Forces and Acceleration

Marvin Orr                     Lincoln-Way High School
                               Rt. 30 
                               New Lenox, IL 60451
                               815-485-7677

Objectives:

1. To measure the time it takes a person to travel a known distance when a 
   constant force is applied.

2. To construct a distance vs. time graph for the data collected for objective 
   #1. 

3. To interpret the distance vs. time graph and make predictions based upon the 
   graph.

4. To learn that when a constant force is applied to a mass a constant 
   acceleration will result.


Materials Needed:

The following materials are needed for each group of 2-4 students:

     1 spring balance
     1 tow rope
     1 coaster cart (an enlarged skateboard)
     1 stopwatch
     4 data sheets
     4 sheets of graph paper

For the entire class the following items are needed:
     
     1 measuring tape 15 m long
     masking tape
     overhead graph
     If time permits, 2 rideable air hockey pucks. (see Physics Teacher, Nov. 
      1989) 

Strategy:

INSTRUCTOR'S PREPARATION:

1. The data table used by the students should contain the following information:
     A. the distance traveled
     B. the amount of force being applied
     C. the time it takes to travel each distance
     D. if each distance is traveled more than once, an average column

2. The graph that each student is to make is a distance vs. time graph.  
Distance should be plotted on the vertical axis and time plotted on the 
horizontal axis. 

3. The coaster carts can be constructed out of 3/4 inch plywood or a similar 
material and roller skate wheels.  Alternatives to constructing these carts are 
roller skates, skateboards, or even wheeled office chairs.

CLASS ACTIVITIES:

1. Have the students pull each other down the hall, on the coaster carts. 
Distances of 5, 10 and 15m will have been measured out on the floor.  While 
doing this another member of the group will record the time that it takes to 
travel each designated distance.  The student pulling the cart must maintain the 
same force reading on the balance throughout the entire distance. 

2. Other group members record on the data table, the distance traveled and the 
amount of time it took to travel that distance.  At least 3 different students 
should ride the cart.

3. Each student is to plot his/her own distance vs. time graph.  (Is the graph a 
straight line?  If not, what is it?  Is your graph like your partner's?  How are 
they the same?  Different? 

4. Based upon their comparisons the students are then to predict what the graph 
would look like for a person who is twice as massive as themselves.  Half as 
massive.

Expected Results:

The first thing the students should notice is that their velocity continued to 
increase.  This will be obvious to them from their experience of riding on the 
cart.  The student will also have a graphical representation of what they had 
experienced while riding on the cart. 

Since each student will have ridden the coaster carts more than once with a 
different amount of force being applied each time, they will also come to the 
understanding that the more force applied the greater the amount of 
acceleration. 

When the students compare their graph with that of their partner's they will see 
that everyone experienced a constantly changing velocity (acceleration). 
Therefore, a constant force produces a constant acceleration. 

Finally, the students will see that the acceleration experienced by each one of 
them was not the same.  The difference in acceleration that each one of them 
experienced is related to the amount of force being applied and the amount of 
mass to which the force was being applied (a = F/m  or F = ma). 

Evaluation:

The student's grade is based upon their data table, graph and their 
interpretation of the results.