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š¬ Think about it
You already know how to describe motion. But is there a cause behind every change in position and velocity? Do all motions need a cause? In this chapter we investigate what changes motion — and Newton's three laws that explain it.
What will we investigate in this chapter?
What We Will Investigate
What causes motion to change?
the net force on an object
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Newton's three laws
first — no net force · second — F = ma · third — equal and opposite
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Apply them
friction, walking, rockets, connected boxes
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From
describing
motion to
explaining
it
- What causes changes in the motion of objects.
- The nature of this cause — force.
- Newton's three laws of motion and how to apply them.
Example:
A ball at rest stays still until a force (a kick) sets it moving.
š Think It Over
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Why does a canoe move forward when the canoeist pushes water backwards, and faster when they push harder?
View Answer
The water pushes the paddle forward with an equal force (Newton's third law); a harder push means a larger forward force, so the canoe speeds up. -
With the same paddle force, which moves faster — an empty canoe or one carrying a passenger?
View Answer
The empty canoe, because it has less mass, and for the same force a smaller mass gets a larger acceleration (Newton's second law).
ā Test Yourself
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What does this chapter say is the cause of changes in motion?
View Answer
Force. -
Whose three laws of motion will we study?
View Answer
Newton's.
