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Newton’s 3 rd Law of Motion

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Newton’s 3rd Law of Motion

Forces

Newton’s Third Law of Motion

Discussion

When your book is on the table what forces are acting upon it?

Newton’s Third Law of Motion

If only the force of gravity were acting on the book, it would be in free fall. The fact that it is at rest is evidence that another force must be acting upon it

Newton’s Third Law of Motion

This other force exactly balances the book’s weight and produces a net force of zero.

Newton’s Third Law of Motion

The “other” force is the support force of the table.

Support force is often called the normal force.

Newton’s Third Law of Motion

The normal force is the force that acts at right angles

(perpendicular)to the surface of contact

Newton’s Third Law of Motion

What is the net force on the object and in what direction would it move?

Newton’s Third Law of Motion

DISCUSSION

If you push with equal and opposite forces on opposite sides of an object at rest, what will happen to the object?

Newton’s Third Law of Motion

DISCUSSION

It will remain at rest!

Newton’s Third Law of Motion

Equilibrium - When the net force on an object is zero.

The object can be moving in a constant speed or motionless

This gymnast is not moving so the net force must be zero.

If the weight of the gymnast is 700 N, then each rope must pull upward with a force of 350 N in order to make the net force zero.

A woman is walking two dogs on a leash. If each dog pulls with a force of 80 newtons, how much force does the woman have to exert to keep the dogs from moving?