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SMC|Academic Programs|Physical Sciences|Acceleration and Frictional Forces

Acceleration and Frictional Forces

Turn on the ULI box which is connected to your computer. Connect a motion sensor into the ULI in Port 2. Log onto the computer and start the program "LoggerPro". (The ULI must be turned on before starting the program.) 

Under "File", click on "Open", "Experiments", "Physics with Computers", "Exp2", and then "Exp 2b". This will load the software that will enable us to gather position, velocity and time information about any object the motion sensor sees. We will start by observing rolling carts.

Cart on a track

Set up the track with the motion sensor at one end. Place the cart close to the sensor. Have one person exert a brief push on the cart toward the other end of the track.

1. Draw a freebody diagram for the cart as it rolls along the track.

 

 

 

 

 

 

2. How do you expect the speed of the cart to vary (if at all) as it moves along the track? How does your answer depend upon the forces acting on the cart? 

 

 

 

Draw a motion diagram below for the cart, showing the acceleration.

 

 

 

Now, place the cart back close to the motion sensor. One person click on the "Collect" button at the top of the computer screen. Another person should then exert a brief push on the cart toward the other end of the track, so it moves off at moderate speed.

On the screen you will see two graphs, one showing the cart's position as time goes by, and another showing the cart's speed as time goes by. At the top of the computer screen, click on "Analyze", and then "Examine". Now when you place the cursor at any point on the graph, a window will show you the value of the distance of the cart from the motion sensor at a specified time, and another small window will show you the value of the velocity of the cart at the same time. Play with this for a moment. 

Find the point on the position/time graph where the cart is 0.7 m from the motion sensor (or as close to that value as you can find). Record the speed and time at that moment in the table below. Do the same for the cart when it is 1.7 m from the motion sensor.

  x1 = 0.7 m x2 = 1.7 m  
velocity v1 =             v2 =             v2 - v1 =               
time t1 =            t2 =            t2 - t1 =             

 

3. What is the cart's acceleration? 

 

Is this consistent with your prediction in Question 2? Explain.

 

 

The mass of the cart is 0.50 kg. Given your answer to Question 3, how much net force is acting on the cart and in what direction is it acting?

4. What is the net force on the cart? 

 

 

Is this consistent with your prediction in Question 1? Explain.

 

 

Forces

Turn the cart upside down and attach the GREEN spring force probe (scale from 0 - 5 N) to the hole in the bottom. Keep the cart UPSIDE DOWN! Practise dragging the cart along the track at a constant moderate, speed. Try to pull on the force probe horizontally.

5. Draw a freebody diagram for the cart as it is dragged along the track at constant speed.

 

 

 

 

 

Record your best estimate of the force required to pull the cart along the track at constant velocity in the table below. Also record the values for the force needed to pull it at constant speed when one mass bar is placed on the cart, and again when two mass bars are on the cart. (Each mass bar has a mass of 0.50 kg.)

On the Track Force Mass
cart alone    
cart + 1 mass    
cart + 2 masses    

Repeat the measurements above for the cart being dragged along upside down on the table this time.

On the Table Force Mass
cart alone    
cart + 1 mass    
cart + 2 masses    

Think about how the forces acting on the cart change when different masses are added. Think about whether they changed in a way that is consistent with your understanding of forces acting on masses, and the motion that forces lead to.  

6. What do you notice about the force required to move the cart and masses, and how does it relate to the frictional force acting on the cart?

 

 

 

 

 

If we could remove that friction between the cart and surface, the cart would accelerate at a rate given by a = F/m. From the data above, what acceleration would the cart have had in each case? (Fill in the table below.)

On the Track Acceleration On the Table Acceleration
cart alone   cart alone  
cart + 1 mass   cart + 1 mass  
cart + 2 masses   cart + 2 masses