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Physical Science Lab Exercise Potential and Kinetic Energy Name_____________________________________________________________________________________ Lab Partner(s)______________________________________________________________________________ Date______________________________________________________________________________________ Lab Station Number_________________________________________________________________________ Materials List: Hot Wheels Track Hot Wheels Cars (4) Connectors (5) Ramp and Catcher Loop Scissors Meterstick Large Table Clamp Small metal rod Duct Tape Stand Along Photogate Objectives: 1) Define, describe and show how to calculate potential energy. 2) Define, describe and show how to calculate kinetic energy. 3) Describe how potential and kinetic energy are related to the principle of conservation of energy. 4) Explain what friction does in the process of conservation of energy. Description of Concepts: Potential energy is energy of position. Kinetic energy is energy of motion. On earth, if an object is moved up to a higher position compared to a certain “reference” lowest position, that object is said to have a potential to do work or provide energy. So, when Wiley E. Coyote is pushing that boulder over the cliff to try and hurt the Roadrunner, that boulder has great potential energy. Specifically, the equation that determines how much potential energy an object has is: P.E. = m*g*h Equation 1 P.E. stands for the potential energy, m is the mass of an object, g is the acceleration due to gravity (9.8 m/s2) and h is the “straight up vertical” height of the object from the lowest reference point. Here is a specific example: Wiley is at the top of a cliff that is 500 meters above the ground. The boulder he is pushing over the cliff has a mass of 200 kilograms. What is the potential energy of the boulder? P.E. = 200 kg*9.8m/s2*500m = 980,000 Joules. Kinetic energy can be considered to be how much energy something has when it is moving. Energy does not just come out of thin air. It has to be transferred from one source to another. So, with the example of the rock falling down over the cliff, it has the most potential energy at the top of the cliff and the most kinetic energy right before it hits the ground. Not considering air friction, the rocks energy converts from entirely potential at the top to entirely kinetic at the bottom when it is also moving the fastest. The equation for kinetic energy is: 10 K.E. = 1/2*m*v2 Equation 2 K.E. is kinetic energy, m is the mass and v is the velocity of the object. So, again, with the same example, we can determine how much kinetic energy the boulder has just before impact and even determine how fast the rock is going then also. Because of something called Conservation of Energy, all of the potential energy is converted to kinetic energy. 1 K.E. * m * v2 , P.E. m * g *h 2 1 1 2 * m * v2 m * g * h , *v g*h 2 2 2 m m v 2*g*h, v 2 * g * h 2 * 9.8 2 * 500m 98.9 s s Procedure: Put the large table clamp at the end of the table top and put a small metal rod so it extends out at a right angle to the clamp. Put the Hot Wheels clamp on the metal rod extending out. Put the longest track on first and attach the next track with the help of a connector. Continue all the way to the floor and a little past so the car can glide a little after it has reached the bottom. Carefully stretch the hot wheels track just a little, so it is not “sagging”. Put a piece of duct tape under the end of the last piece of track so the track does not move. Put the stand alone photogate just past the point where the track starts to touch the ground. Plug the photogate in and turn it on, make sure it reads zero (clear it) and the setting is on “gate”. Put the special index card that is approximately 0.030m wide with the two flaps on top of the car so it stands straight up and will not move while the car is going down the track Measure the maximum height of the track in meters, the mass of the car in kilograms and calculate its potential energy. Carefully mark (with a pencil if possible) lower points on the track to represent 0.75m, 0.50m and 0.25m. Put car #1 at the top of the track and let it go down the track. Put something at the bottom to prevent the car from going too far off the track. Record the reading of the time from the photogate. Clear the photogate (red button) Repeat sending the card down from that height until a consistent time reading is found (do it at least three times for each height). Make sure to clear the reading every time. Send the car down from 0.75m, 0.5mm and 0.25m enough times from each height to get consistent time readings. Repeat this for the other 3 cars. If you need to, borrow a car from another lab station so that you have four different cars total, but let the other lab stations finish with the ones they have first. There should be a total of 16 different trials with 4 different cars. Analysis: The initial energy of the car at the top of the track is the potential energy. That is the accepted energy. At the bottom of the track, the kinetic energy is the experimental energy. For these calculations of energy to be done (automatically), the data needs to be entered into the excel sheet. The data includes: the mass of the car (in kilograms), the maximum height of the car on the track, (meters) the width of the index card (in meters) on the car and the time (in seconds) for each of the 16 trials. Make sure to temporarily save the excel sheet occasionally to prevent loss of data. It would be a very good idea to record the data separately in a notebook and then transfer the data to the excel sheet. It is 11 also a good idea to do one car at a time, and record the data in the sheet after the 4 trials for that car. Look at the results to see if they are less than 10%. If they are, go ahead and continue. If they are not, identify specific sources of error that could be causing the error to be so high. The main cause will be friction. Other sources could be from the track moving and incorrect initial placement of the car on the track. Another is from not clearing the time on the photogate. Lab Report Notes: As, always, include the 5 essential components of the lab report. Because there is a lot of data for this particular lab, the excel data sheet will serve as a list for the data. Just make a short statement referring to the excel sheet for the list of data. That will not always be the case, but for this one, it is. So, make sure to get the data listed correctly on the excel sheet. You need to write out the conclusions of this lab based on your observations and the numerical results. Make sure to list potential sources of error in this section. The excel sheet will also serve as the list of results, because of the extensive error analysis that is automatically done. So, as with the data, make a statement in the lab report that refers to the data sheet for the list of the results. 12
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