Newton’s Three Laws

1
Newtons Three Laws
Sir Isaac Newton 1643-1727
2
Introduction

• Newtons 3 laws define some of the most
  fundamental things in physics including
• Why things fall down
• Why objects move
• And much more!

3
First Law

• Newtons 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.

4
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?

5
First Law

• The force that brings the soccer ball back down
  is
• Gravity
• If it werent 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!

6
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
7
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
8
Second Law

• Newtons second law states states that the Force
  of an object is equal to its mass times its
  acceleration.
• Fma

9
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.
• Lets try a couple problems!

10
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 Fma)
• The answer is 160 N.
• You get this answer by plugging in the values.
• Fma
• F(40kg)(4m/s²)
• F160 N

Note 1 N 1 kg . m s2
11
Second Law

• Suppose you hit a 3 kg textbook with a net force
  of 15 N. What is the books resulting
  acceleration? (Remember Fma)
• The answer is 5 m/s².
• You get this acceleration by plugging in the
  values.
• Fma
• 15N3kg(a)
• a15N/3kg
• a5 m/s²

12
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
13
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.

14
Second Law

• Lets 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)
• Fma Fma
• F(100,000kg)(9.8 m/s²) F(1kg)(9.8 m/s²)
• F980,000 N F9.8 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!

15
Third Law

• Newtons 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
16
Third Law

• Look at the animation. This shows an action with
  an equal but opposite reaction.
• Can you think of any other equal but opposite
  force pairs?

Source http//mw.concord.org/modeler1.3/mirror/me
chanics/NewtonCradle.html
17
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.

18
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.

19
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).

20
In Conclusion

• Newtons Three Laws are important!
• What does his first law state?
• Newtons first law states that something in
  motion will continue in motion until acted upon
  by an external force.

21
In Conclusion

• Newtons Three Laws are important!
• What does his second law state?
• Newtons second law states states that the Force
  of an object is equal to its mass times its
  acceleration.
• Fma

22
In Conclusion

• Newtons Three Laws are important!
• What does his third law state?
• Newtons third law states that for every action
  there is an equal but opposite reaction.