Forces

1
Forces
Chapter 6.1
2
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.

3
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.

4
Types of Forces

• Gravitational
• Electromagnetic
• Normal
• Frictional
• Tension

5
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
  1kgm/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.

6
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.

7
The System and Environment

• The object of interest is called the System.
• The area around the object is called the
  Environment.

8
Free Body Diagram
FN Force of Desk on Book

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.
9
Newtons 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.
• Where F Force in Newtons
• m mass of the object
• a acceleration of the object

10
Newtons 2nd Law The relationship between force
and acceleration

• What will happen to an objects 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?
11
Newtons 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?

Since the slope of the line in a F vs. a graph
equals the mass, the greater the slope, the
greater the mass
m2
m2 gt m1
m1
12
Newtons 2nd Law and velocity vs. time (constant
force)

• Newtons 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.
13
Newtons 2nd LawForce 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?

Force is Constant
m2 gt m1
More Mass Less Acceleration.
14
Newtons 2nd LawMass 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?

Mass is Constant
F2 gt F1
More Force More Acceleration.
15
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?

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Diagram the problem
Fnet Fperson 1 Fperson 2 Fopposing force
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State the Known and Unknowns

• What is known?
• Mass (m) 1850 kg
• Force of person 1 (N) 275 N
• Force of person2 (N) 395 N
• Opposing Force (N) 560 N
• What is not known?
• Acceleration (a) ?

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Perform Calculations

• Fnet Fperson 1 Fperson 2 - Fopposing
• Where
• Fnet ma
• Substitute for Fnet and solve for a
• ma Fperson 1 Fperson 2 - Fopposing
• a (Fperson 1 Fperson 2 - Fopposing)/m
• a (275 N 395 N 560 N)/1850 kg
• a 0.059 m/s2
• If there was no opposing force, how would the
  rate of acceleration change?
• What do you think is the source of the opposing
  force?

It would increase.
Friction.
19
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.
• Newtons 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.