Everyday Forces

1
Everyday Forces
2
Weight

• Weight the magnitude of the force of gravity
  acting on an object.

3
Normal Force

• Normal Force (FN) a force exerted by one object
  on another in a direction perpendicular to the
  surface of contact.

4
Normal Force

• To calculate FN, multiply the weight of the
  object with cosine ?, where ? is the angle
  between the normal force and a vertical line, and
  also the angle between the contact force and a
  horizontal line.

5
Normal Force

• If the surface is horizontal, then ? is 0º, and
  cos 0º 1, and

6
Friction

• Friction opposes the applied force.
• Friction can be categorized as being either
  static or kinetic.

7
Friction

• Static Friction the resistive force that
  opposes the relative motion of two contacting
  surfaces that are at rest with respect to one
  another.
• The force needed to start movement is Fs max.

8
Friction

• Kinetic Friction the resistive force that
  opposes the relative motion of two contacting
  surfaces that are moving past one another.
• The force needed to keep the object moving is Fk.

9
Friction

• Fk lt Fs max
• The force of friction is proportional to the
  normal force.
• Friction depends on the surfaces in contact.

10
Friction

• Coefficient of Friction the ratio of the force
  of friction to the normal force acting between
  two objects.

11
Coefficients of Friction
Fs max 75 N
m 24 kg
Sample Problem 4CA 24 kg crate initially at rest
on a horizontal floor requires a 75 N horizontal
force to set it in motion. Find the coefficient
of static friction between the crate and the
floor.
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Coefficients of Friction
Given m 24 kg Fs max 75
N Unknown µs ?
13
Overcoming Friction
Sample Problem 4DA student moves a box of books
by attaching a rope to the box and pulling with a
force of 90.0 N at an angle of 30.0?, as shown in
the picture. The box of books has a mass of 20.0
kg, and the coefficient of kinetic friction
between the bottom of the box and the sidewalk is
0.50. Find the acceleration of the box.
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Overcoming Friction
Given m 20.0 kg F applied
90.0 N ? 30.0? µk 0.50 Unknown a ?
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Overcoming Friction
Given m 20.0 kg F applied
90.0 N ? 30.0? µk 0.50 Unknown a ?
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Overcoming Friction
F applied y F applied sin? F applied y (90.0
N) sin30.0? F applied y 45.0 N
F applied 90.0 N
? 30.0? F applied x F applied cos? F applied
x (90.0 N) cos30.0? F applied x 77.9 N
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Overcoming Friction
F g mg F g 20.0 kg x 9.81m/s2 F g 196 N
(downward)
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Overcoming Friction
The box does not fly in the air!
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Overcoming Friction
Solving for kinetic friction
20
Overcoming Friction
The box is moving horizontally.
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Section 4 - 4Everyday Forces

• Assignment
• Practice 4C p 145 1-3
• Practice 4D p 147 1-4
• Section Review p 149 1-5