Transcript Explaining Motion

Object Thrown Upward
Explaining Motion
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 constantspeed, 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?
Initial
Final position
Where is the final
position?
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.
The stretch of the spring is a measure of the applied
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
1. 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?
2. 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?
3. 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.
Vectors and Scalars
Addition of Vectors – Graphical
Methods
If the vectors are not along the
same line?
Tail-to-tip method of adding vectors
1. On a diagram, draw one of the vectors – call it V1 –
to scale.
2. 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.
3. The arrow drawn from the tail of the first vector to
the tip of the second represents the sum, or
resultant, of two vectors.
Tail-to-tip method can be extended to
three or more vectors:
Examples
1. 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?
2. How would you determine that two objects have the same
inertia?
3. 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?
4. 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
1. Forces of 4 N and 6 N act on the object. What is the
minimum value for the sum of these two forces?
2. 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.
3. 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
1. Describe the force(s) that allow you to walk across the
room.
2. 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?