Lab Design Force and MotionLEARNING GOALS OF THIS LAB:

Make a prediction based on a hypothesis, and perform an experiment to test this hypothesis. Understand the difference between a hypothesis and a prediction.

Represent a complex situation graphically by constructing a free-body diagram to show the forces exerted on an object.

Collect data from an experiment and find a pattern that will allow you to formulate a relationship.

Hypothesis An object always moves in the direction of the net force exerted on it by other objects.ExperimentEquipment: Dynamics cart, dynamics track, spring scale, masking tape, pulleys, objects to hang, a ramp, a book.Write the following:

State what idea you are going to test. Brainstorm the task and make a list of possible experiments whose

outcomes can be predicted with the help of the idea. Decide whether testing an idea requires that you design experiments to prove the idea or to disprove the idea. How can you convince in your decision?

Briefly describe your chosen design. Include a labeled sketch. Draw a free body diagram of the object while the forces are being

exerted on it.a) Use the idea under test to make a prediction about the

outcome of the experiment. b) Perform the experiment. Record the outcome.c) Did the outcome support the prediction? d) Based on your prediction and the outcome of your experiment, can

you say that the idea is proved, disproved? e) Describe additional assumptions that you used to make a prediction

about the outcome of your testing experiment. How can the assumptions affect your judgment?

II. Observation experiment: Interactions between different objects The goal of this experiment is to find a relationship between the force that an object A exerts on an object B and the force that the object B exerts on the object A when they are interacting with each other. Available Equipment: Force probe sensors with hooks on ends, computers.

Note on equipment: You will be using a new Vernier sensor called a force probe for this experiment. A force probe is a sensor that sends a signal to a computer indicating the force exerted on its tip. The software interface, Logger Pro, helps you plot force as a function of time for two different force probes.Setting up the experiment: Take one of the probes, connect it to the computer and gently pull or push on it. Examine the graph on the screen and make sure it makes sense to you. The force probe is an object. You exerted a force on this object, and this force was recorded as a function of time. Now that you are familiar with force probes, design and perform enough experiments to find a pattern in the readings of the two probes when they record forces that two interacting objects exert on each other. For example: place one probe stationary on the table and tap it with the second probe. Remember that the probes are very delicate and if you use them to tap on each other, you have to do it lightly. Write in your report:

a) For each experiment describe the set-up in words and sketch the graphs that you see on the computer.

b) Find a pattern in the pairs of graphs representing the force-versus-time functions recorded by each probe during an interaction.

c) Formulate a tentative rule relating the force that an object A exerts on an object B to the force that the object B exerts on the object A.

III. Testing experiment: Interaction between different objects The goal of this experiment is to test the rule relating the force that an object A exerts on an object B to the force that the object B exerts on the object A. Remember that the purpose of testing experiment is to reject, not to support the rule under test.Available Equipment: Force probe sensors with hooks on ends, a track, carts, objects of different masses to put on carts, cushions, elastic bands, computers.

a) State what rule you are going to test.b) Brainstorm the task and make a list of possible experiments whose

outcome can be predicted with the help of the rule. Choose 2 experiments.

c) Briefly describe your chosen design. Include a labeled sketch.d) Use the tentative rule to make a prediction about the outcome

of each experiment. e) Perform each experiment. Record the outcome.f) Did the outcome support the prediction? g) Based on the prediction and the outcome of the experiments, what

is your judgment about the rule?

h) Talk to your classmates in other lab groups and find out what results they have. Are they consistent with yours?

i) Reflection question: why was experiment 2 called an observational experiment and experiment 3 a testing experiment? What was the difference?

j) Think of additional assumptions that you used to make a prediction about the outcome of testing experiments. How could the assumptions affect your judgment?

k) Use the rule that you devised and tested to decide who exerts a larger force: a mosquito on a car’s windshield or a car’s windshield when a mosquito smashes into it; a reflex hammer (Taylor hammer) on your knee or your knee on the hammer when a doctor taps your knee with it. Reconcile your answers with your observations of these phenomena.

Before leaving the lab, perform the following experiment:Place an object on an electronic scale and notice the reading, then tilt the scale about 100-200 forward and notice the reading when the same object is on it. On your way home think of an explanation for your observation. To decide whether two experiments give you the same result, you need to pay careful attention to theoretical assumptions and experimental uncertainties. Answering the questions below will help you be more successful.

1) You were measuring how heavy an object is using an electronic platform scale with the smallest increment of 0.1g and got the value of 250 g. Estimate the relative uncertainty of your measurement.

2) After you recorded the reading of the scale, you noticed that the table on which the scale was sitting was tilted a little bit. You measure the angle of the tilt and find it to be about 100. Can you assume that the table is not tilted? To answer this question, compare the change in the reading of the scale due to the tilt with the uncertainty in the scale measurement. Draw a free body diagram and decide what the scale measures.

3) You want to measure the angle of the tilt carefully. You have a protractor and a ruler. Describe two different methods that you can use to measure the angle with the available equipment.

4) Suppose you get an angle of 10º. Estimate the uncertainty of each method. Which of them gives less uncertainty? How could you minimize the experimental uncertainty?