8.1 Forces

1
8.1 Forces

• A force is a push or pull that acts on an object.
• Forces can change the motion of an object.
• Forces can change the shape of an object.

The force applied by the bat can change the
motion of the ball.
Applying a force to an object can cause it to
change shape.
See page 276
2
Types of Forces

• Two categories of forces
• Forces that touch an object are called contact
  forces.
• Tension force in a wire or rope
• Friction slows down motion
• Elastic spring-like object restores itself to
  its normal shape.
• Applying a force without contact are called
  action-at-a-distance forces.
• Gravitational attraction between masses.
• Electrostatic force between charged objects
• Magnetic acts on certain metals

Tension force during a game of tug-a-war
See pages 277 - 278
3
Measuring Mass

• Mass is measured in kilograms (kg).
• 1000 grams (g) 1 kilogram (kg).
• Mass is best measured using a balance scale.
• Mass is the same regardless of gravity.
• Mass does not depend upon the amount of gravity.

See page 279
4
Weight Depends on Gravity

• Weight is the amount of force on an object due to
  gravity.
• The amount of force depends on the amount of
  gravity.
• The measuring unit of force is the newton (N).

A D-cell battery weighs about 1 N.
A typical carton of milk weighs about 10 N.
See page 280
5
Measuring Force

• Force meters usually include a spring or similar
  elastic device that stretches or compresses.
• Common force meters are called newton gauges or
  spring scales.
• 1 kg of mass would have a weight (force of
  gravity) of 9.8 N on Earth (but to make things
  easier, we round up to 10 m/s2)
• Multiply mass by 10 m/s2 to get the weight of the
  object on Earth.

Spring scale
See pages 282 - 283
6
Forces and Motion

• Balanced forces are equal in strength but
  opposite in direction.
• Object will remain stationary or keep moving at a
  constant speed and direction.
• Net force is zero.
• Unbalanced forces cause a change in the speed or
  direction of an object.
• Net force is not zero.

See pages 284 - 285
Take the Section 8.1 Quiz