FORCES
• The term force refers to the interaction of
objects and their environment.
• All forces are exerted on one object by
another object.
• Forces have both size and direction and
are normally classified as “pushes or
pulls”.
• All forces have both size and direction
Types of Forces
• Buoyancy - Is the tendency of an object
to stay afloat.
It is the upward force
that all liquids exert on all matter (a push).
• Electricity is the flow of electrons
producing a current (a push). Negatively
charged particles repel each other.
• Friction is the force that retards the
motion of two touching objects (a push).
Gravity – most familiar force
• Gravity is the basic force of attraction that
is spread throughout the universe. Gravity
pulls objects towards each other.
• Gravity on earth pulls you and all objects
towards the earth.
• You must overcome gravity each time you
lift something.
• Gravitational force on earth is 9.8m/s2
Calculating Force
The relationship between an object's mass m, its
acceleration a, and the applied force F
Force =(mass)(acceleration) or F = ma
• The SI units for force is the Newton (N)
A Newton is equivalent to the units:
N = kg x m
s2
Example
• An object with a mass of 15.0 kg is moving
with an acceleration of 25.0 m/s2. What is
the force acting on that object?
F = ma
= (15.0 kg) x (25.0m/s2) = 375 kg• m/s2
= 375 N
Another Force - Weight
• Weight is a force applied to an object as a
result of gravity.
• Weight = mass x (gravitational force)
Fw = (m) (g)
• On earth, the force of gravity is nearly
constant = 9.8 m/s2
Mass vs. weight
Understand the difference
Mass is:
Weight is:
 A measure of the amount of  A measure of the
matter in an object.
gravitational force on an
object.
 Always constant for an
 Varied depending on where
object no matter where the
the object is in relation to
object is in the universe.
the Earth or any other large
body in the universe.
 Expressed in kilograms,
 Expressed in Newtons (N).
grams, and milligrams.
Weight
• It is different depending on where the
object is located and the amount of gravity
acting on it.
• Weight is expressed in Newtons (N)
• Weight of an object can be determined by
the following formula
Weight = (mass) (gravity)
OR
Fw = (m)(g)
Example
If an object has a mass of 75 kg on earth,
what is it’s weight?
Fw = (m)(g) = (75 kg) x (9.8 m/s2)
= 735 kg • m/s2
= 735 N
= 740 N
How Energy Relates to Work
Energy - the ability to do work
Work - a measure of how
productive an applied force is
Work
• Work is the product of the force applied to
an object time the distance through which
the force acts
• EXAMPLES OF WORK
– Lifting a book
– Pulling a cart
– Pushing a door open
• Sometimes there are easy ways and hard
ways to do the same amount of work.
Work
• The formula for work is:
Work = (force) (distance) or W = Fd
The unit for work is the Joule
J = N * m = kg *m2
s2
It is important that you understand that all units used in
the equation are in Kg, m and seconds. The problem
will not be accurate (or correct) if the units are not in
this form.
Example
• A woman pushes a shopping cart with a
force of 15.0 N a distance of 25.0m. How
much work was done?
• W = Fd
• W = (15N) (25m)
• W = 375 J = 380J
Example
• A book weighing 3.0N is lifted 5.0m. How
much work is done?
• W = Fd
• W = (3.0N) (5.0m)
• W = 15J
Example
• It took 45J to push a chair along the floor a
distance of 7.0m. With how much force
was the chair pushed?
•
•
•
•
•
W = Fd
You need to rearrange the equation to get force.
F=W÷d
F = 45J ÷ 7.0m
F = 6.4N
Example
• A force of 2400N was necessary to lift a
rock. A total of 450J of work was done.
How far was the rock lifted?
•
•
•
•
•
W = Fd
Rearrange for distance
d=w÷F
d = 450J ÷ 2400N
d = 0.19m