To show that acceleration is proportional to force

When a free running trolley accelerates as a result of a force applied to it the acceleration is proportional to the applied force. In this experiment the overall mass of the system is kept constant but masses are moved from the trolley to the end of a string which is hanging over a free moving pulley. As the number of "weights" increases (the accelerating force), the acceleration increases. We will investigate the relationship between acceleration and accelerating force. A ticker tape timer is used to measure the acceleration.

Note: In this simulation the trolley is running on a level track. In the lab a level air track can be used or a simple runway which is tilted sufficiently to overcome friction in the wheels of the trolley and in the pulley

Procedure:

  1. Note the number of weights causing the trolley to accelerate (F). Press "Release Trolley"
  2. Press "Get Tape and Ruler". Drag the marker to a suitable dot near the start (left hand side) of the tape.
  3. Press "Place Mark". Move 5 dots to the right. Press "Place Mark". The distance between the two marks was travelled in 5/50 of a second. Calculate u, the initial velocity. u = distance / 0.1 m/s
  4. Place a mark above the dot on the extreme right of the tape and another on the 5 th dot left of it. The velocity v can be calculated from this section of tape
  5. Count the number of dots from the start of the first interval (u) to the start of the second (v). Time is given by: t = no. of dots / 50
  6. Calculate the acceleration: a = (v-u)/t
  7. Press "Reset" and "Move Mass" to transfer a mass from the trolley to the string. Repeat steps 1 to 6.

Analysis:
Using graph paper, draw a graph of acceleration (y-axis) against force. Note: Start both axes at zero.

Draw a "best fit" straight line. A straight line through the origin shows that acceleration is directly proportional to force.

Precautions:

  • Ensure that the runway is tilted just enough for the trolley to roll at constant speed
  • Ensure that the runway in smooth, free of dust, and does not sag in the middle
  • Ignore the first few dots on the tape. These are unreliable and too close together for accurate measurement.
  • Place a ruler right on top of the tape when measuring distances to avoid parallax error