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Aristotle again

Natural and Violent Motion

Aristotle asserted that natural motion proceeds from the “nature” of an object, dependent on what combination of the four elements: earth, water, air and fire.

The essential thing about violent motion was that it was externally caused and was imparted to objects; they moved not by their “nature” but because of pushes and pulls.

Galileo was the first to suggest that constant-speed, straight-line motion was just as natural as at-rest motion. This property of remaining at rest or continuing to move in a straight line at a constant speed is known as inertia.

Galileo’s inclined planes revisited

Slope upward – speed decreases

Slope downward – speed increases

No slope. Does speed change?

What is the main difference between the everyday usage of the word inertia and its use in Physics?

Newton’s first law of motion

The first law incorporates Galileo’s idea of inertia and introduces a new concept, force.

Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change the state by forces impressed upon it.

Examples

- The room you are sitting in is currently moving at about 400 m/s as a result of Earth spinning about its axis. The walls of the room are attached to Earth but, if you jump up into the air, you are not. Why does the west wall not move across and strike you?
- Assume that you are pushing car across a level parking lot. When you are stop pushing, the car comes to stop. Does this violate Newton’s first law? Why?
- Why does a tassel hanging from the rearview mirror appear to swing forward as you apply the brakes?

If you were traveling toward a distant star and you ran out of fuel, would your spaceship slow down and stop? Explain.

Tail-to-tip method of adding vectors

- On a diagram, draw one of the vectors – call it V1 – to scale.
- Next draw the second vector V2, to scale, placing its tail at the tip of the first vector and being sure its direction is correct.
- The arrow drawn from the tail of the first vector to the tip of the second represents the sum, or resultant, of two vectors.

Examples

- In everyday use, inertia means that something is hard to get moving. Is this the only meaning it has in physics? If not, what other meaning does it have?
- How would you determine that two objects have the same inertia?
- When a number of different forces act on an object, is the net force necessarily in the same direction as one of the individual forces? Why?
- Modern cars are required to have headrests to protect your neck during collisions. For what type of collision are these headrests most effective?

Newton’s Second Law

The net force on an object is equal to its mass times its acceleration and points in the direction of acceleration

Fnet = ma

Units of Force

1 newton (N) = 1 Kg *m/s2

Examples

- Forces of 4 N and 6 N act on the object. What is the minimum value for the sum of these two forces?
- Two ropes are being used to pull a car out of a ditch. Each rope exerts a force of 700 N on the car. Is it possible for the sum of these two forces to have a magnitude of 1000N? Explain your reasoning.
- If the net force on a boat is directed due east, what is the direction of the acceleration of the boat? Would your answer change if the boat had a velocity due north but the net force still acted to the east?

Examples

- Describe the force(s) that allow you to walk across the room.
- You are analyzing a problem in which two forces act on an object. A 200 N force pulls to the right, and a 40 N force pulls to the left. Your classmate asserts that the net force is 200 N because that is the dominant force that is acting. What is wrong with that assertion?

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