Weight Lab

For this lab, you will be calculating force in two different ways, through kinematics and the gravity equation.

Procedure:

  1. It’s physics lab. No safety precautions. Feel free to wail away on partner.
  2. Set the ramp down flat on the desk, with one end perpendicular to the table at the pulley. There should be no incline on the ramp.
  3. Find a cart; obtain its weight, either through measuring with a scale or asking a teacher.
  4. Set the cart on the opposite end of the ramp from the pulley.
  5. Get a piece of string, and tie it to the cart.
  6. Run the string through the pulley.
  7. Find the set of weights. Get them out and figure out how to attach them to the string. But don’t do it yet.
  8. With a yardstick, measure a meter from the front of the cart. Mark it somehow.
  9. Hold the cart, and attach a weight to the end of the string, beyond the pulley. The weight should be hanging straight down, with more than a meter between it and the floor. Use a 100 gram weight.
  10. Get a timer ready, release the cart and measure how long it takes to move one meter. Catch the cart before it flies onto the floor. Also try to catch the weight before it hits the floor. Putting your hand or the yardstick at the one-meter mark is one way to do this.
  11. Repeat the steps 9-10 three times, for statistical significance.
  12. Repeat steps 9-11 for weights ranging from 100 grams to 1 kilogram, with an experiment every 100 grams (100 g, 200g….900g, 1000g).
  1. Record your results in the following data table, and do the following averages: Don’t forget to put in uncertainties as well.

Mass of weight / Trial 1 time / Trial 2 time / Trial 3 time / Avg. time
100 grams
200 grams
300 grams
400 grams
500 grams
600 grams
700 grams
800 grams
900 grams
1000 grams
  1. Clean up and put away all materials. Don’t forget to whack your partner a bit more. (Ben)
  2. Do the following calculations using the averages of time just obtained.

Calculating acceleration with time:

Using your kinematics skills, calculate the acceleration of the cart for each trial using your mad kinematics skills. Specifically, if those skills aren’t quite so mad, this equation:

Xf = Xi + T Vi + ½ A T2

With Xf being the final position (1 meter), Xi being your initial position (0 meters), Vi being your initial velocity (0 m/s), T being whatever average time you record for each trial, and A being the desired value, acceleration in m/s2. Fill out the accelerations in the following table

Now calculate the force being exerted on the cart (and the weight for that matter) by using your inertial skills. Again, if those skills aren’t so great, use the following equation:

F = m A

With A being acceleration calculated in the last step, F being the desired value in Newtons, and m being the combined mass of the cart and the weight. Check your units, what are the units of a Newton? Why must one add the mass of the cart and the weight for this equation to work? Once the force has been calculated, enter it into the data table below, under Experimental force.

Now calculate the theoretical force exerted on the cart using the law of weight shown in the weight section:

W = m g

With W as your weight, in Newtons, mass being the mass of the weight, and g being the gravitational constant, 9.81 m/s2. Why do you use just the mass of the weight, not the mass of the weight + the mass of the cart? Repeat the equation for every experiment run.Enter the value of weight obtained as theoretical weight.

Mass of weight / Experimental Acceleration / Experimental force / Theoretical weight
100 grams
200 grams
300 grams
400 grams
500 grams
600 grams
700 grams
800 grams
900 grams
1000 grams

Looking at the calculated data above, what strikes you? The experimental forces and the theoretical weights should be the same. WHY? What assumptions have we made in these calculations?
-Devon Stork