Download Newton`s Third Law

Unit Two: Dynamics
Newton’s Third Law
Bill Nye… Newton’s Third Law
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https://www.youtube.com/watch?v=cxVBbgFJ
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Start at 1:30
https://www.youtube.com/watch?v=NRKmJgI
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Newton’s Third Law
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When one object exerts a force on a second object,
the second object exerts a force on the first that is
equal in magnitude but opposite in direction.
These forces are called action-reaction forces.
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Ex: If you push against a wall, you don’t go through
it as the wall “pushes back”.
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Only the forces on an object determine its
acceleration.
Newton’s Third Law
• With equal and opposite forces, how does
anything ever move?
 Example: Picking up a ball:
Ball exerts an equal force on your hand, but
this is not on the ball and does not appear in
the free body diagram
Example
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Suppose you are floating around in space
(many km from any planet so that you feel no
gravity) outside of your spaceship. You get
frustrated and decide to kick your spaceship.
Does your foot hurt?
Solution
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Yes, your foot will hurt. Even though there is
no gravity, Newton’s Third law still applies. If
you kick the spaceship, it applies an equal
and opposite force on your foot.
Newton’s Third Law Worksheet
System of Masses
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When two or more masses are attached by a string
or rope and hang over a pulley system, there is a
system of masses.
Some assumptions that must be made:
- Strings only exert pulling forces.
- The tension in the string is the same throughout its
length.
- A frictionless pulley changes the direction of a
string without diminishing its tension.
- Strings do not stretch.
- The strings’ mass is negligible.
Tension
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Tension is the magnitude of the pulling
force exerted by a string, cable, chain, or
similar object on another object.
It is measured in Newtons
It is measured parallel to the string on which
it applies
Example 1
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A 2.0 kg mass, placed on a smooth, level
table is attached by a light string passing over
a frictionless pulley to a 5.0 kg mass hanging
freely over the edge of a table.
A) Draw a free body diagram of the masses
B) Calculate the tension in the string
C) Calculate the acceleration of the 2.0 kg
mass
Answer
Answer continued
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The masses move together – so the
accelerations are the same!
The forces are slightly different. How?
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Ask yourself how the system will move:
First, we know that mass m is falling and
dragging mass M off the table. The force of
kinetic friction opposes the motion of mass M.
However, we know that friction is negligible
here because it is a smooth surface!
We also know, since both masses are
connected by a nonstretching rope, that the
two masses must have the same speed and
the same acceleration.
Example 2
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Two spheres of masses 1.5 kg and 3.0 kg are
tied together by a light string looped over a
frictionless pulley. They are allowed to hang
freely. What will be the acceleration of each
mass?
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http://schools.hwdsb.on.ca/highland/files/201
1/01/System-of-Connected-Masses.pdf
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Systems of Masses Worksheet