s
F = 18N
Be careful
this one’s
harder!
Solve This!
• If the force is 10N and the mass is 5kg
• What is the acceleration?
• Use F = ma
10N
xa
______= 5kg
_________
5kg
5kg
10N = a
5kg
a = 2m/s2
Divide both
sides by 5 and
cancel
Newton’s Third Law
• For every action there is an equal and
opposite reaction
I push on this
wall, the wall
pushes back on
me!
Forces Always Happen in Pairs
Apple 1 pulls
on apple 2
Apple 2 pulls
on apple 1
The force of the
ground pushing
on the guy
The force of the guy
stepping on the ground
Action/reaction Applies to
Earth’s Gravity Too
Force of Earth
pulling down on
the apple
Force of apple
pulling up on Earth
I am
confused!
Is that possible?
The apple pulls on the Earth?!
For every action there MUST be a
reaction; the Earth being pulled by
the apple is the only possibility!
If the force from the Earth moves the
apple, why doesn’t the Earth move?
• The force on the apple is large compared to
the small mass of an apple.
• Therefore, the apple accelerates with a big
acceleration
I’m so little I
have large
acceleration!
•The same force on the hugely massive Earth
does not cause it to accelerate very much
•Therefore, the acceleration of the Earth is not
I’m too huge to
noticeable (but it is there!)
move much.
So what are Newton’s Laws?
?
First Law tells us: An object in motion remains
Also known as:
The law of inertia
in motion an object at rest
remains at rest unless acted
upon by a force!
Second Law tells us:
F = ma
For every action there is an
Third Law tells us: equal and opposite reaction