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chapter 4

# chapter 4 - Chapter 4 Making Sense of the Universe...

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Chapter 4 Making Sense of the Universe Understanding Motion, Energy, and Gravity

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4.1 Describing Motion: Examples from Everyday Life Our goals for learning: How do we describe motion? How is mass different from weight?
How do we describe motion? Precise definitions to describe motion: Speed : Rate at which object moves speed = distance time units of m s æ è ç ö ø ÷ example: speed of 10 m/s Velocity : Speed and direction example: 10 m/s, due east Acceleration : Any change in velocity; units of speed/time (m/s 2 )

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Acceleration of Gravity All falling objects accelerate at the same rate (not counting friction of air resistance). On Earth, g ≈ 10 m/s 2 : speed increases 10 m/s with each second of falling.
Acceleration of Gravity ( g ) Galileo showed that g is the same for all falling objects, regardless of their mass. Apollo 15 demonstration

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Momentum and Force Momentum = mass × velocity. A net force changes momentum, which generally means an acceleration (change in velocity). The rotational momentum of a spinning or orbiting object is known as angular momentum.
Thought Question A car coming to a stop. A bus speeding up. An elevator moving up at constant speed. A bicycle going around a curve. A moon orbiting Jupiter. Is there a net force for each of the following? (Answer yes or no.)

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Thought Question A car coming to a stop. Yes A bus speeding up. Yes An elevator moving up at constant speed. No A bicycle going around a curve. Yes A moon orbiting Jupiter. Yes Is there a net force for each of the following? (Answer yes or no.)
How is mass different from weight? Mass —the amount of matter in an object Weight —the force that acts upon an object You are weightless in free-fall! Apparent Weight

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Thought Question On the Moon, your weight is the same, your mass is less. your weight is less, your mass is the same. your weight is more, your mass is the same. your weight is more, your mass is less.
Thought Question On the Moon, your weight is the same, your mass is less. your weight is less, your mass is the same. your weight is more, your mass is the same. your weight is more, your mass is less.

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There is gravity in space. Weightlessness is due to a constant state of free-fall. Why are astronauts weightless in space? Orbital Trajectories of Cannonballs
What have we learned? How do we describe motion? Speed = distance / time Speed and direction => velocity Change in velocity => acceleration Momentum = mass × velocity Force causes change in momentum, producing acceleration

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What have we learned?
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