Transcript Forces and Motion - Center for Mathematics & Science

Forces and Motion
Steve Case
University of Mississippi
NSF North Mississippi GK-8
September 2005
What words can you think of to
describe the motion of an
object?
NSF North Mississippi GK-8
VELOCITY
The word scientists use to describe how
fast an object is moving
NSF North Mississippi GK-8
Velocity is measured as:
a change in distance per
change in time
change in distance
change in time
What are some things you can think of that are
measured like this?
NSF North Mississippi GK-8
What are some examples of
objects that move at high
velocities?
NSF North Mississippi GK-8
What are some examples of
objects that move at low
velocities?
NSF North Mississippi GK-8
What units would you use
to measure the velocity of
an object moving in this
classroom?
meters per second
m
s
Meters per second tells us how many meters
something can move in one second.
NSF North Mississippi GK-8
What else can we know about an
object’s motion besides velocity?
•What if the object speeds up?
• What if the object slows down?
What if the object’s velocity changes?
NSF North Mississippi GK-8
Scientists have a special way to
measure a change in velocity:
•Acceleration
Acceleration is a change in
velocity.
NSF North Mississippi GK-8
The Way Acceleration Works:
• If you’re speeding up, you have
positive acceleration.
• If you’re slowing down, you have
negative acceleration.
• If you’re changing speed quickly, you
have high acceleration.
• If you’re changing speed slowly, you
have low acceleration.
NSF North Mississippi GK-8
Example:
You’re driving along in
your car and suddenly
have to slam on your
brakes for a red light.
Negative or positive
acceleration? High or
low acceleration?
High negative acceleration because you’re
slowing down quickly.
NSF North Mississippi GK-8
Example:
You’re an astronaut waiting in
your shuttle for takeoff.
Suddenly the rockets fire
and you’re on your way to
Mars. High or low positive or
negative acceleration?
High positive acceleration because you’re
speeding up quickly.
NSF North Mississippi GK-8
Example:
You’re driving down the
highway when your car
runs out of gas and you
slowly roll to a stop on the
side of the road.
Low negative acceleration because you’re
slowing down slowly.
NSF North Mississippi GK-8
Example:
You’re walking to class when
you realize you’re running
just a little bit late. You start
walking a bit fast, but you
still hope to make it on time.
Finally the bell rings so you
break into a jog and then a
run, trying to get to class
before the teacher notices
you’re late.
Low positive acceleration because you’re
speeding up slowly.
NSF North Mississippi GK-8
Challenge Question:
You throw a baseball straight
up in the air. It rises to a
certain height and then falls
back to earth, where you
catch it again in your glove.
First, high positive acceleration because it speeds up quickly as it leaves
your hand.
Then, as it rises, low negative acceleration, because it begins to slow down
slowly because of gravity.
As it begins to fall again, low positive acceleration, because it speeds up
slowly.
Finally, when you catch it, high negative acceleration because it slows
down very quickly in your hand.
NSF North Mississippi GK-8
How Do I Graph the Motion of a
Particle?
Distance (meters)
Distance vs. Time
14
12
10
8
6
4
2
0
Car 1
Car 2
Car 3
0
1
2
3
4
Time (seconds)
NSF North Mississippi GK-8
5
Quick Review
• Velocity: speed of motion
• Acceleration: change in velocity
What causes an object’s
velocity to change?
NSF North Mississippi GK-8
• If you’re riding your bike and want to
speed up, what do you do?
• If you’re driving a car and
want to slow down,
what do you do?
• If you’re playing soccer and
want to move the ball,
what do you do?
NSF North Mississippi GK-8
Pushing pedals, pressing
brakes, and kicking soccer balls
are all examples of forces.
A force is anything that changes the velocity of
an object.
But what’s a change in velocity?
A change in velocity is acceleration, so . . .
A force is anything that gives an object
acceleration.
NSF North Mississippi GK-8
Nothing will change speed
unless acted upon by a force.
If it’s sitting still, it will keep
sitting still. If it’s moving
fast, it will keep moving fast.
Only a force can change the
velocity of an object.
NSF North Mississippi GK-8
Then why do things slow down?
• Flying baseballs slow down and eventually
stop because of the force of gravity.
• Cars and bikes eventually slow down and
stop because of the force of friction.
• Spacecraft in outer space with no friction
and no gravity will keep going at the same
speed forever without a force to slow them
down or speed them up.
NSF North Mississippi GK-8
Final Review
• What is an object’s speed of motion?
• What is a change of an object’s
velocity?
• What is the only thing that can
change an object’s velocity and give it
acceleration?
NSF North Mississippi GK-8