Post on 25-May-2015
description
Forces and Motion
Unit 19
What words can you think of to describe the motion of an
object?
VELOCITY
The word scientists use to describe how fast an object is moving in a
certain direction.
Velocity is measured as:
change in distance
change in time
a change in distance per change in time
What are some things you can think of that are measured like this?
What are some examples of objects that move at high
velocities?
1600 mph
220 mph
Extreme Velocities
17,000 mph 40,000 mph
What are some examples of low velocities?
50 yds/hr (.03 mph)
1.2 mph
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.
Converting Velocities
m/s into km/hr
m
s
Just take the m/s multiply by 1000 and divide by 3600
1000 m/km
3600 seconds/hr
“What kind of mileage does this get?”
Converting Velocities
m/s into km/hr
m
s
Just take the m/s multiply by 1000 and divide by 3600
1000 m/km
3600 seconds/hr
“What kind of mileage does this get?”
1 hwy, 0 city1 highway, 0 city
What else can we know about an object’s motion besides velocity?
• What if the object slows down?
What if the object’s velocity changes?
• What if the object speeds up?
Scientists have a special way to measure a change in
velocity:
Acceleration
Acceleration is a change in velocity.
Visualizing Acceleration
• Here you can see how the ball travels a further and further distance each second.
• That is because its velocity is continuing to increase.
• If you’re speeding up, you have positive acceleration.
• If you’re slowing down, you have negative acceleration.
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?
Answers
• Negative Acceleration• High Rate
You’re an astronaut waiting in your shuttle for takeoff. Suddenly the rockets fire and you’re on your way.
High or low positive or negative acceleration?
Example:
Or you are a drag racer
Answer
High positive acceleration because you’re speeding up quickly.
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.
Example:
Answer Low negative acceleration because you’re slowing down slowly.
Example:• You are at cruising speed on the highway.
You no longer feel any pressure against the seat or are lurching forward.
• What is your acceleration?
Answer Zero, because you are at constant velocity
That velocity can be fast, slow, or not moving at all.
Challenge Question:
You throw a basketball up in the air. It rises to a certain height and then falls back to earth.
Describe its acceleration
Hint: There are multiple steps
Answer
First, high positive acceleration because it speeds up quickly as it leaves your hand.
Answer
Then, as it rises, low negative acceleration, because it begins to slow down slowly because of gravity.
Answer
At the top of the arch, it has zero acceleration, because it briefly stops moving upward and has constant velocity.
Answer As it begins to fall again, low positive acceleration, because it speeds up slowly.
Answer
Finally, when you catch it, high negative acceleration because it slows down very quickly in your hand.
How Do I Graph the Motion of a Particle?
0 1 2 3 4 5 6 70
5
10
15
20
25
30
Car 1 Car 2 Car 3
Car 1 is moving with a velocity of 1 m / sec
There is zero acceleration
0 1 2 3 4 5 6 70
5
10
15
20
25
30
Car 1 Car 2 Car 3
Car 2 has constant velocity throughout at 1 m /sec
Its acceleration remains zero
0 1 2 3 4 5 6 70
5
10
15
20
25
30
Car 1 Car 2 Car 3
Car 3 has a changing velocity throughout, finishing at 7 m / sec
Acceleration is 1 m / sec2
0 1 2 3 4 5 6 70
5
10
15
20
25
30
Car 1 Car 2 Car 3
Quick Review
• Velocity: speed of motion in a direction
• Acceleration: change in velocity
What causes an object’s velocity to change?
• 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?
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.
• That force can act to accelerate it
Only a force can change the velocity of an object.
• That force can act to decelerate it
Only a force can change the velocity of an object.
Then why do things slow down?
• Flying balls 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.
Then why do things slow down?
• In space with no friction and no gravity, objects will keep going at the same speed forever without a force to slow them down or speed them up.