Lecture 5
Newton’s 3rd Law; Gravitational Force
1
Review from Last Time
•
Newton’s Second Law (The Momentum Principle)
–
Free body diagrams  in Newton’s second law it is the
net
force that matters! FBD
help us identify all the forces acting on an object.
–
Once we identify the forces, we can write down Newton’s 2nd Law, writing it out
for each component.
–
Problem Solving Method  a good idea to follow a method when you solve problems!
1. Sketch the scenario to help you visualize it
2. Draw a free body diagram for
each
object in the problem
3. De±ne a coordinate system. If an object is accelerating, one rule of thumb is to
de±ne one of the coordinate axes along the direction of the acceleration.
4. Write F=ma for each component (e.g.
F
x
=
ma
x
;
F
y
=
ma
y
, etc)
5. Solve! If you’re asked to solve for the acceleration or the value of one of the forces
(e.g. the normal force), you can generally solve for this using the equations you
have. If you’re given information about or need to solve for velocity or position,
you’ll need to integrate the acceleration to get
v
(
t
), etc. as we discussed in the
±rst week of class.
2
Newton’s Third Law (Reciprocity)
•
In principle,
any
mechanics problem can be solved by applying Newton’s second law.
But, in order to know how an object will accelerate, we need to know the forces acting
on it. Sometimes the details of the forces are very complicated.
•
Newton told us only two things about the actual forces. First, he gave a complete law
of the gravitational force which we will discuss in a bit. In the case of very complicated
forces between many atoms in a system, for example, he didn’t know the right law for
the forces. He did, however, discover one important law about the nature of forces:
“For every action there is an equal and opposite reaction”
•
If I am on a skate board or ice skates and I push on the wall, what happens? I move
backwards! Why? (Simple DEMO: use chair with wheels)
•
I am accelerating backwards, so there must be a force acting on me. Newton’s second
law tells us this must be so. What provides this force?
•
This force is provided by the wall! When I push on the wall, the wall pushes back on me.
16
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In fact, Newton’s 3rd law tells us that the wall exerts the same force on me that I exert
on the wall (DEMO: if I push harder, I accelerate more)
•
PUZZLER: box sitting on table  are normal force and weight actionreaction pairs? No!
They act on the same object thus cannot be an actionreaction pair.
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 Spring '08
 Millan
 Force, Mass, Momentum, General Relativity, Gravitational Force

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