Experiment 1: Fluid Statics

Physics 4c Lab Aug, 30th, 2012

The purpose of this lab is finding the buoyant for force of a metal cylinder in water by three different ways.

     The first way we used an expression for the buoyant force in terms of the wight of the cylinder and the tension in the string. B=W-T
     For the weight, we used a weighing instrument to get two datas. One is that the metal in air.

     W(in air)=1.03N

     Other is in water. The method to weigh the metal in water is that use a beaker with water as initial zero then put the metal in it, then the weight getting up is the weight of the metal in water.

      W(in water)=1.04N Theoretically, the weight in water should be same as in air.

      Next step is connect the machine which can test the tension of the string to a desktop so that we can have an accurate data.

      Then we got the tension which is T=0.67N.

     So by this way, we got the B=0.37N
     The second way is testing the weight of the water that overflow from an overflow can. Because the weight of the overflow water is equal to the buoyant force. The volume of the overflow water is equal to the volume of the metal cylinder. F=pho*g*V=mg
     So we weigh the empty dry beaker showing as below. m=beaker

     m=0.004kg

     Then fill the overflow can with water and put the metal cylinder into it so that the water overflow. We caught the water by the beaker. After a few seconds, we can take out the can of beaker, but make sure you do not outflow the water from can.

     Follow to the next step, weigh the mass of the can with water. M=beaker+water

     M=0.041kg

     mass of water=M-m=0.041-0.004=0.037kg

     weight of water=0.037*9.8=0.36N

     so the W=B=0.36N

     The third way to find the buoyant force is find the volume of displaced water. So we used the micrometer caliper to measure the height and diameter of cylinder show as below.

h=0.076m

     D=0.025m   

     Then we derive the expression and plug in all data we measured to find the volume of cylinder.

     Final, we got buoyant force=0.36 which is very closed to the front two ways.
     Summery: Theoretically, these three datas must be the same. But anyway, in the first way, we can ignore the string which exists errors from it. This string must has its mass and buoyant force. So we have errors from this part. And as you can see, the weight of cylinder in water is differ in air. In the second way, the important thing is when we put the metal cylinder into the water, whether we make sure there is no extra water flowing out or not. And we must fill it exactly full so that the volume of overflow water we got exactly equal to the volume of cylinder. And there is a little water sticking on can yet. This is serious in this part. In the third way, definitely we must exist some errors we measure the height and diameter. I thought if the cylinder be abraded or not even a little bit. It will impact the result directly. All these three ways, we ignore the hook in fact. That is the point existing errors. In my opinion, I prefer the first way. Because the mass of string is closed to zero to metal cylinder, so it can be existing too much errors. And the volume of the string is pretty small too. So we put it into the water. It seems like it does not displace to water. From the result we got, the first one is bigger than others. Because it is so hard to control the water cant be overflow for extra.  In part A, if the cylinder had been touching the bottom of the water container, the value changed to higher. Because when the cylinder touching bottom, the tension we measure will get smaller actually, there exists error in here. That is why the B getting higher because of B=W-T, the W doesn't change and T get smaller. Then B is getting higher. That's it. (Feel really sorry about my pool English, and hope you can enjoy it. Thanks)

   Theoretically, the buoyant force we get from these three ways are exact the same!