Glider on air-track with varying masses: Quantitative Observational Experiment

Aim

To understand how the acceleration of an object is affected as we vary its mass while the net force exerted on it by other objects is held constant.

Prior Knowledge

• Kinematics: position, velocity and acceleration.
• Newton's first law.

Description of the Experiment

A fan is mounted on top of a glider travelling on a level air track. The fan pushes the air, and in turn the air exerts a constant force on the glider. The glider is allowed to move along the air-track after the fan is turned on. The experiment is repeated with objects of different masses placed on the glider.

Addtional Information

Focus on the motion of the glider relative to the ruler attached to the side of the air-track. You may observe that the card appears to be going "backwards" (relative to the camera at some points. This is because the camera is panning faster than the cart is moving. Do not be confused by this: the motion of the cart relative to the ruller mounted on the air-track is what is important

Fan glider:	659g
Glider with 200g mass:	859g
Glider with brass plate:	1010g
Glider with black block:	1161g

Youtube movies can be stepped frame by frame using the , and . keys on your keyboard. If you want to download the movie to your computer, right-click or control-click HERE.

Questions

1. Analysing the motion of the glider.

   • For one particular mass, draw a motion diagram of the glider, analysing the video frame by frame.
   
   
   
   • Is the glider's speed constant or changing? How can you tell?
   
   
   
   • Next, plot a graph of the glider's postion as a function of time.
   
   
   
   • What is the shape of the curve that you get?
   
   
   
   • What does this tell you about the acceleration of the glider?
   
   
   
   • Using the position and time data that you have, what sort of graph could you plot in order to determine the acceleration of the ball most accurately?
     
     
     • Consider the different variables you might choose to get a straight line graph whose slope would yield the acceleration.
     • What problems might arise? Consider particularly possible sources of systematic error in your analysis. Where do they come from? Can you eliminate all of them?
   
   
   
   • The end result of this discussion is that it is possible to eliminate the systematic errors if you are careful, but it is going to be extremely difficult. It is easier to fit the correct polynomial curve to a graph of position versus time and obtain the acceleration from the best fit curve. This can be done using excel or your favourite data analysis software.
   
   
   
   • Repeat this process finding the acceleration of the glider for each of the different masses placed on the glider.

2. Relationship between mass and acceleration

   • First decide which variable is the dependent one and which is the independent one. If you are unsure, ask yourself: "Does a change in mass cause a change in acceleration?" (Acceleration depends on mass.) Or, "does a change in acceleration cause a change in mass?" (Mass depends on acceleration.)
   
   
   
   • Next, plot a graph of mass vs acceleration or acceleration vs mass according to which variable is the dependent one. Do you get a straight line?
   
   
   
   • If you get a straight line, how are mass and acceleration related?
   
   
   
   • If you did not get a straight line, what sort of relationship would you guess between mass and acceleration, based on the shape of the curve?
   
   
   
   • Test your guess by plotting the new variables and seeing if you get a straight line.
   
   
   
   • If a straight line is not obtained, try again.