Blog 8-Seesaws

After a swim meet at Punahou, some of my friends and I went to play on the playground.  One of the fun things to play on at Punahou's playground are the seesaws.  Instead of rocking back and forth on them, as all the other children were doing, we, being physics/AP physics students, tried to balance the seesaws.

At first, we tried with only two people on the seesaw.  In this picture, Kevin and I are on the seesaws.  Since our weights are about 30 pounds different, he had to sit much closer to the fulcrum than I did.  By doing this, we were able to balance the torques on both sides of the seesaw.  If we were in a classroom, we would have been able to use the equation: TORQUE(ccw)=TORQUE(cw).  Since torque=force x radius, we could have set up the following equation: (106 N)x(2.5 m)=(135 N)x(r).  Kevin had to shift his center of mass to 1.9 meters away from the fulcrum in order for the seesaw to be balanced.

After we balanced the seesaw with two people, my other friend jumped onto my side of the board.  This time, Kevin had to shift his center of mass backwards in order to balance the board. Again, if we were in a classroom, we could determine the different torques on each side of the board: Fr + Fr=Fr.  Thus, (106 x 2.5) + (95 x .15) = (135 x r).  In this case, Kevin's center of mass is at 2.06 meters.

Blog 7-Liquid Fury

At the Kalani Invitational on Saturday, my friends and I participated in the Liquid Fury relay.  As intense as this may sound, it is actually a "fun" relay where teams of six (although the number of participants really doesn't matter) compete to be the most entertaining for a blue trophy. My friend brought a stand-up surfboard, two paddles, and an inflatable walrus.  Our plan was to have one swimmer (Kacy Johnson) drag the rest of us around while we balanced on them. 

For the first leg of the race, Kacy pulled Kevin across on the board, and for the second, he pulled another guy (Jacob) on the walrus while Jacob paddled.  However, Kevin was much more successful at keeping his balance for several reasons.  The board has a much greater surface area, its center of mass was central and low, and it was meant for people to be on it. In the above picture, it is easy to see that Kevin can balance easily and move quickly through the water.  The inflatable walrus, however, was much smaller, had a higher center of mass (since it had a head and is in a sitting position), and was not intended for humans to ride it while paddling.  Jacob struggled to maintain his balance on the walrus, and fell off of it several times.  The picture below is one of the times that Jacob fell off of the walrus.

For the last leg of the race, five girls piled onto the surfboard and made Kacy and Kevin push/pull us.  The picture at the very top of the page shows the five of us on the board.  However, part way through the lap Kevin got distracted by the walrus floating by so he let go of the board.  His upward push on the board had kept the torques balanced (net torque had been zero).  Since he let go, the back end of the board began to sink, so we tried to scoot forward towards Kacy.  

If we were to draw a FBD for this, Kacy would be the fulcrum and his arm would have tension at a diagonal (since he is pulling us forward and providing an upward push to help keep us afloat).  The water/board provides an upward push due to buoyancy.  Each girl would have a different torque since we all have different masses and were different distances from the fulcrum.  The board’s center of mass would also have a downward push. 

Thankfully, we did not flip (lengthwise, later we flipped sideways because my friend started dancing while standing on the board).  The torques were unbalanced, yet we were still able to stay afloat due to the buoyancy of the board and some good physics (the front end of the board was well supported but the back end was sinking). You can see the uneven support in the picture above.