Force of Friction
Force of Friction
Force of Friction
FORCE OF FRICTION
The magnitude of the force of static friction between any two surfaces in contact
can have the values
𝑓𝑠 ≤ 𝜇𝑠 𝑛
where the dimensionless constant 𝜇𝑠 is called the coefficient of static friction and
n is the magnitude of the normal force exerted by one surface on the other. The
equality in the equation holds when the surfaces are on the verge of slipping, that
is, when 𝑓𝑠 = 𝑓𝑠,𝑚𝑎𝑥 = 𝜇𝑠 𝑛. This situation is called impending motion.
The inequality holds when the surfaces are not on the verge of slipping.
𝑓𝑠 ≤ 𝜇𝑠 𝑛
𝜇𝑘 is the coefficient of kinetic friction. Although the coefficient of kinetic friction
can vary with speed, we shall usually neglect any such variations in this text.
FORCE OF FRICTION
FORCE OF FRICTION
QUICK QUIZ 1
You press your physics textbook flat against a vertical wall with your hand. What is
the direction of the friction force exerted by the wall on the book?
(a) downward (b) upward (c) out from the wall (d) into the wall
QUICK QUIZ 2
You are playing with your daughter in the snow. She
sits on a sled and asks you to slide her across a flat,
horizontal field. You have a choice of
(a) pushing her from behind by applying a force
downward on her shoulders at 30° below the horizontal
or
(b) attaching a rope to the front of the sled and pulling
with a force at 30° above the horizontal.
Which would be easier for you and why?
ANSWER: (b) Attaching a rope
FORCE OF
FRICTION
Experimental Determination of
static friction and kinetic
friction
The following is a simple method
of measuring coefficients of
friction. Suppose a block is
placed on a rough surface
inclined relative to the horizontal
as shown in the figure. The
incline angle is increased until
the block starts to move. Show
that you can obtain ms by
measuring the critical angle uc at
which this slipping just occurs.
FORCE OF
FRICTION
Example 1
A hockey puck on a frozen
pond is given an initial speed
of 20.0 m/s. If the puck always
remains on the ice and slides
115 m before coming to rest,
determine the coefficient of
kinetic friction between the
puck and ice.
FORCE OF FRICTION
Example 2 (Seatwork)
A block of mass 𝑚2 on a rough, horizontal
surface is connected to a ball of mass 𝑚1
by a lightweight cord over a lightweight,
frictionless pulley as shown in the figure. A
force of magnitude 𝐹 at an angle 𝜃 with
the horizontal is applied to the block as
shown, and the block slides to the right.
The coefficient of kinetic friction between
the block and surface is 𝜇𝑘 . Determine the
magnitude of the acceleration of the two
objects.
CONCEPTUAL QUESTIONS