Transcript Newton`s Three Laws

Newton’s Three Laws
Sir Isaac Newton
1643-1727
Introduction
 Newton’s 3 laws define some of the
most fundamental things in physics
including:
 Why things fall down
 Why objects move
 And much more!
First Law
 Newton’s first law states that
something in motion will
continue in motion until acted
upon by an external force.
 This is also called the Law of Inertia.
First Law
 That means that something moving
would move FOREVER if another force
did not act on it.
 Think of a soccer ball being kicked.
Do you know what force acts on it to bring
it back down to earth?
First Law
 The force that brings the soccer ball
back down is:
 Gravity
If it weren’t for this force, when you kicked
a soccer ball it would never stop flying
and never come back down to earth.
That would be the longest kick ever!
First Law
 Take a look at the animation. Why
does the person continue to move
when the car stops?
Source: http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/newtlaws/cci.html
First Law
 The person keeps moving because no
force pushed on it. The force of the
wall only acted on the car.
Source: http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html
Second Law
 Newton’s second law states
states that the Force of an
object is equal to its mass
times its acceleration.
F=ma
Second Law
 This law is so important because it
allows for calculations to be made.
With this law you can actually
calculate how strong a force is.
Let’s try a couple problems!
Second Law
 Imagine you are riding on a sled. You
are accelerating at a rate of 4 m/s²
and have a total mass of 40 kg.
What is the net force exerted?
(Remember: F=ma)
 The answer is 160 N.
You get this answer by plugging in the values.
F=ma
F=(40kg)(4m/s²)
.
Note: 1 N = 1 kg m
F=160 N
s2
Second Law
 Suppose you hit a 3 kg textbook with
a net force of 15 N. What is the
book’s resulting acceleration?
(Remember: F=ma)
 The answer is 5 m/s².
You get this acceleration by plugging in the values.
F=ma
15N=3kg(a)
a=15N/3kg
a=5 m/s²
Second Law
 Look at the animation.
Why do the elephant and
feather hit the ground at
the same time?
(assume no air resistance)
Source: http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/newtlaws/efff.html
Second Law
 The elephant and feather hit the ground at
the same time because they have equal
accelerations- gravity (9.8 m/s²).
 What will be different between the elephant
and the feather when they hit the ground?
Look at the following example.
Second Law
 Let’s assume the mass of the elephant is
100,000 kg and the mass of the feather is 1
kg. They both accelerate (due to gravity) at a
rate of 9.8 m/s². What is their net forces?

(elephant)
(feather)
F=ma
F=ma
F=(100,000kg)(9.8 m/s²)
F=980,000 N
F=(1kg)(9.8 m/s²)
F=9.8 N
This means the elephant will hit the ground with a force
100,000 times bigger than the feather. That will definitely
leave a mark!
Third Law
 Newton’s third law states that
for every action there is an
equal but opposite reaction.
Source: http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4a.html
Third Law
 Look at the animation. This shows an
action with an equal but opposite
reaction.
Source: http://mw.concord.org/modeler1.3/mirror/mechanics/NewtonCradle.html
 Can you think of any other equal but
opposite force pairs?
Third Law
 Perhaps you came up with:
 When a baseball hits a bat
 The bat exerts a force on the ball and the
ball exerts an equal but opposite force on
the bat.
Third Law
 Perhaps you came up with:
 When you step from a boat to a dock
 You can feel the boat drift backwards. This
is because as we move the direction of the
dock, the boat moves with in an equal but
opposite force.
Third Law
 Perhaps you came up with:
 Shooting a gun
 The force forward the rifle puts on the
bullet is matches with the equal but
opposite force backwards the bullet puts
on the rifle. This is what causes the gun to
recoil (kick).
In Conclusion
 Newton’s Three Laws are important!
 What does his first law state?
 Newton’s first law states that
something in motion will continue
in motion until acted upon by an
external force.
In Conclusion
 Newton’s Three Laws are important!
 What does his second law state?
 Newton’s second law states
states that the Force of an object
is equal to its mass times its
acceleration.
F=ma
In Conclusion
 Newton’s Three Laws are important!
 What does his third law state?
 Newton’s third law states that for
every action there is an equal but
opposite reaction.