Chapter 5:
forces
I. What are forces?
A. Characteristics:
1. Forces result from the interaction of
objects. A FORCE is a push or a pull
that one object exerts on another.
2. How are forces measured:
a. in metric units  Newtons (N)
b. in English units  pounds (lbs)
(force of 1 lb = 4.448 N)
3. Forces are VECTORS (size & direction):
- have positive values (e.g. +10N) when
force is directed: up/North/right/East
- have negative values (e.g. -10N) when
force is directed: down/South/left/West
- forces can be represented in diagrams
by using arrows.
(length = strength)
(points in direction
of the force)
4. Forces influence motion:
- have the ability to change an object’s
velocity (i.e. speed and direction)
- Newton’s Laws of Motion
5. Forces are not always noticeable:
- gravity pulls object’s toward earth
- the ground pushes up on you
6. All forces are placed into TWO main
categories:
a. Contact Forces – those that result when
two objects are in direct contact w/
each other. Examples…
Applied Force – they push or pull
Tension
Elastic/Spring
Friction
Air Resistance
Normal Force
b. Action-through-space Forces
(a.k.a. field forces – create a “force field”)
– those that result even when two objects
are not in direct contact w/ each other.
Examples…
Strong Nuclear – strongest force of all
Electromagnetic
Weak Force – related to radioactivity
Gravitational
II. Types of Forces
 The Contact Forces
1. Applied Force
- a force that is applied to an object
by a person or another object.
2. Tension Force
- a force that is transmitted through a string
rope, cable, or wire when it is “pulled”
tight from opposite ends
- the force is directed along the entire
length of the rope and pulls equally on
the objects on both ends
3. Spring Force
- the force exerted by a spring upon the
object that is compressing or stretching
the spring
- The spring will push/pull back on the
object w/ a force that is needed to return
the compressed/stretched spring to its
resting position
 the magnitude of the force is
proportional to the amount of stretch
or compression
Examples:
A force of 16 N is required
To stretch a spring a
Distance of 40cm from its
Rest position.
What force is needed to
stretch the spring:
a)Twice the distance 32 N
b)3x the distance
48 N
c) Half the distance? 8 N
4. Gravitational Force (a.k.a. Weight)
- the force w/ which a massively large
object attracts another object toward itself
- all objects on earth experience the force
of gravity pulling them “downward”
toward center of the earth.
- on earth, gravity exerts a force of 9.8 N on
every kg of mass, thus:
 the strength of gravity (g) = 9.8 N/kg
- on the moon, the strength of gravity (g) is
1/6 that of earth (g = 1.6 N/kg)
- the force of gravity is equal to the weight
of an object
- to calculate your weight, which is the
force of gravity:
Fgravity = m * g
(where g = 9.8 N/kg)
(weight)
1 kg
W = 1(9.8)
= 9.8 N
2 kg
W = 2(9.8)
= 19.6 N
3 kg
W = 3(9.8)
= 29.4 N
- DON’T confuse mass and weight
a. Mass – the amount of matter in an object
 measured in kg
 the same everywhere in the universe
b. Weight – the force of gravity acting on
an object’s mass (i.e. related to the pull
of the earth)
 changes depending on location
The Statue of Liberty has a mass of 225,000
kg. How much does she weigh?
To calculate the weight of an object you have to
multiply it’s mass times the acceleration of
gravity.
Write the formula:
W = m
*
g
Substitute known values:
W = (225,000 kg) * 9.8 m/s²
Present solution with units: W = 2,200,000 N
What the heck is 2,200,000 N?
The Statue of Liberty weighs 2,207,250 Newtons, which is
495,000 lbs pounds!
- Example: An astronaut has a mass of 120 kg.
a) what is his weight?
b) what is his mass on the moon?
c) what is his weight on the moon?
Question: Can an object have mass but no
weight when that object is located
on earth?
YES…
- Remember Principles of Free Fall:
 when objects are in free fall, gravity will
accelerate them toward the earth with a
speed of 9.8 m/s2
 when in free fall, an object has no weight
W=0N
weightlessness
5. Friction
a. the force that opposes the motion of an
object (i.e. acts in the opposite direction
of motion).
b. It results from two surfaces being
pressed together
c. friction depends on two things:
1. types of surface in contact; due to
microscopic imperfections
Example:
 puck on sandpaper = high friction
 puck on ice = low friction
2. amount of force pressing the surfaces
together (more mass, more friction)
Ffr
Ffr
d. Three general types of friction:
- Static, Sliding, and Rolling
1. Static friction (greatest)
- results when the surfaces of 2 objects
are at rest; prevents an object from
starting to move (static friction cancels
out applied force).
- eventually reach a point where applied
force overcomes static friction and
object will begin to move
2. Sliding friction
- resists sliding motion of object across
a dry surface
- to keep an object moving, there must be
another force present to overcome sliding
force
 Remember:
Static friction is much stronger than
sliding friction.
It is much harder to start an object
moving than to keep an object moving
3. Rolling friction:
- similar to sliding friction, but applies to
objects that are rolling
- example: tires rolling on the ground.
e. Reducing friction:
- Why? To decrease NRG loss (e.g. heat,
noise) and to keep surfaces from wearing
down.
1. Add lubricant b/n surfaces
- puck slides on ice
- oil in car engine
2. Reduce roughness of surfaces:
- sanding wood
3. Ball bearings:
- Two moving surfaces in a machine avoid direct contact
by placing small metal balls b/n them. This way,
sliding friction is changed to rolling friction.
4. Cushion of air or magnetic levitation:
- air hockey
- maglev trains
6. Air Resistance
- the force that acts upon objects as they
travel through air
- most noticeable for object’s traveling at
high speeds (e.g. skydiver) or large
surface area
- usually not taken into account b/c of its
negligible magnitude
7. Normal Force (a.k.a. support force)
- the force that a surface exerts on an object that
is pressing on it.
- it has an equal strength and opposite direction
to the force pressing the object into the surface
III. Forces and Equilibrium
- Multiple forces are acting simultaneously
on objects at all times
A. Free-Body Diagrams
- used to show the relative magnitude and
direction of all forces acting upon objects
- remember:
 size of arrow = magnitude
 tip of arrow = direction of force
A free-body diagram illustrates the
relative magnitude and direction of all
forces acting upon an object. The
object must be isolated and “free” of its
surroundings.
This is a free-body diagram of the Statue of Liberty. She
is represented by a simple box. The forces acting on her
are labeled with a magnitude and the arrow shows
direction. Notice the surrounding objects are stripped
away and the forces acting on the object are shown.
496210 lb
496210 lb
“FW” : the force of the weight of the statue.
“FN” : the normal force, which represents the force
FW = 495,000 lb
of the pedestal pushing back up on the statue.
FN =495,000 lb
Create a free body diagram (FBD) of the gorilla:
FN
Gorilla
FW
Free Body Diagram of the Sitting
Gorilla (The box represents the
gorilla, W = weight of the gorilla, N =
Normal force)
Sitting Gorilla
Draw a FBD of the wooden swing:
Where are the forces
on the swing?
FT1
FT2
Swing
FW
Free Body Diagram of the wooden swing (The
box represents the wooden swing, W = weight
of the swing and the parrot, T represents the
ropes that are in tension supporting the weight)
Parrot on wooden
swing hung by ropes
Draw a FBD of bucket the bungee jumper
leaped from:
Where are the forces
on the bucket?
FT
bucket
FW
Free Body Diagram of the bucket (T represents the
tensile force of the cable the bucket is suspended
from, and W is the weight of the diver and the
bucket)
Bungee jumping
from crane
Draw a FBD of the ring at point C:
Where are the forces on the ring?
A
B
FTCA
FTCB
C
D
FTCD
Traffic Light
supported by cables
Free Body Diagram of the ring at
point C (T represents the force of the
cables that are in tension acting on
the ring)
Draw a FBD of the traffic light:
A
B
C
Where are the
forces on the
light?
FTCD
Light
D
FW
Traffic Light
supported by cables
Free Body Diagram of the traffic light
(FTCD represents the force of the cables
acting on the light and FW is the weight
acting on the light)
B. Determining Net Force:
- the vector sum of all
forces acting on an
object is called net
force
- Rule:
Forces that are equal
in magnitude and
opposite in direction
will cancel out each
other. Examples:
-Examples: Determine the net force in each
of the following diagrams
C. Balanced v. Unbalanced Forces
- when multiple forces act on an object,
the net force will either be:
1. balanced (net force = zero)
2. unbalanced (net force is not zero)
 What is the importance of balance and
unbalanced forces?
 the basis of Newton’s First Law of Motion
1. Balanced Forces
- objects are at equilibrium
- Net force = 0 and the object will be at
rest or maintaining a constant motion
(i.e. object has NO ACCELERATION)
 The importance of Balance & Unbalanced
forces (cont.):
1. Unbalanced Forces
- Net force will be > 0 and will cause an
object to change its state of motion
- Cause positive/negative acceleration
in which objects change their velocity
- examples…
Net force = +40 N, object will accelerate right
A book is sliding across table,
There is no applied force to cancel
the force of friction (-5N), thus the
book will begin to decelerate and
slow down.