136
Self-Check
Find the direction of each of the forces in the figure above.
Fluid friction
Try to drive a nail into a waterfall and you will be confronted with the
main difference between solid friction and fluid friction. Fluid friction is
purely kinetic; there is no static fluid friction. The nail in the waterfall may
tend to get dragged along by the water flowing past it, but it does not stick
in the water. The same is true for gases such as air: recall that we are using
the word “fluid” to include both gases and liquids.
Unlike solid kinetic friction, the force of fluid friction increases rapidly
with velocity. In many cases, the force is approximately proportional to the
square of the velocity,
F
fluid friction
.
c
.
Av
2
,
where A is the cross-sectional area of the object,
.
is the density of the fluid,
and c is a constant of proportionality that depends partly on the type of
fluid and partly on how streamlined the object is.
Discussion questions
A. A student states that when he tries to push his refrigerator, the reason it
won’t move is because Newton’s third law says there’s an equal and opposite
frictional force pushing back. After all, the static friction force is equal and
opposite to the applied force. How would you convince him he is wrong.
B. Kinetic friction is usually more or less independent of velocity. However,
inexperienced drivers tend to produce a jerk at the last moment of deceleration
when they stop at a stop light. What does this tell you about the kinetic friction
between the brake shoes and the brake drums.
(1) Normal forces are always perpendicular to the surface of contact, which means right or left in this figure. Normal
forces are repulsive, so the cliff’s force on the feet is to the right, i.e., away from the cliff. (2) Frictional forces are
always parallel to the surface of contact, which means right or left in this figure. Static frictional forces are in the
direction that would tend to keep the surfaces from slipping over each other. If the wheel was going to slip, its
surface would be moving to the left, so the static frictional force on the wheel must be in the direction that would
prevent this, i.e., to the right. This makes sense, because it is the static frictional force that accelerates the
dragster. (3) Normal forces are always perpendicular to the surface of contact. In this diagram, that means either
up and to the left or down and to the right. Normal forces are reulsive, so the ball is pushing the bat away from
itself. Therefore the ball’s force is down and to the right on this diagram.
Chapter 5Analysis of Forces
3. the ball's normal
force on the bat
1. the cliff's normal
force on the
climber's feet
2. the track's static fric-
tional force on the wheel