3. LEARNING OUTCOMES
Identify forces acting on an object and
draw free-body diagrams
Solve problems for a static-point mass
Recall and apply the equation F = ma to
new situation or to solve related problems
4. FREE-BODY DIAGRAMS
A diagram which shows all the forces
acting on a body
Consists primarily of a sketch of a body
and arrows representing the forces
acting on it
Weight
Contact force
Tension
6. WEIGHT
Weight of a body is the gravitational force
exerted by the Earth on the body. The weight
acts at the center of gravity of a body. It is
always directed towards the center of the Earth
The free-body diagram of a body falling without
air resistance is shown below
7. CONTACT FORCE
Contact force is the force exerted by
the contact surface on the body
Acts perpendicularly to the contact surface
Two forces act on the body: they are the
contact force and the weight
8. RECALL: WEIGHT VS.
CONTACT FORCE
Contact forces are forces
that act on a body at the
point of contact between
two different bodies
Non-contact forces is a
force that acts on a body
such as the weight which
acts through the center of
gravity (CG) of an object
Contact forces Non-Contact forces
9. WHAT ARE THE FORCES ACTING
ON THE BODIES?
The pair of action
and reaction forces
is the gravitational
force by the Earth
on the man and the
gravitational force
by the man on the
Earth
There are 2 free-
body diagrams : One
for the man and the
other for the Earth
10. FREE-BODY DIAGRAM
The free-body
diagram of the man
shows the force
acting on him only
The free-body
diagram for the Earth
shows the force that
acts on the Earth only
11. WHAT ARE THE FORCES ACTING
ON THE BODIES?
The first pair of action
and reaction forces is
the gravitational force
by the Earth on the
man and the
gravitational force by
the man on the Earth
The second pair of
action and reaction
forces is the contact
force by the man on
the Earth and the
contact force by the
Earth on the man
12. FREE-BODY DIAGRAM
The free-body
diagram shows two
forces : F1 and F2
acting on the man.
Since the man is at
rest , the two
forces are equal
and opposite in
direction
13. FREE-BODY DIAGRAM
The free-body
diagram for the
Earth shows two
forces , F1 and F2
acting on the
Earth
Since F1 = F2 , the
Earth is also at
rest
20. TENSION
The tension is directed along the length of the
string and pulls equally on the support and the
pendulum bob
The diagram below is not a free-body diagram as it
shows forces acting on three bodies: the pendulum
bob, the string and the support. It will not be useful in
analysis.
21. TENSION
Isolate the pendulum bob as a free-body
Only contact force acting on the bob is
the tension
Free-body diagram of the pendulum bob
consists of the tension and weight only
22. PUTTING IT TOGETHER
Weight and contact force are equal. No
resultant force acting vertically
Acceleration of the crate is due entirely to
the tension force
32. UNBALANCED FORCES
When 3 forces are
applied to a point of an
object , the resultant
force (due to it being
unbalanced) acting on
the object can be
obtained by adding the
force vectors
Vectors A,B and C
represent three
forces
34. BALANCED FORCES
• When 3 forces acting on an object is
balanced , resultant force is ________.
• There is no _________.
• An object will either ________ or
____________.
35. BALANCED FORCES
Consider a picture hanging on the wall.
What are the forces acting on the
picture frame?
48. EFFECTS OF FRICTION ON
MOTION
As a useful force
To move forward, you place one foot in
front of you and push your other foot
backwards on the ground. The friction
between your shoe and the ground
prevents you from slipping.
49. EFFECTS OF FRICTION ON
MOTION
As each wheel
turns, friction is
created at the point of
contact between the
wheel and the road
At the point of
contact, the wheel is
moving backwards
Friction acts in the
opposite direction and
provides the forward
force that drives the
car
50. WHEN THERE IS NO FRICTION
Without friction, the wheels would spin
without helping the car to move forward.
An example of this is when a car gets
stuck in mud or fine sand as shown above
51. FACTORS THAT AFFECT
FRICTION
Depends on the nature of the surfaces in
contact
The rougher the surface the greater is the
friction
Is proportional to the force pressing the
surfaces together (in the case of
horizontal movement, the pressing force
is the weight).
Pushing heavy vs. light objects
• Is independent of the area of contact
