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Force

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... Kg car is traveling along a level highway with a force due to the engine of 4500 ... an engine and two cars is accelerating due to the engine's 22000N force. ... – PowerPoint PPT presentation

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Title: Force


1
Force
  • Unbalanced forces are the reason objects
    accelerate.
  • If the unbalanced portion of all forces acting on
    an object is determined, Newtons second law can
    predict the acceleration of that object.
  • Force is measured in newtons (N)

2
Unbalanced Force!!!
SF ma
SF ma means that you must find the vector sum
of the forces. This vector sum is the unbalanced
portion that accelerates the object.
3
Types of Forces
  • Force due to gravity (Fg) is always pointed down
    and is equal to the (mass)(9.8 m/s2)
  • 9.8 m/s2 9.8 N/Kg
  • So, Fg mass(Kg) 9.8 N/Kg

4
Types of Forces
  • Normal Force (FN) is the force due to the
    presence of a surface.
  • It is always perpendicular to the surface.
  • (FN) is what your scale tells you!! Its the
    surface pushing back.

5
Types of Forces
  • Friction Force (Ff) resists motion.
  • Always opposite the direction of motion
  • mf is coefficient of friction
  • mf (Ff) / (FN)

6
Types of Forces
  • Other forces F?
  • Other forces are a result of a push or pull
    from an outside source.

7
Free Body Diagrams
Free body diagrams are simply diagrams that help
to identify any unbalanced force that will
accelerate an object.
All forces acting on an object act as though
they are acting on the center of gravity of the
object.
8
Free Body Diagrams
  • Represent the object as a point in space. This
    dot is the center of gravity of the object

Fg is always down
Fn always perpendicular to the surface.
9
Creating Free Body Diagrams
Imagine that you are pulling straight forward on
a 10 Kg wagon across a horizontal field with a
force of 61 N. The wagon has a coefficient of
friction of 0.23. Find the net force on the
wagon.
10
Creating Free Body Diagrams
  • Represent the object as a point in space. This
    dot is the center of gravity of the object

11
Step Two
  • Draw and label all of the forces acting on the
    object.
  • The forces should be shown as Pulls on the
    center of gravity

Typical Order Fg, Fn, Ff, other forces.
12
GSFO!!!!
  • Ask yourself four questions IN THIS ORDER!!!!
  • Gravity? If yes, Fg straight down.
  • Surface? If yes, Fn perpendicular to the
    surface.
  • Friction? If yes, Ff opposite motion.
  • Other Forces? F(other)

13
Step Three
  • Choose a coordinate system.
  • Find the magnitude of each force

Fp 61 N
Ff mfFn
14
Step Four
Find the magnitude and direction of the net force
SF ma SFy may 0 SFy Fg Fn 0 SFx
max SFx Fp Ff ma SFx 61N -22.5N 38.5N
(10 Kg)a
Fp 61 N
Ff -22.5 N
15
Examples
  • A ball at rest on a table
  • A ball free falling
  • A cart being pulled across a frictionless
    horizontal surface

16
Examples
  • A 15 Kg sled that is pulled with a horizontal
    force of 17 N across low friction snow (mf
    0.04) with a constant velocity. Find the net
    force.

17
Examples
  • A 1200 Kg car is traveling along a level highway
    with a force due to the engine of 4500 N
    forward yet there is a friction coefficient of
    0.12. Find the acceleration of the car.

18
Examples
A train with an engine and two cars is
accelerating due to the engines 22000N
force. Find the net force on each car and the
trains acceleration. Assume no friction.
  1.2 x 105 Kg 1.4 x 104 Kg 2.2 x 104 Kg
22000 N
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