Forces
Chapter 6.1
What You Already Learned
• Velocity is the ratio of the change in
position of an object over a period of
time.
• Acceleration describes a change in
velocity of an object.
Acceleration
• What may cause an object to
experience a change in velocity?
– Force
• A force is a push or pull.
– A force may cause an object to speed up.
– A force may cause an object to slow down.
– A force may also cause an object to change
direction.
Types of Forces
• Gravitational
• Electromagnetic
• Normal
• Frictional
• Tension
Force
• Force is a vector quantity, therefore it
has both magnitude and direction.
• The SI unit for force is the Newton.
– The Newton is a derived unit where:
1N = 1kg•m/s2
– 1 Newton is the amount of force required
to accelerate a 1 kg mass 1 m/s2.
– 1 Newton is about the weight of a
medium sized apple.
Contact vs. Long Range
• Some forces act over distances while
others act only when two objects are in
contact with one another.
– Contact forces exist when two objects
are in contact with one another.
– Long-range (FIELD) forces act over
distances without a need for direct contact.
Electromagnetic forces and gravity are
long-range forces.
The System and Environment
• The object of interest is called the
System.
• The area around the object is called the
Environment.
FN
System
Environment
Fg
Free Body Diagram
FN
FN = Force of
Desk on Book
System
=
Fg
Fg = Force of
Gravity on Book
Free-Body diagrams provide a means by which all the external forces
acting on a system can be summarized and accounted for, and the
resultant vectors determined.
Newton’s 2nd Law
• Newton determined that the acceleration of an
object is directly proportional to the net
unbalanced external force applied to move
it and inversely proportional to the mass of the
object.
Fnet
a
m
Where F = Force in Newtons
m = mass of the object
a = acceleration of the object
Newton’s 2nd Law: The relationship
between force and acceleration
• What will happen to an object’s acceleration if
you increase the net force acting on it?
– As the force on an object increases, the rate of
acceleration will increase.
– i.e. Bigger Force = Bigger Acceleration
What does the slope of the line
in an a vs. F graph equate to?
1
Slope 
mass
Force
Newton’s 2nd Law: The relationship
between force and acceleration
• How does the graph change if you plot
force vs. acceleration instead?
– The slope now equals the mass.
• Which object on the graph below has a
greater mass and how do you know?
m2
m1
Acceleration
m2 > m1
Since the slope of
the line in a F vs. a
graph equals the
mass, the greater
the slope, the
greater the mass
Newton’s 2nd Law and
velocity vs. time (constant force)
• Newton’s 2nd Law indicates that a constant
force applied to an object will cause the
speed to change at a constant rate.
• i.e. Acceleration is constant
Note: Remember from
kinematics that the slope
of the line is equal to a.
Time
Newton’s 2nd Law
Force Constant / Different Mass
• If the applied force to a system is held constant and
the mass is increased from m1 to m2, what would a
velocity vs. time graph look like?
m1
Force is
Constant
m2
m2 > m 1
Time
More Mass = Less Acceleration.
Newton’s 2nd Law
Mass Constant / Different Force
• If the mass is held constant and the force applied to
a system is increased from F1 to F2, what would a
velocity vs. time graph look like?
F2
Mass is
Constant
F1
F2 > F1
Time
More Force = More Acceleration.
Example: What is the rate of
acceleration?
• Two people are pushing a stalled car. The mass of
the car is 1850 kg. One person applies a 275 N force
while the other applies a 395 N force. A third force
of 560 N acts in in the opposite direction compared
to the two people. What is the acceleration of the
car?
Diagram the problem
Fnet = Fperson 1 + Fperson 2 – Fopposing force
State the Known and Unknowns
• What is known?
– Mass (m) = 1850 kg
– Force of person #1 (N) = 275 N
– Force of person#2 (N) = 395 N
– Opposing Force (N) = 560 N
• What is not known?
– Acceleration (a) = ?
Perform Calculations
• Fnet = Fperson 1 + Fperson 2 - Fopposing
Where:
o Fnet = ma
• Substitute for Fnet and solve for a:
o ma = Fperson 1 + Fperson 2 - Fopposing
o a = (Fperson 1 + Fperson 2 - Fopposing)/m
o a = (275 N + 395 N – 560 N)/1850 kg
o a = 0.059 m/s2
o If there was no opposing force, how would the rate
of acceleration change? It would increase.
o What do you think is the source of the opposing
force? Friction.
Key Ideas
• Force is a vector quantity that is a measure of the
magnitude of a push or pull in Newtons.
• Forces exist as contact or long range.
• Use free body diagrams to represent forces when
problem solving.
• Newton’s 2nd Law of Motion states that the rate of
acceleration of an object is proportional to the force
applied and inversely proportional to its mass.
– A constant force applied to an object will cause it to accelerate
at a uniform rate.
– As force increases, acceleration increases.
– As mass increases, acceleration decreases.