2. Action-Reaction Pairs: Act On Different Bodies
Forces exerted BY a body DO NOT (directly) influence its motion!!
Forces exerted ON a body (BY some other body) DO influence its motion!!
When discussing forces, use the words “BY” and “ON” carefully.

3. Example “Free Body  Diagram” for the monitor.
Shows all forces
on it, in proper
directions.
Figure 5.6: (a) When a computer monitor is at rest on a table, the forces acting on the monitor are the normal force n and the gravitational force Fg . The reaction to n is the force Fmt exerted by the monitor on the table. The reaction to Fg is the force FmE exerted by the monitor on the Earth. (b) The free-body diagram for the monitor.
Monitor at rest on table. Force of monitor on table ≡ Fmt. Force of
table on monitor ≡ Ftm. Ftm keeps monitor from falling. Ftm & Fmt
are 3rd Law action-reaction pairs. Forces on monitor are “Normal
Force” n & weight Fg. 2nd Law for monitor in vertical direction:
∑Fy = 0 = n - Fg. So, n = Fg = mg. So n = mg. They are equal &
in opposite directions, BUT THEY ARE NOT action-reaction pairs
(they act on the SAME object, not on different objects!)

4. Normal Force 2nd Law  ∑F = ma = 0
Suppose object is at rest on table. Does the force of gravity stop? OF COURSE NOT!
But, object does not move:
2nd Law  ∑F = ma = 0
 There must be some other force acting besides gravity (weight) to have ∑F = 0.
That force  The Normal Force = n
“Normal” is math term for perpendicular ()
n is  to the surface & opposite to the weight (in this simple case only!)
Caution, the normal force is not always = & opposite to the weight, as we’ll see! n mg

5. NO!!! Normal Force Where does the normal force come from?
From the other body!!!
Does the normal force ALWAYS equal the weight?
NO!!!

6. Example m = 10 kg The normal force is NOT always equal to the weight!!

7. Example m = 10 kg ∑F = ma FP – mg = ma Note: The normal force
is zero here because the
mass isn’t in contact with
a surface!