Bell Work 2/2-2/3
Please get out a sheet of paper and write your
name, the class period, and the date. Then
answer the following questions:
1. What has been your favorite part of ICP?
2. What are your interests?
3. What is your all-time favorite dessert?
4. What do you already know about motion?
Vocabulary
Force: a push or a pull
Friction: a force that opposes motion
Inertia: the property of objects that states that objects tend to remain as they are
Speed: distance covered per unit of time
Speed and Velocity
Speed
◦ Speed=distance/time
◦ Average speed vs instantaneous speed
Velocity: specifies direction and speed
PROBLEM:
◦ What is the average speed of a cheetah
that sprints 100 m in 4 s? How about if it
sprints 50 m in 2 s?
Motion is Relative
When discussing how an object
moves, we mean its motion
relative to something else
◦ All speeds are relative to the surface
of the Earth
Can the bird drop down and catch the
worm if Earth moves at 30 km/s?
Everybody’s Talking about MOTION!
Aristotle (384-322 BCE)
◦ Said that there were two types of motion: natural motion and unnatural motion
◦ Natural motion occurred without a force
◦ Unnatural motion required force, like a person pushing a cart
Galileo (1500s)
◦ Questioned Aristotle
◦ Said that force is required to start an object moving, but no force is required to KEEP it moving—except
for the force needed to overcome friction
Sir Isaac Newton (1643-1727)
English Physicist, Philosopher,
Mathematician, Astronomer
Played HUGE role in the
scientific revolution of the
seventeenth century
Published Mathematical
Principles of Natural
Philosophy
https://www.youtube.com/watch?v=dan
YFxGnFxQ
Newton’s First Law of Motion
Often called “The Law of Inertia”
The big idea: Every object
continues in a state of rest or of
uniform speed in a straight line
unless acted on by a nonzero net
force
Things tend to keep on doing
what they’re already doing
Understanding the Law of Inertia
How does an applied force, a push,
affect the motion of an object that is
not moving?
How does a force directed opposite to
a moving object affect the motion of
that object?
Is it possible to make an object move
in a curved path by giving it a simple
push?
Sports
Inertia
An object moves by its own inertia
If you stick something into a place where
there is no friction, such as in the vacuum
of outer space, it will move forever in a
straight-line path
SO, when the space shuttle moves in a
nearly circular orbit around the Earth, is a
force needed to maintain its high speed?
If suddenly the force of gravity were cut
off, what type of path would the shuttle
follow?
https://www.youtube.com/watch?v=08B
FCZJDn9w
Bell Work 2/4 through 2/5
1. Why do you lurch forward in a bus that suddenly slows? Why do you lurch backward
when it picks up speed? What law applies here?
2. What is your average speed if you run 50 m in 10 s?
3. As Earth rotates about its axis, it takes 3 hr for the United States to pass beneath a
point above Earth that is stationary relative to the Sun. What is wrong with the
following scheme? To travel from Washington, DC, to San Fransisco and use very little
fuel, simply ascend in a helicopter high over Washington, DC, and wait 3 hr until San
Fransisco passes by.
4. A walker’s speed is 4 m/s. How far did the walker go in 5 s?
a)
10 m
b)
4m
c)
20 m
d)
5m
Net Force: The Sum of All Forces on an
Object
Net force: the sum of all forces on an object
The net force is what changes an objects state
of motion
Newton (N): scientific unit of force
Vector: an arrow showing the direction of the
quantity
◦ Arrow’s length represents magnitude and its
direction shows the direction of the quantity
Mechanical Equilibrium
Mechanical equilibrium:
when the net force on an
object is zero
◦Its state of motion does
not change
Why Aren’t We Falling through the
Floor? The support force.
Equilibrium
A bowling ball at rest is in equilibrium. But it is
also in equilibrium if it moves steadily, without
changing its state of motion. ONLY WHEN
TWO OR MORE FORCES ACT ON AN OBJECT
CAN IT BE IN EQUILIBRIUM
Exit Ticket
1. Correct your friend who says, “The race-car driver rounded the curve at a
constant velocity of 100 km/hr.”
2. Who was the first to discover the concept of inertia, Galileo or Newton?
3. When only a pair of equal and opposite forces act on an object, what is the
net force acting on it?
4. Give an example of something moving when a net force of zero acts on it.
5. If a huge bear were chasing you, its enormous mass would be very
threatening. But if you ran in a zigzag pattern, the bear’s mass would be to
your advantage. Why?
Bell Work 2/6/15
1. Consider a pair of forces, one having a magnitude of 20 N and the
other a magnitude of 12 N. What maximum net force is possible
for these two forces? What is the minimum net force possible?
2. You whirl a yo-yo in a flat circle above your head. If you let go of
the string, Newton’s first law tells you that the yo-yo will
a. Continue on a circular path
b. Drop toward the ground
c. Move in a straight line at a constant speed until a force acts to change its
motion