This slide show is a work in progress. I am putting together a short introduction to drawing free body diagrams for physics and engineering problems. Hint - learn Newton's 3rd law first. It makes learning the other two a breeze. I'll be updating and adding slide shows soon.

1. ENGINEERING
Statics

2. NET FORCE
When the net force on an
object is zero, the
ACCELERATION of the object is
zero, However, the object may
still be moving IN A STRAIGHT
LINE

3. FUNDAMENTAL
FORCES
How many forces are involved
in a free body diagram?
1
2
3
4

4. FUNDAMENTAL
FORCES
How many forces are involved
in a free body diagram?
1 Gravity
2 Electromagnetic
3 Strong Nuclear Force
4 Weak Nuclear Force

5. FUNDAMENTAL
FORCES
How many forces are involved
in a free body diagram?
1 Gravity
2 Electromagnetic
3 Strong Nuclear Force
4 Weak Nuclear Force

6. FUNDAMENTAL
FORCES
How many forces are involved
in a free body diagram?
1 Gravity
𝐹 = 𝐺
𝑚1𝑚2
𝑑2
2 Electromagnetic
Coulomb force
Magnetic force
Friction
Tension
Bouyant
Normal
Spring
Intermolecular

7. FUNDAMENTAL
FORCES
How many forces are involved
in a free body diagram?
1
2
3
4

8. FUNDAMENTAL
FORCES
How many forces are involved in a free
body diagram?
𝐹 = 𝐺
𝑚1𝑚2
𝑑2

12. WHY IS THIS SO
HARD?
1.Resolve all force
vectors into their x y
and z components
2. Σ𝐹𝑛𝑒𝑡 = 0 𝑁
3.Draw each pair of
forces involved ONE
PAIR AT A TIME
Let’s make it simpler

13. Gravi
ty
(assume the ropes have
negligible weight. Then the
force of gravity on them is
zero. You may exclude these two
pairs of forces)
Intuitively, we might
assume that the force
through each rope is
50N. But we can also
prove it
mathematically.
𝑇1 = 𝑇2
𝑇1 + 𝑇2 = 100𝑁
A simple substitution
gives us the answer.
𝑇1 + 𝑇1 = 100𝑁
𝑇1 = 𝑇2 = 50𝑁

14. Gravi
ty
(assume the ropes have
negligible weight. Then the
force of gravity on them is
zero. You may exclude these two
pairs of forces)

15. Gravi
ty
(assume the ropes have
negligible weight. Then the
force of gravity on them is
zero. You may exclude these two
pairs of forces)
According to
Newton’s 3rd Law
of Gravity,
forces always
occur in pairs.
• What object
does the
“reaction
force” act
on?
• What is the
magnitude and
direction of
this
“reaction

16. Gravi
ty
(assume the ropes have
negligible weight. Then the
force of gravity on them is
zero. You may exclude these two
pairs of forces)
Since we are only
concerned with the
forces acting on the
sign and not the
earth, we can
exclude the earth
and the reaction
force on it.

17. If the only
force acting on
the sign were
the weight of
the sign, there
would be a NON-
ZERO net force
acting on the
sign, and it
would
accelerate in
the direction
of the net
force (in this
case, towards
the ground)

18. Tensi
on
Fortunately,
there are 2
ropes
supporting the
weight of the
sign.
The SUM of
these forces
that are acting
on the sign ADD
UP TO ZERO. We
know this
because the
sign is NOT
ACCELERATING

19. CHECK YOUR WORK
Did we include all
relevant forces?
Suggestion – always start
with gravity
So far, we have drawn the
diagram. In the next step, we
will calculate the horizontal
and vertical components of the
tension forces in each rope.

20. 𝐹𝑛𝑒𝑡 = 0𝑁
𝐹𝑛𝑒𝑡,𝑥 = 0𝑁
𝐹𝑛𝑒𝑡,𝑥 = 0𝑁
AND

21. WHAT DO WE KNOW
𝑇1=𝑇2cosΘ
We know this because the acceleration
(and hence the net force) in the
horizontal direction is zero.
Don’t confuse these forces for
action/reaction pairs of forces!
Remember that the “reaction” forces
are acting on THE SIGN. An easy way
to remember this is to pretend the sign
were made of a stretchy material.

22. WHAT DO WE KNOW
In this problem, it is
ESSENTIAL to solve for
T2 before proceeding to
the horizontal forces.
Once you’ve done enough
of these problems,
you’ll see that each one
is a puzzle to be
solved.
𝑇1=𝑇2cosΘ