Lab 1 Force and motion I

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					Lab 1: Force and motion I
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Physics 193 Fall 2007

                               Lab 1: Force and motion I

Welcome to your first Physics 193 lab. Each week you and your lab partners will work
together designing experiments to accomplish specific goals. You will also have a virtual
partner, whose name is Saalih, who will not be present in the lab but will want to learn about
it by reading your lab reports. Saalih has knowledge of mathematics but not of physics, so
you’ll have to be very complete in your writing.


  1) Observe, record, and represent different types of motion.
  2) Develop a qualitative rule relating an object's change of motion to the unbalanced
     force exerted on it by other objects.
  3) Represent your ideas in multiple ways to help understand what you are trying to

I. Observation experiment: Recording and representing motion

The goal of the experiment is to learn how to record the motion of an object and represent its
motion using a motion diagram.
Available equipment: You have a toy car, a tennis ball, sugar packets, a meter stick and a
Motion of a toy car

Place a toy car at rest on the floor. Practice having one group member call out a signal every
second (for example “Now!”). Get the toy car moving in a straight line on the floor. Walk
beside it and when you hear the signal, place sugar packets to mark the locations of the car
each second. Write the following in your report:
    a) Decide where you will choose the origin. Measure the positions of the sugar packets
        relative to that origin. Record the data.
    b) Do you think that if you repeat the experiment you would get the same results? Is it
        worth repeating the experiment again?
    c) How would you classify the object’s motion: motion with constant rate, increasing
        rate/decreasing rate? How do you know?
    d) Now imagine that you want to describe the experiment to Saalih, so Saalih can
        understand the procedure and the results. Is it better to use words or there is another,
        more concise way? Think about what parts of the experiment and the results are
        important and what are not. Once you have thought about this, describe the outcome to
        Saalih in the most efficient way.
Motion of a tennis ball

Next, roll a tennis ball along the floor so that it moves at a reasonable speed. Walk beside it
and place sugar packets to mark the locations of the ball every second. Make sure to roll the
tennis ball carefully so that it rolls in a straight line. If the ball starts to curl near the end of its
motion, ignore that part of the motion. Repeat steps a)-d) above for the tennis ball.
Lab 1: Force and motion I
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Drawing a motion diagram

Read the box ‘Reasoning Skills: Constructing a Motion Diagram’ in the Active Learning
Guide (ALG) on page 1-8. Then draw a motion diagram for the car and another for the tennis
ball. Remember to include dots, v arrows and Δv arrows.
How is the motion diagram different from your written descriptions of the results of the two
experiments? Which is more informative? Which is more efficient? Why?

II. Observation experiment: Forces exerted on an object by other objects

The goal of this experiment is to learn to represent forces exerted on an object by other
objects in a clear and efficient way.
Available equipment: Bowling ball and tennis ball.
Pick up a tennis ball and hold it stationary in your hand. Then pick up a bowling ball and hold
it the same way. Do you feel any difference? Now we will learn to represent this difference
graphically. For each situation (tennis ball and bowling ball) do the following (A more
detailed explanation of free-body diagrams can be found in the ALG on pages 1-13 to 1-14):
     a) List all the objects that interact with the ball (to interact, the objects need to touch the
         ball—however the Earth is an exception – it can interact from a distance). Now decide
         how you can represent these interactions on a diagram so that Saalih can understand
         and also see your reasoning concerning the interactions. After doing this, read on.
     b) Represent the ball with a dot (a point particle) and use an arrow to represent each
         interaction of another object with the ball. Connect the tails of the arrows to the dot.
         Label each force arrow with an F that has two subscripts. The first subscript is for the
         object that exerts a force on the object of interest, and the second subscript is for the
         object of interest. For example, the force that the hand exerts on the ball can be written
         as FH on B . Pay attention to the lengths of the arrows on each free-body diagram.
     c) Indicate what forces “cancel” or “balance” each other. Indicate if there is an
        “unbalanced” force. Is the diagram that you constructed easier to understand than the
        text above?
     d) Why are forces represented by arrows and not just by numbers or lines?

III. Testing experiment: Does an object’s motion always occur in the direction of the

unbalanced force exerted on it by other objects?
The goal of the experiment is to test two ideas: 1) whether an object always moves in the
direction of the unbalanced force exerted on it by other objects; 2) whether the object always
changes its motion in the direction of the unbalanced force exerted on it by other objects.
Available equipment: Bowling ball, mallet, sugar packets.
    a) First, think about the two competing ideas that you need to test. Think about how you
        can use the available equipment to design experiments relevant to both of them. Also,
        think what the words “test an idea’ mean in real life. Does “testing” mean trying to
        prove or trying to disprove an idea? When you come up with possible experiments,
        call your TA over and discuss your experiments with him/her.
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     b) After your discussion with the TA, record the experiments you are going to perform.
        Draw pictures and free body diagrams for each situation.
     c) Make predictions of the outcome of each experiment based on both ideas.
     d) Make a table to record the following information for the ball: (1) the direction of the
        motion; (2) the direction of the change in motion; (3) the direction of the unbalanced
        force. Then perform each experiment, record the outcome in the table and decide
        whether one or both of the ideas gave you the predictions that matched the outcomes
        of the experiments. Make a judgment about each idea.
     e) How would you convince Saalih that your judgments of the ideas are reasonable?
     f) Experiment 1 was called an observational experiment and experiment 3 was called a
        testing experiment. How do these names reflect the differences in performing these
        two kinds of experiments? (What aspects does one kind have that the other type does
     g) Design, perform and describe two more experiments in which the object does not
        move in the direction of the unbalanced force. Draw a motion diagram and a force
        diagram for each experiment.
     h) You have represented motion and forces in different ways. Explain how these
        representations helped you to find a pattern/relationship between motion and forces.

IV. Why did we do this lab?

    a) In a paragraph, summarize what you have learned during this first lab in terms of
       physics content and in terms of the purpose of the two kinds of experiments you
       designed and performed.
    b) Describe how your understanding of the relationship between force and motion is
       different from your understanding before.
Lab 1: Force and motion I
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Physics 193 Fall 2007

V. Homework

Lab 2 is a quantitative lab. To be successful in lab 2, you will need to first learn about
experimental uncertainties. To help you learn about them there is a document on the course
website. Please download the document and read the document carefully. After you finish
reading, solve the following problems. They will help you learn how to estimate uncertainties
and how to use them to evaluate experimental results. Please complete the homework at home
on a separate sheet of paper before coming to lab 2. We will read your answers and provide
feedback. If you are not satisfied with the grade, you can always improve it (see the syllabus).
1. The following instruments are available in your laboratory. What would be the absolute
uncertainties in the measurements made with these instruments?

     a. Ruler                                        b. Protractor

c. Watch                                        d. Scale
2. Measure the length of the pencil. Estimate the relative uncertainty in the measurement.

3. By using the instruments from the task 1, you have determined that it takes 3 s for a toy car
to travel through the distance of 162 cm. What are the relative uncertainties of the distance
and time measurements? Which measurement is the least uncertain?
     4. Calculate the average speed of the toy car. Decide whether you can use the weakest link
     rule to determine the relative uncertainty in speed estimation. Determine the range of possible
     values for the actual average speed of the car.
     5. Why do we need to calculate relative uncertainty? Why isn’t absolute uncertainty enough?