The Car

1
The Car Ramp

• CPO Science

2
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?

3
Overview

• Timer Functions
• Using the Timer
• Measuring Speed
• Graphing Speed
• Predicting Speed from our Graph

4
CPO Timing System
5
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?

6
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.

7

• How does the timer work like 3 internal
  stopwatches?

8
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?

9
Motion Investigation 1

• Why does the car have a tab on the side?

10
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

11
What Happened?

• What are the variables in this experiment?
• Distance between A B
• position of A B
• Weight
• starting point
• friction
• start technique
• Ramp angle

12
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!

13
Controlling Variables

• Now Lets try that experiment again, and this
  time we will do our best to control all variables
  except ramp height.

14

• The One-Foot Race

15
How Fast? Match them up! (m/s)

• A. 1.2 x 101
• B. 4.5 x 101
• C. 7.1 x 102
• D. 2.8 x 101
• E. 2.0 x 102
• F. 3.0 x 10-9
• G. 1.0 x 10-3
• H. 1.3 x 100
• I. 1.0 x 10-9

• 1. Human fast walk
• 2. Snail
• 3. Hair growth
• 4. Continental drift
• 5. Concorde SST
• 6. Winner of 100 m dash
• 7. Tsunami (tidal wave)
• 8. Running cheetah
• 9. Fastball pitch (Nolan Ryan, 1974)

16
How Fast? Match them up! (m/s)

• H. 1.3 x 100
• G. 1.0 x 10-3
• F. 3.0 x 10-9
• I. 1.0 x 10-9
• C. 7.1 x 102
• A. 1.2 x 101
• E. 2.0 x 102
• D. 2.8 x 101
• B. 4.5 x 101

• 1. Human fast walk
• 2. Snail
• 3. Hair growth
• 4. Continental drift
• 5. Concorde SST
• 6. Winner of 100 m dash
• 7. Tsunami (tidal wave)
• 8. Running cheetah
• 9. Fastball pitch (Nolan Ryan, 1974)

17
Using a model to predict speed of car

• Turn to investigation 2.1, Foundations of
  Physical Science Investigation Manual

18
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

19
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

20
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 DONT TURN THE TIMER
  OVER, THATS 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!

21
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?

22
Series of Trials

• Place photogate A at the top of the ramp, but be
  sure the wing doesnt break the beam while the
  car is at rest. Dont 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

23
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?

24
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?

25
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?

26
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

27
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

28

• What is different about the look of this graph
  when compared to the other two graphs we created?
• Its a line! What equation describes the
  relationship between x and y variables for a
  straight line?
• ymxb

29
Using the Line Equation

• Substitute variable names from our experiment for
  each of the letters in the equation ymxb.
• 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

30

• What does m represent?
• 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!

31
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)

32
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?

33
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?

34

• See handout with explanation of finding area
  under speed/time graph