Forces

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Chapter 3

• Forces

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Lesson 3.1Newtons Second Law

• The net force acting on an object causes the
  object to accelerate in the direction of the
  force.
• Fma

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F
m
a
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Example

• Throw a softball (.2kg) and a baseball (.14 kg)
  both with a force of 40N. Do they accelerate at
  the same rate?

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Baseball aF/m a40N/.14kg a286m/s2
Softball aF/m a40N/.2kg a200m/s2
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Friction

• The force that opposes motion between two
  surfaces.
• Depends on
• Kind of surface
• Forces pushing them together

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3 Types of Friction

• Static friction- When two surfaces are not moving
  past each other.
• Such as when you push a box and it wont move.

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• 2. Sliding friction- When two surfaces are moving
  past each other.
• Such as a box being pushed across the floor will
  move as long as there is a continuous pushing
  force applied.

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• 3. Rolling Friction- When a rolling object moves
  past a surface.
• Such as a train on tracks.
• A continuous force is not necessary.

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Air Resistance

• Acts in the opposite direction of the motion of
  the object.
• Throw a ball forward, air resistance is backward.

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Lesson 3.2Gravity

• Anything that has mass is attracted by the force
  of gravity.
• You to your pencil, you to the ground, your
  pencil to your book.

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Law of Gravitation

• Any two masses exert attractive forces on each
  otherthis attraction depends on
• Masses of the objects
• Distances between them

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When objects are affected by gravity, the a
becomes g for gravity, causing F to become
w for weight.
Fma
wmg
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Gravity and You

• Generally, the larger the planet, the larger the
  force of gravity, therefore the more you weigh.

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Projectiles

• Anything shot or thrown through the air. Always
  flow in a curved path.

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Centripetal Acceleration

• Acceleration toward the center of a curved path
  because of the centripetal force.

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Lesson 3.3Newtons 3rd Law of Motion

• For every action force, there is an equal in size
  and opposite in direction reaction force.

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Action/reaction Pairs

• Always act on different objects.
• A swimmer in the wateraction force on the water,
  reaction force on the swimmer.

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Momentum
Momentum mass velocity
p
m
v
Momentum (p) is tricky because you would think
that momentums variable would be an m.
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Momentum

• Since velocity has direction, momentum has
  direction.
• Momentum can be transferred from one object to
  another, but not lost nor gained
• (conservation of momentum)

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Momentum Example
QIf you have a 3kg ball traveling 20 m/s
and it hits a 2 kg ball, how fast will the 2
kg ball go (if gravity, friction, and
air resistance are not factored in)?
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m1 v1 m2 v2 3 20 2 v2 v2
30 m/s