5.10 - Newton’s third law
Newton's third law: “To any action there is
always an opposite and equal reaction; in other
words, the actions of two bodies upon each
other are always equal and always opposite in
direction.”
Newton’s third law states that forces come in pairs and that those forces are
equal in magnitude and opposite in direction. When one object exerts a force on
another, the second object exerts a force equal in magnitude but opposite in direction
on the first.
For instance, if you push a button, it pushes back on you with the same amount of
force. When someone leans on a wall, it pushes back, as shown in the illustration
above.
To illustrate this concept, we use an example often associated with Newton, the falling
apple shown in Example 1. The Earth’s gravitational force pulls an apple toward the
ground and the apple pulls upward on the Earth with an equally strong gravitational
force. These pairs of forces are called action-reaction pairs, and Newton’s third law is
often called the action-reaction law.
If the forces on the apple and the Earth are equal in strength, do they cause them to
accelerate at the same rate? Newton’s second law enables you to answer this question.
First, objects accelerate due to a net force, and the force of the apple on the Earth is
minor compared to other forces, such as those of the Moon or Sun. But, even if the
apple were exerting the sole force on the Earth, its acceleration would be very, very
small because of the Earth’s great mass. The forces are equal, but the acceleration for
each body is inversely proportional to its mass.
Newton’s third law
Forces come in pairs
Equal in strength, opposite in direction
The forces act on different objectsFab = íFba
Force of a on b = opposite of force of b
on a
The weight of the apple is 1.5 N.
What force does the apple exert
on the Earth?
1.5 N upward
5.11 - Normal force
Normal force: When two objects are in direct
contact, the force one object exerts in response
to the force exerted by the other. This force is
perpendicular to the objects’ contact surface.
The normal force is a force exerted by one object in direct contact with another. The
normal force is a response force, one that appears in response to another force. The
direction of the force is perpendicular to the surfaces in contact. (One meaning of
"normal" is perpendicular.)
A normal force is often a response to a gravitational force, as is the case with the block
shown in Concept 1 to the right. The table supports the block by exerting a normal force
upward on it. The normal force is equal in magnitude to the block’s weight but opposite
in direction. The normal force is perpendicular to the surface between the block and the
Normal force
Occurs with two objects in direct contact
Perpendicular to surface of contact