Experiments
Plunger Cart
012-14781A
21
Experiment 6: Sliding Friction and Conservation of Energy
Purpose
In this lab, the Plunger Cart will be launched
down a ramp, as shown in Figure 6.1, while
riding on a friction block. The initial elastic
potential energy and gravitational potential
energy of the car are converted to thermal
energy as the car slides to a stop. The thermal
energy generated on the surfaces is the same
as the work done against sliding friction.
Theory
Using the principle of conservation of energy, we can equate the initial energy of the system with
the final (i.e. thermal) energy of the system. This leads to:
(elastic P.E.) + (Gravitational P.E.) = (work done against friction)
where k is the spring constant of the plunger (from Experiment 4), x is the distance that the plunger
is pushed in, m is the mass of the cart plus the friction block, D is the distance that the block slides
after the cart’s plunger is released,
is the angle of the ramp to the horizontal, and µ
k
is the coefficient
of kinetic or “sliding” friction.
In this experiment, you will use the principle of the conservation of energy to predict D, given certain
measurements you will make and the value of k determined in Experiment 4. First you will need to
determine the coefficient of kinetic or “sliding” friction for the friction block.
Determining µ
k
: If the angle of the ramp is high enough, the friction block will slide down the ramp
with uniform acceleration, due to a net force on the block. The net force on the block is the difference
between the component of the gravitational force (mgsinø) that is parallel to the surface of the ramp
and the friction force (-µkmgcosø) that retards the motion. The angle ø is the angle of the ramp
when the block slides down the ramp with uniform acceleration. The acceleration down the ramp
is given by:
The average acceleration down the ramp is given by:
where d is the total distance the block slides and t is the time required to slide through that distance.
Equipment Needed
Plunger Cart
Stopwatch
Metric Measuring Tape
Brick or Block
Long Board (ramp)
Friction Block
Protractor
q
Figure 6.1
Friction Block
Eqn. 1
2
k
1 kx mgDsin
mgD cos
2
Eqn. 2
k
a mgsin
cos
Eqn. 3
2
2d
a
t