A force is a push or pull that acts on an object. 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 (c) McGraw Hill Ryerson 2007

Two categories of forces 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. Gravitation – 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 (c) McGraw Hill Ryerson 2007

Mass is measured in kilograms (kg). 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 (c) McGraw Hill Ryerson 2007

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 (c) McGraw Hill Ryerson 2007

Common force meters are called newton gauges or spring scales. 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. Multiply mass by 9.8 m/s2 to get the weight of the object on Earth. Spring scale See pages 282 - 283 (c) McGraw Hill Ryerson 2007

Balanced forces are equal in strength but opposite in direction. 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 (c) McGraw Hill Ryerson 2007