8.1 Forces1. A force is a push or pull that acts on an object.

a) Forces can change the motion of an object.b) Forces can change the shape of an object.

See page 276

Two Categories of Forces

1) Forces that touch an object are called contact forces.a) Tension – force in a wire or ropeb) Friction – slows down motionc) Elastic – spring-like object restores itself to its normal shape.

See pages 277 - 278

2) Applying a force without contact are called action-at-a-distance forces.

a) Gravitational – attraction between masses.

b) Electrostatic – force between charged objects

c) Magnetic – acts on certain metals

Measuring Mass

1) Mass is measured in kilograms (kg). 1000 grams (g) = 1 kilogram (kg).

2) Mass is best measured using a balance scale.

3) Mass is the same regardless of gravity. Mass does not depend upon the amount of gravity.

See page 279

Weight Depends on GravitySee page 280

1) Weight is the amount of force on an object due to gravity. The amount of force depends on the amount of

gravity.

2) The measuring unit of force is the newton (N).

Measuring Force See pages 282 - 283

1) Force meters usually include a spring or similar elastic device that stretches or compresses.

2) Common force meters are called newton gauges or spring scales.

3) 1 kg of mass would have a weight (force of gravity) of 9.8 N on Earth. Multiply mass by 9.8 m/s2 to get the weight of the object on Earth.

Forces and Motion1) Balanced forces are equal in strength but

opposite in direction.a) Object will remain stationary or keep moving at a

constant speed and direction.b) Net force is zero.

2) Unbalanced forces cause a change in the speed or direction of an object. Net force is not zero.

See pages 284 - 285