Skating. Question. Yes No

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1 Skating A rotary lawn mower spins its sharp blade rapidly over the lawn and cuts the tops off the grasses. Would the blade still cut the grasses if they weren t attached to the ground? Yes No

2 Mike at the skating rink When I step onto the ice rink, I remain stationary unless someone pushes me. If someone pushes me, I start moving in the direction of the push. I don t stop until I hit the wall. Why do they put all that ice on the floor? Can you describe my motion? Inertia and Coasting A body at rest tends to remain at rest. A body in motion tends to remain in motion. Newton s First Law: An object that is free of external influences moves in a straight line and covers equal distances in equal times.

3 Why the ice? Real-world complications mask simple physics Solution: minimize or overwhelm complications To demonstrate inertia: work on level ground (goodbye gravity) use wheels, ice, or air support (goodbye friction) work fast (goodbye friction and air resistance) Describing motion Position an object s location Velocity its change in position with time Force a push or a pull Newton s First Law: An object that is not subject to any outside forces moves at a constant velocity.

4 A rotary lawn mower spins its sharp blade rapidly over the lawn and cuts the tops off the grasses. Would the blade still cut the grasses if they weren t attached to the ground? Yes No What happens when someone pushes? A push changes the skater s velocity. The harder the push the bigger the change. The bigger the skater the smaller the change.

5 Physical Quantities Position an object s location Velocity its change in position with time Force a push or a pull Acceleration change in velocity with time Mass a measure of an object s inertia Newton s Second Law An object s acceleration is equal to the force exert on it divided by its mass. That acceleration is in the same direction as the force. acceleration = force/mass force = mass! acceleration

6 Summary Physical Quantities Position an object s location Velocity its change in position with time Force a push or a pull Acceleration change in velocity with time Mass a measure of an object s inertia Newton s First Law: An object that is not subject to any outside forces moves at a constant velocity. Newton s Second Law:An object s acceleration is equal to the force exert on it divided by its mass. That acceleration is in the same direction as the force. acceleration = force/mass Vectors Magnitude how much there is. Direction which way it is pointing. Position I live three miles east of the campus. Magnitude = 3 miles (units are important). Direction = east Reference point = campus

7 I m riding on a train that is coasting along at constant velocity when I drop my pen. Where does it land? In front of me Behind me Below me Reference frames Inertial frame of reference one that is not accelerating. Newton s Laws of motion work in any inertial frame of reference. A dropped ball on a coasting train falls down.

8 A ball is suspended from a thread. A second thread is attached to the bottom of the ball. If I pull down on the bottom thread slowly, which thread is more likely to break? The bottom thread The top thread Both are equally likely A ball is suspended from a thread. A second thread is attached to the bottom of the ball. If I pull down on the bottom thread quickly, which thread is more likely to break? The bottom thread The top thread Both are equally likely