The Car & Ramp
CPO Science
Key Questions
 How do we measure and describe
the world around us?
 What is speed and how do we
measure it?
 Can you predict the speed of the
car at any given point on the
ramp?
Overview
 Timer Functions
 Using the Timer
 Measuring Speed
 Graphing Speed
 Predicting Speed from our Graph
CPO Timing System
CPO Timing System
 How can we measure time accurately?


Using the timer in stopwatch mode; Who can get
the fastest time?
The 100 meter race
 One runner has a time of 10.01 seconds
 Another runner has a time of 10.00 seconds
 Who wins?
Photogates
 How does the Photogate start and stop
the timer? Do the speed challenge.
 What happens when you block the light
beam several times in succession; does
the timer reset, or does it add the times?
 Plug the second photogate into the B
port.
 How does the timer work like 3 internal
stopwatches?
Review
 How do you start the timer?
 How do you stop the timer?
 If you block the light beam several times in a row,
does the timer start from zero each time, or are the
times added?
 What does the timer measure when the A light is on?
 What does the timer measure when the B light is
on?
 What does the timer measure when both lights are
on?
Motion Investigation #1
 Why does the car have a tab on the side?
Light Beam
Car
Photogate Clamp
Car
Ramp
Foot
Physics
Stand
Ramp Height
 Design a quick experiment to see
what effect ramp height has on the
TIME it takes the car to move from
Photogate A to Photogate B.
 Ramp hole #: 3, 5, 7, 9
What Happened?
 What are the variables in this
experiment?
1. Distance between A & B
2. position of A & B
3. Weight
4. starting point
5. friction
6. start technique
7. Ramp angle
Technique
 Practice your drop technique until you
get three identical times in a row! This
is very important for data collection in
the next investigation!
Controlling Variables
 Now Let’s try that experiment again,
and this time we will do our best to
control all variables except ramp height.
 The One-Foot Race
How Fast? Match them up!
(m/s)
 1. Human fast walk
 A. 1.2 x 101
 2. Snail
 B. 4.5 x 101
 3. Hair growth
 C. 7.1 x 102
 4. Continental drift
 D. 2.8 x 101
 5. Concorde SST
 E. 2.0 x 102
 6. Winner of 100 m dash
 7. Tsunami (tidal wave)
 8. Running cheetah
 9. Fastball pitch (Nolan Ryan,
1974)
 F. 3.0 x 10-9
 G. 1.0 x 10-3
 H. 1.3 x 100
 I. 1.0 x 10-9
How Fast? Match them up!
(m/s)
 1. Human fast walk
 H. 1.3 x 100
 2. Snail
 G. 1.0 x 10-3
 3. Hair growth
 F. 3.0 x 10-9
 4. Continental drift
 I. 1.0 x 10-9
 5. Concorde SST
 C. 7.1 x 102
 6. Winner of 100 m dash
 7. Tsunami (tidal wave)
 8. Running cheetah
 9. Fastball pitch (Nolan Ryan,
1974)
 A. 1.2 x 101
 E. 2.0 x 102
 D. 2.8 x 101
 B. 4.5 x 101
Using a model to predict
speed of car
 Turn to investigation 2.1, Foundations
of Physical Science Investigation
Manual
Make a Graph of Speed vs.
Displacement
 Why do we start with this graph?
 Only need 1 photogate
 Can make predictions with graph
 What is the dependent variable, and do we assign it to
the X or Y?
 What is the independent variable?
 Should we connect the data points?
 What does the graph tell us about the speed of the car as
it rolls down the ramp?
 Explain why the graph is a curve
Test the Graphical Model
 Connect the data points on your graph
 Without using the car/ramp setup, predict what
the speed of the car at clamp B would be if the
photogates were 27 cm apart.
 Test your prediction!
 Calculate % error
The amazing Carnak
 Place the Photogate at the 38 cm mark
 Turn the timer face down on the table
 Run the car down the ramp; DON’T TURN THE TIMER
OVER, THAT’S CHEATING
 Use your graph and a little algebra to predict the time on
the display
 Write the time on your white board
 Turn the timer over! How close were you?
 Calculate % error
 THIS IS YOUR GRADE!
Position vs. Time
 Suppose we want to collect data and
graph the relationship between
displacement of the car and time
(distance vs. time graph).



How do we measure the distance?
How do we measure the time?
What change in our setup is required?
Series of Trials
 Place photogate A at the top of the ramp, but
be sure the wing doesn’t break the beam while
the car is at rest. Don’t move A!!!
 Place Photogate B at 6 different places along
the ramp.
 Measure:
 Displacement (distance from A to B)
 Time A, Time B, Time from A to B
Graphing Data
 What is the dependent variable?
 Displacement; the distance the car moves
depends on how much time has elapsed
 What is the independent variable?
 The time it took the car to move from A to B
 Create the d/t graph.
 What does the graph tell us about the motion of the car?
 Why is the graph a curve?
Using a Graph for Predictions
 Time to make another prediction!
 Place the photogates 55 cm apart.
 Turn the timer over and run the car
down the ramp
 What will the timer read? Make your prediction,
check it, and calculate % error
 What is your grade on this investigation?
Acceleration
 What is acceleration?
 How could we find the acceleration of the
car on the ramp?
 Place photogates 20 cm apart at different
places on the ramp, and find acceleration
 How do accelerations compare at different
places on the ramp?
 How could I make the acceleration
greater?
Testing Different Variables
 What other combination of variables have we not
yet graphed and investigated?
 Speed of car vs. elapsed time
 Do we need to run more trials to collect data for
this?
 No, we need to calculate speed at B
from previous data
 Calculate speed at B for each of the trials in
investigation #3
More Graphing
 What is the dependent variable?

Speed at B; it depends on the time elapsed
 What is the independent variable?

Time elapsed from A to B
 Create a graph of Speed vs. Time
What is different about the “look” of this
graph when compared to the other two
graphs we created?
 It’s a line! What equation describes the
relationship between x and y variables for a
straight line?

y=mx+b
Using the Line Equation
 Substitute variable names from our experiment for
each of the letters in the equation y=mx+b.
 What does y represent?

Speed at b, or VB
 What does x represent?

Time elapsed, or tAB
 What does b represent?


This is a challenge! Check out the other data we
collected and see if you can figure it out
Speed at A, or VA
What does m represent?
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Slope of the line
How do you find the slope?
Change in y over change in x
What quantity is defined as the
change in speed over time?
 Acceleration!
Rearanging the Equation
 Write the equation of the line using the physics
variables
 VB = at + VA
 Physical Science teachers will
recognize this as a= (Vf – Vo)/t
 You have just used a graph to show the
relationship between 4 different physical variables!
You derived the equation for finding acceleration!
 Use your graph to find b (VA) & m (a)
Prediction Vs. Experiment
 For each of the following times, use your equation
to find the speed at B and plot these data points
on your experimental graph of speed vs. time
 T= 0.2000, 0.3000, 0.4000, 0.5000
 Find VB for each of these times
 Plot the ordered pairs on your
experimental graph
 How close does your prediction match your
experiment?
Summary
 In many situations, like the car/ramp, the
distance, speed, time, and acceleration are all
important variables.
 We know how to relate speed, distance, and
time s = d/t; but without acceleration.
 We know how to relate speed, time, and
acceleration a = (Vf – Vo)/t; but without
distance.
 How do we relate all four variables for a more
general description of motion?
See handout with
explanation of finding area
under speed/time graph