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Forces
Force: A push or a pull on an object. A vector quantity.
Two Types of Forces:
Contact Forces: When the object is directly pushed or pulled.
Field Forces: When the object is not ‘touched’.
Free Body Diagrams
When working with forces, it is helpful to use a free body diagram. It’s simply a
drawing that includes the forces acting on a system.
Combining Forces
When two or more forces act on an object, they can be combined / simplified into one
force. This is known as the Net Force.
If the net force on an object is zero, the object is said to be in Equilibrium, or Static.
What is the net force in each of the situations below?
Sir Isaac Newton
First person to relate force to motion. Has
three laws bearing his name.
Newton’s First Law: An object in motion
will remain in motion and an object at rest will
remain at rest unless a force acts upon it.
Also known as the Law of Inertia
Inertia: The tendency of an object to resist
change. More mass = more inertia.
Newton’s Second Law: The acceleration
of an object is directly proportional to the net
force acting on it and indirectly proportional to
it’s mass.
F = ma
Newton’s Third Law: For every action,
there is an equal and opposite reaction.
(1642-1727)
Since F = ma, the units of force are mass times acceleration, or kg  m
This is also known as a Newton (N).
Example Problem:
Three confused sled dogs are trying to pull
a sled across the Alaskan snow. Alutia pulls
east with a force of 35 N, Seward also pulls
east but with a force of 42 N, and Kodiak
pulls west with a force of 53 N.
A) What is the net force on the sled?
B) Ignoring friction, with what acceleration
can the dogs propel the 113 kg sled?
C) With this acceleration, how fast can they
cover 100. m from a dead stop?
s2
Example Problems
Common Forces We’ll Be
Encountering This Year
Weight vs. Mass
Mass is a scalar quantity.
Weight is a vector or Force quantity. It
depends on acceleration.
SI
English
Mass
Kg
lb
Weight
N
lbf
Your mass on earth is the same as your mass
on the moon. Your weight on earth is different
than your weight on the moon. Weight
depends on the acceleration due to gravity.
Example Problem 2
Drag Force: A force exerted by a fluid on an object in the opposite direction of
motion.
http://www.boreme.com/boreme/funny-2006/tomahawk-p1.php
Terminal Velocity: The constant velocity that is reached when the drag force
equals the force of gravity.
A 50.0 kg bucket is being lifted by a rope. The rope will not break if the tension is 525 N
or less. The bucket started at rest, and after being lifted 3.0 m, it is moving at 3.0 m/s. If
the acceleration is constant, is the rope in danger of breaking?