Newton's Second Law of Motion

Cynthia D'Souza                De La Salle Institute
                               3455 S. Wabash Ave.
                               Chicago IL 60616
                               (312) 842-7355

Objectives:

To verify Newton's Second Law of motion by  a) subjecting a body to multiples of 
a force; b) subjecting two bodies to the same force and qualitatively describing 
what happens; then quantitatively, the acceleration produced by a gentle ramp. 
Data is collected, analyzed and graphed. 

Materials Needed:

Coffee can, stop watch, meter stick-one each per group of five members.  For the 
demonstration, a lab cart and strong slinky and pulleys, toy carts and weights. 

Strategy:

The laboratory cart is kept fixed while a student sits on it holding one length 
of slinky which will be pulled by teacher till the cart begins to move.  This 
approximates a constant external force.  Now two such lengths are held by the
student and motion studied.  Next the cart is accelerated under three lengths.
Conclusions are put up on the board.

The group gathers around the table to compare the accelerations produced by the 
same force acting on two masses in the ratio 1:2.  The arrangement used is a 
fixed mass in a scale pan falling under its own weight.  Conclusions are posted.

Now the groups are handed their equipment and led to the ramp.  Marks are made 
at distances of 0, .50, 1.0, 2.0,meters on the ground and observers with 
stopwatches stationed there.  The coffee can is let go and timings noted. 

A graph of d/t is made showing accelerated motion.  Each group computes average 
velocity and acceleration for each mark.  To find out if the acceleration is
constant, students graph vavg/t.  Results are discussed.

Performance Assessment:   
                                                      
Groups answer these questions:
1) What visual evidence have you that the three objects were not moving at 
   constant speed?
2) How does your first graph support this?
3) Find the slope of your second graph.  What does this represent?
4) What would happen to the value of acceleration if the ramp were made 
   successively steeper?  What setup would you use to show this? 
5) What is the value of a when the ramp is vertical?
6) Show by a diagram all the forces acting on the coffee can and which 
   constitutes the unbalanced force.

Conclusions:

1) a a F  for constant m 
2) a a 1/m  for  constant F
3) Graph of d/t is not a straight line but vavg\t is, showing that acceleration 
   produced on a fixed mass by a fixed force is constant. 

References:

Personal Bank


 distance from  |  time to roll  |   average velocity |     Dv/Dt 
  x=0 in meters |  to new mark(s)|   from x=0         |                  
  0             |                |                    |
  0.5           |                |                    |
  1.0           |                |                    |
  1.5           |                |                    |
  2.0           |                |                    |
  2.5           |                |                    |
  3.0           |                |                    |
  3.5           |                |                    |
  4.0           |                |                    |