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Newton's Third Law of Motion
Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first.
When getting the net force to find acceleration, it must be the net force
on a particular object.
Extremely Important Point
Third Law Forces must be acting on different objects!!!!!
| F 1 |
Force on Student exerted by the
ground. |
| - F 1 |
Force on ground exerted by the
student. |
| F2 |
Force on student exerted by the
cart. |
| - F 2 |
Force on cart exerted by the
student. |
| F3 |
Friction force on cart exerted
by the ground. |
| - F 3 |
Force on ground exerted by the
cart. |
Student moves when F1 > F2
Cart moves when  -F2
> F3
weight = force of gravity on a body = FG = mg FN is
the contact force perpendicular to the surface.
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FG = Force of Gravity on the Bowling Ball. It is not
the force of the Bowling Ball on the Table.
Since the Bowling Ball is not accelerating, the Table must be
exerting and opposite and equal force of contact on the Bowling Ball. This creates
a next Force of 0 to match the 0 acceleration. We are using the Second Law
of Motion not the Third Law of Motion.
The Second Law of Motion says F = ma. Where Force is
the total of all forces acting on the same body. FG and FN
are both acting on the Bowling Ball keeping if from accelerating.
Remember: The Third Law of Motion is for forces on two
different objects.
F/N is the reaction force to FN.
If FN is the Force the table exerts on the Bowling Ball, then F/N
is the force the Bowling Ball exerts on the Table. The Third Law says they are
equal and opposite directions.
FG is the Force of Earth’s gravity on the Bowling
Ball. F/G would be the force the Bowling
Ball’s gravity
exerts on the Earth. It is not shown in the diagram. |
Be very careful. The Third Law of Motion refers to forces on different
objects. The Second Law of Motion refers to forces on a single object
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