NSTA Web SeminarForce and Motion

Bill RobertsonFebruary 1, 2006

Newton’s third law

Circular motion

Newton’s second lawΣF = ma

ΣF: represents the net force acting on an object.

m: represents the mass of an object, which is a numerical measure of its inertia.

a: represents the acceleration of the object.

image

Which best describes Newton’s third law?

• If one object is causing another to accelerate, then the first object is pushing harder on the second object than the second object is pushing on the first object.

• If you push something hard enough, it will fall over.

• Every force exerted on an object results in some kind of reaction.

Which best describes Newton’s third law?

• If one object is causing another to accelerate, then the first object is pushing harder on the second object than the second object is pushing on the first object.

• If you push something hard enough, it will fall over.

• Every force exerted on an object results in some kind of reaction.

Newton’s third law• Action and reaction

• Whenever object A exerts a force on object B, object B exerts an equal and opposite force back on object A.

Balloon clouds in back

Balloon clouds at bottom

1 donkey

4 donkeys

A B

C D

Split donkey carts

A B C D

How does a propeller plane work?

Single Airplane

How does a propeller plane work?

Single Airplane

How does a rocket work?

Rocket Pushes Gas

How does a jet plane work?

You catch a baseball. Which of the following are true about the forces exerted?

• You exert a larger force on the baseball than the baseball exerts on you.

• The baseball exerts a larger force on you than you exert on the baseball.

• The force you exert on the baseball is equal to the force the baseball exerts on you.

You catch a baseball. Which of the following are true about the forces exerted?

• You exert a larger force on the baseball than the baseball exerts on you.

• The baseball exerts a larger force on you than you exert on the baseball.

• The force you exert on the baseball is equal to the force the baseball exerts on you.

For the baseball:

F = ma

For you:

F = ma

Animation of carts on air track

Circular Motion—Which of the following are true?

• Objects moving in a circle at a constant speed are not accelerating

• The force that causes objects to move in a circle is called the centripetal force.

• The force that causes objects to move in a circle is called the centrifugal force.

• Centrifugal force doesn’t really exist.

Circular Motion—Which of the following are true?

• Objects moving in a circle at a constant speed are not accelerating

• The force that causes objects to move in a circle is called the centripetal force.

• The force that causes objects to move in a circle is called the centrifugal force.

• Centrifugal force doesn’t really exist.

YES NO

Suppose you want to figure out all the forces acting on something that's moving in a circle, such as a race car rounding a curve at constant speed.

Should you do the following? Figure out all the forces of contact on the car, such as friction, and then add the centripetal force to these forces. 

The centripetal force is a name given to forces that are already present, that happen to cause something to

move in a circle.

In this case, the friction between Einstein and the record is the force causing Einstein to move in a circle. Therefore, friction is the centripetal

force.

Now look at things from Einstein’s point of view

When you view things from a rotating frame of reference, a

centrifugal force appears.

This force is very real to someone in that frame of reference.

Another example

As viewed in the rotating frame of reference

Is the centrifugal force a real force?

YES NO

Know of any other forces that come into being in a rotating frame of

reference?

Upcoming NSTA Web Seminars: • Doing Good Science II

February 8, 2006

• Watershed Dynamics II

February 16, 2006

• Investigating Safely II

February 22, 2006

6:30 PM Eastern Time