Physics 105

March 24, 2003

The results of experiment P10a. demonstrate one of the most important conservation laws in Physics: Conservation of Energy. Conservation of energy means that the total energy of a system is conserved; however this does not prevent the transformation of energy from one form into another form. The results of experiment P10a. illustrate the transformation of energy: potential energy (in this case the energy associated with the height above the surface of the earth) is transformed into kinetic energy (the energy associated with motion):

• The potential energy of an object (associated with the force of gravity) is mgh where m is the mass of the object (in kg), and h is the height of the object above the surface of the earth (in m).
• The kinetic energy of an object is m*v^2/2 where m is the mass of the object (in m) and v is its velocity (in m/s)

When the potential energy of the object decreases (decreasing height) we see that the kinetic energy increases (increasing speed).

Conservation laws allow us to make predictions on the outcomes of our experiments, but not necessarily provide detailed information on each aspect of the actual motion (e.g. conservation of energy allows us to determine the speed of our cart at each height, but does not tell us how long it will take for the cart to move to that position from its initial starting point).

A second important conservation law is the conservation of linear momentum. This conservation law can be used to predict the outcome of collisions. During a collision, the collision force will (significantly) change the motion of the objects involved. By studying the change in the velocity of the objects involved, we can learn a great deal about the collision force. The parameter that is frequently used to describe the collision is the linear momentum of the objects involved. More details about the description of collisions can be found in the class notes.

In our first experiment we will look at elastic collisions and test the predictions that can be made about the outcome of the measurements. In order to do this, carry out experiment P17 in your handout folder.

© Frank L. H. Wolfs, University of Rochester, Rochester, NY 14627, USA

Last updated on Monday, March 24, 2003 8:01