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# PH 121 7.4-7.5

Newton's Laws
by

## Richard Datwyler

on 24 May 2018

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#### Transcript of PH 121 7.4-7.5

Newtons Third Law
Stated: Whenever one object exerts a Force on a second
object, the second exerts an equal force in the opposite
direction on the first.
Finishing off Mechanics and Newtons Laws
We worked with interacting pairs on Monday.

We then discussed a system vs. an environment

Finally constraints.
Example
Two blocks 10 kg and 5 kg pushed with 20 N. compare net forces on blocks.
These constraints happen when objects are joined
either by a rope, or by being in contact.
It also works around pulleys if they are frictionless

Let us talk a bit more about ropes or strings

When we have them in problems, we have said
the force is constant and equal on both ends.
This is only true if they are massless.
What if that was false, what would you do?
Example two blocks and a heavy rope
When we do assume that the rope is massless then
from newtons law, F= ma, the sum of forces will be
zero, thus equal in magnitude and opposite in direction.
This transfers to the forces between the two objects
the rope joins.

It basically turns them into interaction pairs just as we have done with contact forces in other Newton's 3rd law problems.
Likewise, pulleys, so long as they have no friction let the
massless strings
turn directions
, and change the forces on the
different objects into interaction pairs.
Now together ropes and pulleys.
5 kg
If this block is stationary
what is the force on the
end of the string?
F
10 kg
T
T
T
T
F
1
2
3
4
What are the
tensions and
Force in picture?

With these principles, and all of Newton's laws
we can start to have real fun solving different
types of problems.

10 kg
5 kg
20 N
A
B
rope
F
pull
"Can you explain how a pulley works?"
"Can you show us how not having a massless string would effect a free body diagram and calculations? I want to see an example because in real life ropes and cables can sometimes be very heavy. "
"How is it that pulleys make it easier to lift stuff. For example, more pulleys means less effort to lift. "