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PH 121 7.1-7.3
Transcript of PH 121 7.1-7.3
As we have used Newton's
first and second laws
they mainly deal with single
particles or objects.
These objects were acted upon,
but we didn't discuss the source.
This view is not right, there is much more
to it than that.
When an object is acted on,
a different object was the agent: the source
of that force.
Think hammer and Nail.
When the hammer hits the nails it exhorts a
force on it.
Likewise the nail exhorts a force back on the
This interaction is what your book calls a
Things to note
The forces occur simultaneously
also there is no implication on cause and effect
With this idea in mind we need to label some different
Object is any individual body or particle
it has mass, shape, limits
When we represent it, we only label forces acting
not the forces it creates
Example a Car is pushing a truck, its tires
exhort a horizontal force of 2000 N on the ground, what does its FBD look like?
A system is composed of objects that are
interacting. Specifically the objects we need to
consider to solve the problem.
They will not be the only interacting objects.
On previous example If the car is 1000 kg
and the truck 3000 kg.
what objects are there in the problem
Which make up my system
The Environment composes everything else
outside of the system.
When the environment acts on any object inside
the system, that interaction is called an external force.
most common here is gravity.
We don't care how much the earth moves, but it does
influence our systems. Thus gravity is an external force.
Newton's Third Law
Catch phrase: every action, equal and opposite reaction
Stated: Whenever one object exerts a force on another object, the second object exerts and equal force on the first in the opposite direction
As noticed in these questions, Newton's third law only equates Forces not acceleration.
There is one final important note in this last section.
When two objects move together they have the same acceleration.
" 4. Which is a better statement
Objects are in a system and everything else is the environment"
" Could you go over example 7.2 on page 171?"
"Can we review how to draw a free body diagram? I didn't understand how to draw the interacting forces."
"Could you explain the whole, object, system, Environment idea?"
"How does the normal force apply to an action/reaction pair of forces?"
"Can you explain about the friction force acting on the person in example 7.1?"
"Can you go over the tactics box on page 169?"
Let us draw an interaction diagram for a person pushing a crate.