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Chapter 7

PHYA11H3 Chapter 7: Chapter 7 Newton’s Third Law

Physics and Astrophysics
Course Code
Salam Tawfiq

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Chapter 7 Newton’s Third Law
7.1 Interacting Objects
- Any time an object A pushes or pulls on another object B, B pushes or pulls back on A
- Interaction: the mutual influence of two objects on each other
o If object A exerts a force
A on B on object B, then object B exerts a force
B on A on object A
This pair of forces, is called an action/reaction pair: two objects interact by
exerting an action/reaction pair of forces on each other
- The same idea holds true for long-range forces such as gravity
o If you release a ball, it falls because the earth’s gravity exerts a downward force
earth on ball
but does the ball really pull upward on the earth with a force
ball on earth?
Newton was the first to realize that, indeed, the ball does pull upward on the earth
Objects, Systems, and the Environment
- System: objects whose motion we want to analyze
- Environment: objects external to the system
- External forces: interactions with objects in the environment
7.2 Analyzing Interacting Objects
- Propulsion: the force that a system with an internal source of energy uses to drive itself forward
o I.e.) The friction force
o The distinction between you and the crate is that you have an internal source of energy that
allows you to straighten your leg by pushing backward against the surface
In essence, you walk by pushing the earth away from you and the earth’s surface
responds by pushing you forward these are static friction forces
7.3 Newton’s Third Law
- Newton’s third law: Every force occurs as one member of an action/reaction pair of forces
o The two members of an action/reaction pair act on two different objects
o The two members of an action/reaction pair are equal in magnitude but opposite in
AonB = -
- The two members of an action-reaction pair have equal magnitudes
o F A on B = F B on A this is the quantitative relationship that will allow you to solve problems
of interacting objects
Reasoning with Newton’s Third Law
- Forces are equal but the accelerations are not
o This is why when you release a ball, the ball and the earth exert equal and opposite forces
on each other, but the earth does not fall up to meet the ball
Acceleration Constraints
- If two objects A and B move together, their accelerations are constrained to be equal: A= B
- Acceleration constraint: a well-defined relationship between the accelerations of two or more
- In practice, acceleration constraints are expressed in terms of x- and y-components of
aCx = aTx = ax
Therefore, since the accelerations of both objects are equal, we can drop the subscripts C and T and
call both of them ax
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