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NSTA Web Seminar Force and Motion Bill Robertson February 1, 2006

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NSTA Web SeminarForce and MotionBill RobertsonFebruary 1, 2006

Newton’s third law

Circular motion

ΣF = 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.

- 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.

- 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.

- Action and reaction
- Whenever object A exerts a force on object B, object B exerts an equal and opposite force back on object A.

B

A

C

D

C

D

A

B

How does a propeller plane work?

How does a propeller plane work?

- 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 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.

- 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.

- 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.

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.

YES

NO

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.

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.

NO

YES

- 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