Building Robots

This week in robotics we completely dismantled the bots we took to the Oahu Regional and began building two new ones for this week's Pan Pacific Championship. As I was trying to loosen a screw that was accidentally placed in the wrong place, I realized that my new knowledge of torque came in handy. In robotics we tighten (and loosen) screws with L shaped Allen Wrenches and I first I attempted to remove the screw by holding the short end of the wrench an twisting with the long end in the screw. Sadly, I realized that the screw was in too tight and I could not get it out. Then, naturally I flipped the wrench over and tried twisting the screw while holding the longer end. This worked well and immediately after it occurred to me that I had just used the principles of torque to solve my problem.

Torque is the product of force and a lever arm (perpendicular length or sometimes the radius), and by turning the wrench over, I got a grip on the wrench that was farther away than it had been, and therefore even though I was applying the same amount of force, I was getting more torque and was able to loosen the screw.

About ten minutes after this incident, we took the robot over to our practice field to see if it would be tall enough to knock a football down from the post that is on the field. While the robot was almost tall enough, it did not have quite enough force to knock down the ball. The ball sits on the post with a static friction force that is equal to the coefficient of static friction times the normal force exerted by the post on the ball. In order to knock down this ball, our robot would have to exert enough force to overcome the static friction and make the ball move.

Driving in Circles

For the past few months I have had my driver's permit and have been learning to drive. Learning about circular motion these past few weeks has helped me to explain some of the phenomena I feel while driving. Every time I drive I have to go through a roundabout on Ford Island in order to get home or leave the island. Here I have experienced circular motion. While turning through the curve I always seem to be pulled to the opposite side as the way I am turning. This is because when I turn my body continues in the straight line that it was originally moving while the car turns. The reason that I am able to turn with the car is because I am wearing a seat-belt which applies a force to pull me in the direction that the car is turning.

Also, as the car is turning, it has a tangential velocity that is equal to the radius of the circle times the angular velocity. Even though I may stay the same speed while turning in the circle, my acceleration is changing because acceleration is change in angular velocity which is speed and direction. Because I am continually changing direction, my acceleration is constantly changing.

Also, there is a net force directed radially inward of mv^2/r which is the centrepetal acceleration in this case.

Moving the Mats in the Lower Gym

This Wednesday was the East Oahu Robotics Tournament. It was a lot of fun and the day was filled with physics. The competition was in the Iolani Lower Gym, so afterward we stayed to help clean up. Our task was to roll up the floor coverings and put them into a box located by the door. It turns out that those mats are pretty heavy and take at least two people to lift one.

The mat has a weight (mass times gravity) directed downward and when it is on the ground, a normal force pointing upwards that is equal (but opposite) to the weight. In order for a person (or two

) to lift this mat they have to apply a force that is greater than the weight, thus resulting in an upward acceleration of the mat. This is shown in Newton's second law, F=ma (force =mass times acceleration.

However, once the mat is in the air a force equal and opposite to the weight must be applied to keep the mat stationary (F=ma, a=0, when upward and downward forces are equal). Also, though it may seem as though work is being done as the mat is moved to its box, the force is applied perpendicularly to the motion of the mat so no work is being done.

A Spinning CD

Have you ever opened a CD player while the CD was still spinning? I know I have, and with physics I can now explain in more detail the properties of the spinning CD. First, as you open the player, the CD begins to slow down, or accelerate in the negative direction, but continues to spin until it is stopped by the friction between it and the bottom of the CD player.

If you look at the words or pictures on the face of the CD, and track, lets say one letter as it spins, you can determine its tangential acceleration. Tangential acceleration is the radius of the circle on which the letter is spinning times the angular acceleration (the change in angular velocity over a certain time). Because the CD is slowing down the Tangential Acceleration vector is in the opposite direction as the Tangential Velocity vector and is tangent to the circle of motion.

Also, if there are words closer to the center of the CD than the word you are following, they are spinning with the same angular velocity (delta theta/time) as the letters father away, but because they have different radii, their tangential velocity (vt=(r)omega) and therefore tangential acceleration (at=r(alpha)) are different.

Small Boat Big Load

This morning I looked out my window and saw this small boat pulling this rather large dock. Let's analyze how this was done. The boat and its load were traveling to the right, in the positive direction so naturally the boat applied a force in the positive direction on the dock. There was only horizontal acceleration (not vertical) so the weight of the boat was equal to the normal force of the water. The acceleration of the boat is determined by Newton's Second Law that states that net force equals mass times acceleration. In this case the net force on the dock was the tension applied by the boat on the rope connecting it and the dock. This force was strong enough to overcome the (minimal) force of the resistance of the dock in the water.Also the current was flowing in the same direction as the boat was traveling so the current added to the velocity of the boat and dock together.

Here brings up another interesting point. I know realize that while I was watching the boat I was standing in a certain reference frame and that my perception of the velocity of the dock and boat was different than that of say someone either in the boat or in another boat floating in the current of the water.