2.1 Wheels

2.1 Wheels - Observations About Wheels Key Concepts: 1....

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1 Key Concepts: 1. Friction 2. Energy Observations About Wheels • Without wheels, objects slide to a stop • Friction is responsible for the stopping effect • Friction seems to make energy disappear • Wheels eliminate friction, or so it seems • Wheels can also propel vehicles, but how? Question 1 Why does a wagon need wheels? – Why do sleds work well only on snow or ice? Friction is a Force Force on person by box Force on floor by box Force on box by floor It’s the sum of all the forces that determines the acceleration. Force on box by person Frictional Forces • A frictional force – opposes relative sliding motion of two surfaces – points along the surfaces – acts to bring the two surfaces to one velocity • Frictional forces always come in 3 rd law pairs: – Pavement’s frictional force pushes cart backward – Cart’s frictional force pushes pavement forward Question 2 Why do sleds seem to “break free” and then slide easily when you shove them hard enough?
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2 Types of Friction • Static friction: when nothing is sliding – Acts to prevent objects from starting to slide – Forces can vary from zero to an upper limit •S l id i n g f r i c t i o n – Acts to stop objects that are already sliding – Forces have fixed magnitudes Static Friction N W F f Prevents two surfaces from starting to slide against each other Static Friction (f) • A force in the direction opposite to the direction of the your push/pull (F) • Is always exactly equal to the force of your push/pull. • Force of static friction is limited
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2.1 Wheels - Observations About Wheels Key Concepts: 1....

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