PreLab

PRELAB:

Prediction

Everyone has his/her own "personal theories" about the way the world works. One purpose of this lab is to help you clarify your conceptions of the physical world by testing the predictions of your personal theory against what really happens. For this reason, you will always predict what will happen before collecting and analyzing the data. Your prediction should be completed and written in your lab journal before you come to lab. The “Method Questions” in the next section are designed to help you determine your prediction and should also be completed before you come to lab. This may seem a little backwards. Although the prediction question is given before the method questions, you should complete the method questions before making the prediction. The prediction question is given first so you know your goal.

Spend the first few minutes at the beginning of the lab session comparing your prediction with those of your partners. Discuss the reasons for any differences in opinion. It is not necessary that your predictions are correct, but it is necessary that you understand the basis of your prediction.

Make a sketch of how you think the acceleration-versus-mass graph will look for carts with different mass released from rest from the top of an inclined track.

Do you think the acceleration of the cart increases, decreases, or stays the same as the mass of the cart increases? Make your best guess and explain your reasoning.

Sometimes, as with this problem, your prediction is an "educated guess" based on your knowledge of the physical world. In these problems exact calculation is too complicated and is beyond this course. However, it’s possible to come up with a qualitative prediction by making some plausible simplifications. For other problems, you will be asked to use your knowledge of the concepts and principles of physics to calculate a mathematical relationship between quantities in the experimental problem.

Method Questions

Method Questions are a series of questions intended to help you solve the experimental problem. They either help you make the prediction or help you plan how to analyze data. Method Questions should be answered and written in your lab journal before you come to lab.

The following questions should help with your prediction and the analysis of your data.

1. Make a sketch of the acceleration-versus-time graph for a cart released from rest on an inclined track. On the same graph sketch how you think the acceleration-versus-time will look for a cart with a much larger mass. Explain your reasoning? Write down the equation that best represents each of these accelerations. If there are constants in your equation, what kinematic quantities do they represent? How would you determine these constants from your graph?

2. Write down the relationship between the acceleration and the velocity of the cart. Use that relationship to construct an instantaneous velocity versus time graph for each case. The connection between the derivative of a function and the slope of its graph will be useful. Write down the equation that best represents each of these velocities. If there are constants in your equation, what kinematic quantities do they represent? How would you determine these constants from your graph? Can any of these constants be determined from the constants in the equation representing the acceleration? Which do you think best represents the velocity of the cart? Change your prediction if necessary.

3. Write down the relationship between the velocity and the position of the cart. Use that relationship to construct a position versus time graph for each case. The connection between the derivative of a function and the slope of its graph will be useful. Write down the equation that best represents each of these positions. If there are constants in your equation, what kinematic quantities do they represent? How would you determine these constants from your graph? Can any of these constants be determined from the constants in the equation representing the velocity? Which do you think best represents the position of the cart? Change your prediction if necessary.