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Motion and Force

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Chapter Three: Motion 3.1 Position and Velocity 3.2 Graphs of Motion 3.3 Acceleration

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Acceleration is the rate at which velocity (speed) changes. If your speed increases by 1 m/s each second, then your acceleration is 1 m/s per second. What is the acceleration for the steeper hill?

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3.3 Speed and acceleration Speed and acceleration are not the same thing. Acceleration is the change in speed divided by the change in time.

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3.3 Speed and acceleration An acceleration in m/s/s is often written as m/s 2 (meters per second squared). It is better to think about acceleration in units of speed change per second (that is, meters per second per second).

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3.3 Acceleration and motion graphs A speed vs. time graph is useful for showing how the speed of a moving object changes over time. If the line on the graph is horizontal, then the car is moving at a constant speed. There is no change in speed so there is no acceleration.

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3.3 Acceleration and motion graphs Like velocity, acceleration can be positive or negative. Positive acceleration adds more speed each second. Speed is increasing at the same rate.

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3.3 Acceleration and motion graphs Negative acceleration subtracts some speed each second, so things get slower. People sometimes use the word deceleration to describe slowing down. Speed is decreasing at the same rate.

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3.3 Acceleration and motion graphs The position vs. time graph shows acceleration more clearly. This graph is a curve when there is acceleration.

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3.3 Free fall An object is in free fall if it is accelerating due to the force of gravity and no other forces are acting on it. Objects in free fall on Earth accelerate downward.

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3.3 Free fall Falling objects increase their speed by 9.8 m/s every second, or 9.8 m/s 2 The letter “g” is used for acceleration due to gravity.

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3.3 Acceleration and direction Acceleration occurs whenever there is a change in speed, direction, or both.

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3.3 Acceleration and direction Individual vectors can be drawn to scale to calculate the change in direction.

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3.3 Acceleration and direction A car driving around a curve at a constant speed is accelerating because its direction is changing.

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3.3 Curved motion A soccer ball is an example of a projectile. A projectile is an object moving under the influence of only gravity. The path of the ball makes a bowl-shaped curve called a parabola.

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3.3 Curved motion Circular motion is another type of curved motion. An object in circular motion has a velocity vector that constantly changes direction.

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Technology Connection Moments in Motion Some things move too quickly for us to see what is really happening. Special photography can capture fast action in still images that reveal a whole new world to our eyes.

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Activity Speed is how fast something moves in relation to a reference point without regard to the direction. In this activity you and a partner will each calculate your average speed in different units. How Fast are You?

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