Force and Motion Experiments

Time Estimate:2 hours, about 10 minutes per experiment/question

Goal: To quantify the connection between interactions and motion.

Strategy: Use what we have learned about kinematics to measure motion. Measure force as well and see how force and motion are connected. Similar to the motion graphs experiments, do this in simple situations to discover the basic principles and then extend them to the more complicated problems.

You will need a track, a wireless cart, a pulley, string and weights. The cart has a built in force probe. You remove the magnet from one end and attach a hook for your string to pull on. These experiments are easiest to do if the cart if fairly heavy, about 1 kg or more. There should be some masses you can add. You can apply a constant force by attaching a string to the force probe hook with a small amount of weight, 10 or 20 g, hanging on the other end over the pulley.

The net force is the vector sum of all the individual forces acting on a particle.

Important Note:The force sensor reading tends to drift over time due to technological limitations. It can give you a force reading when nothing is pulling on it and that will throw off all your measurements. You should get in the habit of zeroing it before every run. In the hardware setting under the force sensor settings there is a button labeled "Zero Sensor Now." Click that and it should be OK at least for the duration of your run.

Experiment 1:

The cart, with very small (negligible) friction, is pulled with a constant force so that it moves in the positive direction, speeding up at a steady rate. Predict the graphs of velocity, acceleration, and force vs. time. Then observe them and reconcile any problems in your predictions.

1. Draw an accurate motion diagram for this problem.
2. Draw an accurate free body diagram for this problem (sections 4.6 in your text may help).
3. Explain why the force applied by the string and the net force are the same in this situation.
4. When a constant force is applied, which kinematic quantity is constant - velocity or acceleration? They are vectors so compare their directions as well. Explain how your motion and free body diagrams show this.

Experiment 2:

Add a substantial amount of friction to the cart on the track (a folded-up sheet of paper taped to the bottom of your cart works nicely). Predict the graphs of velocity, acceleration and force vs. time as they compare to the previous experiment. Then observe them and reconcile any problems in your predictions.

1. Draw an accurate motion diagram for this problem.
2. Draw an accurate free body diagram for this problem (sections 4.3 and particularly tactics box 4.2 in your text may help)
3. Explain why the force applied by the string and the net force are not the same in this situation.
4. Does the force probe measure the net force? Explain, and produce a graph that you think would represent the net force.
5. When a constant force is applied, which kinematic quantity is constant - velocity or acceleration? Explain how your motion and free body diagrams show this. Does it go with the string force or the net force?
6. What properties of the velocity and acceleration changed relative to Experiment 2? Explain why they changed.

For the remaining experiments, remove the friction as well as the string and hanging mass. You should have only a force probe attached to your cart.

Experiment 3:

Give the cart a short, sharp pull on the force probe and then release it and let it coast down the track. Assume that friction is negligible. Predict the graphs of velocity, acceleration and force vs. time. Your prediction should include both the pull and the coasting. Then observe them and reconcile any problems in your predictions.

1. Draw an accurate motion diagram for this problem.
2. Draw accurate free body diagrams for this problem (sections 5.1 and 5.2 in your text may help). You'll need one while pulling and one after letting go.
3. Explain the relationship between the force applied by your hand and the net force in this situation.
4. Discuss the relationship between velocity, acceleration, and net force throughout this experiment. Explain how your motion and free body diagrams show this.
5. Does the force continue after the pull is over? Does the acceleration?

Experiment 4:

Orient the cart so that the positive direction is downhill on a tilted track. Then push the force sensor to send the cart up the track from the bottom. Let it approach the top, slowing down at a steady rate, stop, and go back again, speeding up at a steady rate. Stop it before it falls off the track by touching the force sensor. Predict the graphs of velocity, acceleration and force vs. time, including the push and the stop. Then observe them and reconcile any problems in your predictions.

1. Draw an accurate motion diagram for this problem.
2. Draw accurate free body diagrams for this problem (sections 5.1 and 5.2 in your text may help).
3. Explain the relationship between the gravitational force and the net force in this situation. Constructing F_net from a free body diagram will help.
4. Discuss the relationship between velocity, acceleration, and net force throughout this experiment. Explain how your motion and free body diagrams show this.
5. Why does the acceleration not change ? How can the cart be sometimes slowing down and sometimes speeding up when then acceleration doesn't change? Look at this two ways, using the motion diagram and the free body diagram.
6. Is the velocity ever zero? What about the acceleration? Explain how both those facts can be consistent and connect them to the net force.
7. Is the net force ever zero? Explain how that is consistent with your observed velocity and acceleration.
8. Is the net force after the push and before the catch the same as the force of gravity? Explain.
9. Why does the acceleration not change direction when the cart comes back down the ramp? Connect your answer to the net force.

Experiment 5:

Repeat experiment 2 but try several different values of cart mass, keeping the string force constant. To make the effect, if any, of cart mass easy to see, you should add substantial masses to it. They should be significantly larger than the hanging mass, something on the same order as the mass of the cart. Predict the changes you expect to see in the kinematic variables as you increase the mass of the cart.

1. Does the net force change? Explain.
2. Does the acceleration change? Explain.

Questions

1. Look up Newton's first and second laws in your text. Justify each of them using your experimental data. Look at all of the experiments you have done, not just some. You need to address both the magnitude and the direction since Newton's laws are vector laws.

2. Is it necessary to have a nonzero net force when an object is moving? Use your data to justify your answer.

3. In the football collision observations, we saw that you can survive a very high acceleration if it only lasts a short time. The same acceleration, lasting for a longer time, would cause a concussion or even death. If you look up a little anatomy about how the brain is situated inside the head, you should be able to explain this