Newton’s Three Laws

Sir Isaac Newton1643-1727

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=maF=(40kg)(4m/s²)

F=160 NNote: 1 N = 1 kg . m

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=ma15N=3kg(a)a=15N/3kga=5 m/s²

Second Law

Look at the animation.Why do the elephant andfeather hit the ground atthe 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=(1kg)(9.8 m/s²)F=980,000 N 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.

Can you think of any other equal but opposite force pairs?

Source: http://mw.concord.org/modeler1.3/mirror/mechanics/NewtonCradle.html

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.

Webquest

Now it is time to apply what you have learned!

Webquest

Case:Your space shuttle from planet Newtonian made a crash landing on planet Earth.

Webquest

Case:On your planet, everyone floats. But on Earth everyone is stuck to the ground! And no one knows why!

Webquest

Task:King Martian has asked you to use the following internet sites and research Newton’s three laws. Then, fill out the corresponding worksheet and (according to the teacher’s instructions) create a presentation.

Webquest

Links:

http://teachertech.rice.edu/Participants/louviere/Newton/

http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml

http://www.sciencenetlinks.com/interactives/gravity.html

Best of luck!