Lab 5: Frictional Force
Purpose
The purpose of this activity is to measure frictional force coefficients for an object
sliding across a surface. Also, we explore and test the model (theory) of constant
frictional coefficients.
Equipment Provided
Required: (1) mass "sleds", (2) mass sets, (3) spring force gauges, (4) PASPORT force sensors, and
(5) PASPORT PowerLinks.
Theory
SIMPLE CASE (ALL FORCES VERTICAL AND HORIZONTAL)
A horizontal force is used to pull a weighted sled across a level table. The forced need to
START the sled in motion is equal in magnitude to the MAXIMUM static friction exerted by the
table surface on the bottom of the sled. The force needed to keep it moving at a constant
velocity is equal in magnitude to the kinetic friction exerted by the tabletop on bottom of the
sled. We can easily draw diagrams to show these forces and relate them to the normal force
exerted by the table on the sled bottom.
W = (ms + mw ) g
ax = ax = 0 (equilibrium)
n = W and P = f
Also, f = μ n
(μ – coeff. of friction)
When the sled is NOT moving the coefficient of friction is the STATIC coefficient and
when the sled is moving at constant velocity the coefficient is the KINETIC friction
coefficient. In the above diagram the force P is the force pulling the sled (measured by
a force gauge), the force f
is the friction force, W is the weight of the sled and added masses, and n is the
normal force from the table on the sled.
, NOTE that above force diagram is treating the added mass PLUS the sled as a single object (SLED
+ MASS). The four forces (f,n,W, and P) are acting on this combined object. If the velocity is zero
or constant, then the acceleration is zero and, therefore, the NET FORCE is also zero. Also, the
STATIC friction is equal to its maximum value only when the applied P is sufficiently large.
GENERAL CASE
The applied force P may not be horizontal and, in fact, the surface may not be level.
For a general situation an accurate analysis of force components must be done to
determine the normal force and the relationship between the pulling force, weight,
and the frictional force.
Purpose
The purpose of this activity is to measure frictional force coefficients for an object
sliding across a surface. Also, we explore and test the model (theory) of constant
frictional coefficients.
Equipment Provided
Required: (1) mass "sleds", (2) mass sets, (3) spring force gauges, (4) PASPORT force sensors, and
(5) PASPORT PowerLinks.
Theory
SIMPLE CASE (ALL FORCES VERTICAL AND HORIZONTAL)
A horizontal force is used to pull a weighted sled across a level table. The forced need to
START the sled in motion is equal in magnitude to the MAXIMUM static friction exerted by the
table surface on the bottom of the sled. The force needed to keep it moving at a constant
velocity is equal in magnitude to the kinetic friction exerted by the tabletop on bottom of the
sled. We can easily draw diagrams to show these forces and relate them to the normal force
exerted by the table on the sled bottom.
W = (ms + mw ) g
ax = ax = 0 (equilibrium)
n = W and P = f
Also, f = μ n
(μ – coeff. of friction)
When the sled is NOT moving the coefficient of friction is the STATIC coefficient and
when the sled is moving at constant velocity the coefficient is the KINETIC friction
coefficient. In the above diagram the force P is the force pulling the sled (measured by
a force gauge), the force f
is the friction force, W is the weight of the sled and added masses, and n is the
normal force from the table on the sled.
, NOTE that above force diagram is treating the added mass PLUS the sled as a single object (SLED
+ MASS). The four forces (f,n,W, and P) are acting on this combined object. If the velocity is zero
or constant, then the acceleration is zero and, therefore, the NET FORCE is also zero. Also, the
STATIC friction is equal to its maximum value only when the applied P is sufficiently large.
GENERAL CASE
The applied force P may not be horizontal and, in fact, the surface may not be level.
For a general situation an accurate analysis of force components must be done to
determine the normal force and the relationship between the pulling force, weight,
and the frictional force.
