1. Forces – Helpful Tips and Review
Physics 12
Forces – Helpful Tips and Review

2. Free Body Diagrams – Be careful!
Fg is directly downwards
FN is perpendicular (90˚) to the ground
In linear equilibrium (no accelerarion, FNET = 0):
Ff = Fgx
FN = Fgy
If the object is accelerating down the ramp:
Ff < Fgx
FNET = Fgx - Ff
There is no FA here! FA refers to an APPLIED force. This means it is a force made by a human or machine. In this case, the only force that may or may not make the object accelerate is GRAVITY!
Only draw the forces acting on ONE object (in this case, the flower pot) FN Ff
Fgx θ Fg
Fgy θ

3. Example: Draw 2 free body diagrams – 1 for the boy and 1 for the girl
FN(girl)
FN(boy)
FA(boy)
FA(girl)
Fg(girl)
Fg(boy)
Fg(boy) = |FN(boy)|
Fg(girl) = |FN(girl)|
FA(boy) = |FA(girl)|

4. Pulley systems and masses – Be careful!
The force due to tension in the rope (FT, or just ‘T’ here) is the same on both sides of the pulley ( T = T )
Because m2 > m1, the object will accelerate downwards on the right, and upwards on the left
Since m2 is accerating downwards:
FNET(m2) is downwards
m2g > T
therefore, FNET(m2) = m2g – T
Since m1 is accerating upwards:
FNET(m1) is upwards
m1g < T
therefore, FNET(m1) = T – m2g m1 m2
m1g T
m2g

5. Pulley systems – Be careful of force directions!
For the mass hanging over the side of the table,
If mg > T, the object will accelerate downwards
so FNET = mg – T
For the mass on the table,
Mg = N
so FNET = 0 in the vertical plane
So the object does not accelerate up or down.
The net force on this object will be HORIZONTAL.
If the object is accelerating to the right,
FNET = T – Ff
Remember, T is uniform throughout the rope,
so, the motion of this system depends on only 2 other forces:
-the mass of the hanging object
-the friction force on the object sitting on the table (which in turn relies on the coefficient of friction (µ) and the normal force (N)
You could draw a free body diagram for the system Ff
Total Mass mg