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Champlain - St. Lawrence
Winter 2017
Physics 203-NYA-05
Mechanics Lab
Physics 203-NYA-05
Mechanical Energy
I. Introduction
An object of mass m which is moving at speed v carries energy in its motion, called kinetic
energy ( ). The formula for computing kinetic energy is:
1
2
In the MKS system of units
is measured in Joules (J). Kinetic energy is also found in
the rotation and vibration of an object (i.e., its internal motions) besides its motion through
space.
An object moving under the action of a conservative force such as gravity also has energy
in the form of potential energy ( ) whose value changes with the position of the object. In
the case of gravity, the formula for computing the potential energy is:
where is the height measured from a fixed reference (ground, beginning of the motion,
origin of a coordinate system, or something else). Potential energy may also be present in
the internal motions of an object.
The total mechanical energy of an object at any instant is the sum of its kinetic and potential
energies:
may or may not remain constant during the motion of an object. If the only forces acting
on an object are conservative, will stay constant. If other, non-conservative, forces act
on the object, may change. Such forces include friction and forces such as those between
objects during a collision. When mechanical energy is lost it is transformed into other
types of energy such as heat, sound, chemical energy, electrical energy, etc. The general
principle of conservation of energy in physics states that energy is never created nor
destroyed. It only changes form.
Energy Conservation
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Champlain - St. Lawrence
Winter 2017
Physics 203-NYA-05
Mechanics Lab
II. Objective
The general objective of this experiment is the study of total mechanical energy in the
motion of a puck on an inclined air table. Specifically, we will verify whether total
mechanical energy is conserved during the projectile motion of the puck and whether
keeps the same value before and after a collision.
III. Procedure
For this experiment you will use the spark sheet obtained in the study of projectile motion
at the beginning of the semester. (If you need to redo the experiment, please go and see
the technician).
IV. Calculations
For each of the first two parts of the puck’s motion (the initial slide in straight line and the
first bounce), record the position , and the time for about a dozen points that span
each trajectory and are equally spaced in time. This may imply that you will record every
second point or every third point along the trajectory. Reset the value of to zero at the
beginning of each part of the motion.
For each point:
1. Compute the velocity and of the puck following the method used in the study
of projectile motion. Then compute the kinetic energy of the puck in Joules. (If
you did not record the mass of the puck, use
500 . This will have no impact
on whether the total mechanical energy is conserved or not during the puck’s
motion).
2. Compute the potential energy of the puck in Joules with the formula .
Here is measured with respect to the origin of the , axes using the coordinate
of the puck as shown below:
sin
( : angle of the incline)
3. Compute the total mechanical energy of the puck in Joules.
Energy Conservation
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Champlain - St. Lawrence
Winter 2017
Physics 203-NYA-05
Mechanics Lab
Then:
4. Make a graph of , and as a function of time. (Put the three curves on the
same graph). Perform a detailed linear regression for as a function of time.
Is the total mechanical energy conserved (i.e., constant) during each part of the motion?
Using the total mechanical energy just before and just after the collision with the bumper
(use the last computed before the collision and the first computed after the collision),
compute the fraction of the total mechanical energy which is lost during the collision.
Discuss what happens to the lost energy.
V. Report
Your instructor will inform you of the structure and content of the lab report.
Energy Conservation
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