Pre-Lab 7B: Conservation of Energy

Purpose A car launched up the hill at a given speed will never go higher than a certain point. A car rolling downhill will only reach a certain speed. Why? The answer is that nature keeps an exact balance of energy: the law of conservation of energy Speed uses one form of energy and height uses another. This investigation explores the exchange of energy.

Key Question What limits how much a system may change?

Background Define: law of conservation of energy formula for drop height formula for predicted velocity

Drawing Use the changed lab set up: drawing on the board; note there is NO string used!!

Section 1: Set the bent ramp to the steepest angle Bottom section of ramp must be horizontal Photogate must be in the correct location: look at the DRAWING right now!!! Do not use any steel marbles Do not use the string to determine drop height (resolution counts and is required) Instead, look at the picture for the car’s center: pg 41 measure with a meter stick h1: from the table to the center of the car and measure h2: from the table to the lazer beam Drop height is h = h1 – h2

Drop the car from each 5cm mark on the hill Section 2: Graph the speed of the car vs. the drop height.

Section 3: Use the formula for potential energy to fill in the second column of Table 2. due to the law of conservation of energy, the potential energy at the top must be equal to the kinetic energy at the bottom: Ep = Ek: Thus, mgh = 1/2mv2; but masses will cancel so: gh = 1/2v2 predict the velocity of your car at the bottom: v = the square root of 2gh Use the formula you just derived to fill in the column for the predicted speed of the car. Plot the line for the predicted speed on the same graph as you made in part 2a above (key?)

Section 3 cont: Note table 2: is missing a row so, ADD A ROW Section 4: Let the car roll downhill, bounce off the rubber band and go back up hill again. Does it reach the same height as it was dropped from? required: Challenge experiment: Use a rubber band to launch the car uphill so it goes through the photogate with the same speed as it had going down.

Post Lab 7B: conservation of energy

Purpose nature keeps an exact balance of energy: the law of conservation of energy Speed uses (Ke) one form of energy and height (Ep) uses another. This investigation explored the exchange of these two energies.

Background Define: law of conservation of energy

Graph the speed of the car vs. the drop height: what are the labels of the axis? describe the graph describe the predicted graph how can you explain the predicted is higher

Section 4: Let the car roll downhill, bounce off the rubber band and go back up hill again. Does it reach the same height as it was dropped from, why? Challenge experiment. Use a rubber band to launch the car uphill so it goes through the photogate with the same speed as it had going down. What did you find and why?