San Diego State Department of Physics

Lab 3: Equations of Motion
San Diego State University
Department of Physics
Physics 182A/195L
Lab partner 1:
Lab partner 2:
Most people are familiar with the idea of objects in motion. We drive cars, play catch and watch
objects fall. But it was not until the sixteenth and seventeenth centuries that our modern
understanding of motion was established with contributions from Galileo Galilei and Isaac
Newton. In this section we will review the physics that describes objects in motion.
All kinematic equations can be derived from the basic equation of kinematics,
which says that when a force is acting on an object with mass , that object experiences an
acceleration . To see how this equation yields other equations of motion, we need to first
understand the relationships between acceleration, velocity , and displacement .
Displacement is the easiest of these quantities to measure, since we can measure it with a
meter stick; it is just the change in position of an object. If an object’s position is changing with
time, then we say it has a nonzero velocity, and that velocity is, on average, given by
Similarly, if an object’s velocity at any given moment is changing with time, then we say it has a
nonzero acceleration, and that acceleration is, on average, given by
These equations are useful for averages, but they fail to answer the question: what is the exact
acceleration, velocity, and position of the object at the time ?
To answer this question, we need to describe acceleration, velocity and position not as
averages, but as functions of time:
1 Department of Physics
These functions are related to each other through the limit in the averages given
above. You may know that such limits are called derivatives; if you have not taken calculus this
is not required information however. Speed is nothing but the derivative, or algebraically the
instantaneous rate of change, of position:
Acceleration is the derivative of speed (the instantaneous rate of change of speed):
Constant acceleration
Knowing how , , and are related, we can consider a specific physical example.
One of the most common examples explored in this course is a system with constant force. A
quantity which does not change with time is called a constant. In other words,
for whatever variable we are claiming is constant. Recall that , so if is a constant,
then acceleration must also be constant:
What happens to the speed and position functions when acceleration is constant? To find out,
let’s first name the constant acceleration for brevity. Using the equation from above, we know
that acceleration is the derivative of speed:
We can invert this equation by integrating from 0 to :
which yields , or
We can repeat the same process for the relationship between speed and position:
Integration yields: , or
In this lab we will check whether or not the equations of motion we derived in the theory section
actually work. To test these equations, we will conduct an experiment where a cart rolls down a
track with constant acceleration. By recording the speed and position of the cart as a function of
time, we are able to visualize the functions and . Then, using curve fitting, we will be
able to test whether or not the functions we observe match those predicted by the theory.
Set up
1. Connect the Smart Cart to PASCOâ„¢ via Bluetooth in the Hardware Setup Window
(located to the left).
2. Set up the track as shown in Figure 1 with an end-stop located at both ends of the track.
3. Attach the Smart Cart’s magnetic bumper (if you do not have a magnetic bumper, press
the trigger on top of the cart to extend the plunger).
4. Use two Stackable Masses to incline the track. Place the Smart Cart on the track and
make sure that the magnetic bumper or plunger faces downhill.
5. Position the back of the Smart Cart at the top of the track. This is the starting position for
the Smart Cart.
Figure 1: The Smart Cart has the plunger extended on an elevated track by two masses.
Part 1: Position vs. Time
1. Click on Record and then release the cart. Note that data collection will not start until the
cart travels at least 10 cm due to the initial starting condition. This will give you better
looking data.
2. You can stop recording at any time, but there is an automatic stopping condition that
stops the recording after the cart travels 80 cm. You can delete unwanted runs using the
Delete Run feature in the Experiment Control Bar.
3. Use the highlighter tool to select only data recorded while the cart is in motion up until it
reaches the end of the track.
4. Select the User Defined Curve Fit from the Graph Tool Palette.
5. The curve fit will currently look like . You can edit this (in the Curve Fit
Editor on the left) and change it to . You can either type ‘o’ for
and or you can right click and add a subscript. Capstone can read both.
6. Click on Apply.
3 Department of Physics
7. Record the initial position , the initial velocity , and the acceleration in Table 1.1.
8. Include your Position versus Time in Figure 1.1 in the Data section. Right click on the
edge of the PASCO™ graph object and select “Copy Display”. Paste into this document
with “Ctrl+v”.
Part 2: Velocity vs. Time
From the same run, we can also look at the velocity as a function of time.
1. Select a User Defined Curve Fit from the Graph Tool Palette.
2. The Curve Fit will currently look like . You can edit this (in the Curve Fit
Editor on the left) and change it to
3. Click on Apply.
4. Record the initial velocity , and acceleration in Table 2.1.
5. Include your Velocity versus Time in Figure 2.1 in the Data section. Right click on the
edge of the PASCO™ graph object and select “Copy Display”. Paste into this document
with “Ctrl+v”.
Figure 1.1: Position versus time graph
Table 1.1: Variables of motion from Part 1.
Initial Position (m) Initial Velocity (m/s) Acceleration (m/s2
5 Department of Physics
Figure 2.1: Speed versus time graph
Table 2.1: Variables of motion from Part 2.
Initial Velocity (m/s) Acceleration (m/s2
1. Using , write down the equation y(t) using the coefficients
recorded in Table 1.1.
2. Using , write down the equation v(t) using the coefficients recorded in
Table 2.1.
3. Write down :
4. Compare the answers found from questions 2 and 3. Write down a general statement
that relates the velocity and position curves.
1. Explain the difference between speed and velocity.
2. What happens to the speed and position functions when acceleration is constant?
3. If you were to model through a Position vs Time graph the act of walking forward, what
would happen to the slope if you were to walk faster?
4. We observed a constant acceleration of our cart down the cart track. This implies that
there was a constant force being applied to the cart. (Think about .) What was
generating this constant force?
5. Is it possible, using this experimental setup, to get a negative position value on the
Position vs Time graph? Why or why not?
7 Department of Physics

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