Lego Mindstorm Activity: Pulley Systems

Today was Lego Mindstorm Experiment numero dos (that’s #2 for all you non-Spanish speakers). We were to use the Lego Mindstorm motor to control a pulley to lift weights. Doing so we would explore:

1. Force and energy: Newton’s 2^nd Law of Motion
2. The law of conservation of energy – energy can’t be
   created or destroyed only transferred or transformed.
3. Velocity and Acceleration
4. Power – measures the time rate at which work is done

However do to some technical difficulties with the Labview program, our experiment was limited to testing force, mass, and acceleration. Here is the main equation used in the experiment:

Force = mass x acceleration

F=ma

A few of things to know before getting into the nitty gritty of the experiment:

• Force (F) is measured in Newtons (N)
• Mass was converted from grams (g) to kilograms (kg)
• Meters (m) was converted from inches (in)

We ended up conducting twoexperiments – one in which we made force constant and the other in which we made mass constant.
Experiment 1

As previously stated, force remained constant at 10 N for all trials.

The Labview program runs the motor works the pulley to lift the weight. We plugged the number 10 (constant force) into the program so the motor would “know” to lift the weight with that amount of force for each trial.

Because acceleration is dependent on the change in velocity or the change in distance over time, we wanted to see how acceleration changes at different levels of mass. So we altered the weight five times to calculate those differences. This is what our numbers looked like in the end:

Force (N)          Mass (kg)         A (m/s^2)
10                       0.05                      200

10                       0.1                        100

10                      0.15                     66.66

10                       0.2                        50

10                      0.25                       40

I know, you’re looking at this, scratching your head without a clue as to what any of this means. But don’t worry, I’m here to explain it to you.

As I already told you, acceleration is a change in velocity over time. This is to say that acceleration is calculated by diving velocity by time or better yet, diving meters per second by second  which is the same as dividing distance (meters) by time twice (seconds squared). The end equation is this:

Acceleration = meters / seconds squared
A=m/s²

In the above chart you can see that acceleration is greater when mass is lesser. This is due to a simple fact – the lesser the mass, the greater an object can travel in a shorter amount of time and vice versa.

Still don’t get it? I’ll use a real world example for you.

You have a golf ball, a baseball, and a bowling ball. If you are a pitcher throwing at a constant force for each throw, what do you think will happen when you try to throw each ball? The golf ball will have the greatest acceleration because you will throw in the farthest in the shortest amount of time since it has the least amount of mass. The bowling ball will have the least acceleration because its great mass will cause you to throw it the shortest distance in the longest amount of time. And finally the baseball will have the middle acceleration. Its mass is greater than the golf ball but less than the bowling ball so you will throw it somewhere between each; it will go the median distance in the median amount of time.

So going back to Lego Mindstorm and the chart, when mass of the weight was smallest at 0.05 kg, acceleration was greatest at 200 N. And as mass increased, acceleration decreased.

Experiment 2

In our second experiment mass was kept constant at 0.25 kg at five different levels of force. The same concept introduced in Experiment 1 applies to this one. again acceleration is the change in velocity over time so our goal was to see how different levels of force,  intead of mass, affected acceleration. Here’s a chart of our results:

Force (N)          Mass (kg)         A (m/s^2)
10                            0.25                      40

20                           0.25                      80

40                           0.25                     160

80                           0.25                     320

120                         0.25                     480

Alright, either your scalp is really itchy or you’re once again confused. I’ll use the baseball example from earlier to break it down for you.

Now you have a single baseball that has a mass that doesn’t change. You’re a pitcher warming up for a game. You’re a little rusty so the first time you throw it you do so with a small amount of force and the ball takes a while to go practically nowhere. You build your confidence with each throw. Now when you throw it you use more force and the baseball goes much farther much quicker. You’re excited but the game’s about to start and you only have time for one last pitch. You give it your all, using all the force you have. In what seems like a split second the ball goes flying – it’s going, going, gone. You’re ready to win.

This example shows how added force on a given object increases its acceleration. Take another look at the chart. You can see that at the constant mass the greater the force, the greater the force the grater the acceleration. At a force of 10, acceleration is a measly 40 but at a force of 120, acceleration is a whopping 480.

Hopefully you all learned a lot about force, mass, and acceleration through this experiment and how each can be found in everyday life. Maybe now you’ll recognize it and the next time your favorite pitcher is throwing with an injured rotator cuff, you’ll know what might be in store for your team.

Thanks for reading!