Download Section 5.2 Friction

Mr. Borosky
Physics Section 5.2 Notes
Page 1 of 3
Section 5.2 Friction
Objectives
Define the friction force.
Distinguish between static and kinetic friction.
Read intro paragraph p. 126
Friction – force opposing relative motion of 2 objects that are in
contact. Examples tires on road, shoes on ground. The direction of
the force is parallel to the surface and in a direction that opposes
the slipping of the 2 surfaces.
STATIC AND KINETIC FRICTION
Read Section.
Kinetic Friction – the force exerted on one surface by a second
surface when the 2 surfaces rub against one another because one or
both of the surfaces are moving. Also called Sliding Friction.
From Old Book
Sliding Friction – force between 2 surfaces in relative motion.
The force of sliding friction is less than that of static
friction. Thus a car will stop faster if the wheels are not
skidding.
To keep a book sliding at constant velocity on a desk you must
exert a constant force that is just the same size as the
frictional force but in the opposite direction.
Experimentally it has been found that the force of friction
depends primarily on the force pushing the surfaces together,
FN, and on the nature of the surfaces in contact. FN is the
force pushing the surfaces together (Normal or Perpendicular
Force)
Static Friction – the force exerted on one surface by a second
surface when there is no motion between the 2 surfaces.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 5.2 Notes
Page 2 of 3
From Old Book
Static Friction – force that opposes start of motion between 2
surfaces. Static friction forces have maximum values.
When the magnitude of your push on the box is greater than the
max value of the static friction between the floor and the box,
the box starts moving. When the box starts to move the force
of friction decreases.
Once your force is greater than this maximum STATIC Friction, the
couch begins moving and KINETIC Friction begins to act instead of
static friction.
You have Static Friction acting when trying to get an object moving
and once it is moving you have Kinetic or Sliding Friction acting.
Go Over example from book dealing with figure 5.8 p. 126.
A MODEL FOR FRICTION FORCES
On what does a Frictional Force depend?
1. The materials that the surfaces are made of play a role.
2. The normal force between the 2 objects. The harder one object
is pushed against the other, the greater the force of Friction.
If you pull a block along a surface at a constant velocity,
according to Newton’s Laws, the Frictional Force must be equal and
opposite to the force with which you pull.
Coefficient of Kinetic Friction - ratio of frictional force and the
normal force between 2 surfaces. It is indicated by the symbol “”.
It is a constant that depends on the 2 surfaces in contact. This is
for an object that is already moving.
Kinetic Friction Force – the force exerted on one surface by a
second surface when the 2 surfaces rub against one another because
one or both of the surfaces are moving. It is equal to the product
of the coefficient of kinetic friction and the normal force.
FFKinetic = μKFN
FF = μFN
Coefficient of Static Friction - ratio of frictional force and the
normal force between 2 surfaces. It is indicated by the symbol “”.
It is a constant that depends on the 2 surfaces in contact. This is
for an object you are trying to get to move.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 5.2 Notes
Page 3 of 3
Static Friction Force - the force exerted on one surface by a second
surface when there is no motion between the 2 surfaces. It is less
than or equal to the product of the coefficient of static friction
and the normal force.
FFStatic ≤ μSFN
FF = μFN
The static Friction Force will increase up to a maximum value before
it is overcome and motion starts.
Do Example Problem 3 p. 128
Fg = mg = 25(9.8) = 245 N and Fg = FN
FPush = μFN = .2(245) = 49 N
(note .2 come from table 5.1)
Do Practice Problems p. 128 # 17-21
Show and Go Over Table 5.1 p. 129
Do Example Problem 4 p. 129
FPush doubles to 98 N
and FF = original FPush
FNet = FPush – FF
ma = 98 – 49
25a = 49
a = 1.96 m/s2
Do Practice Problems p. 130 # 22-26
Important Things to remember about Friction
1. Friction acts in the direction opposite to the motion.
2. The magnitude of the force of friction depends on the magnitude
of the normal force between the 2 surfaces.
3. Multiplying the coefficient of static friction and the normal
force gives you the maximum static friction force.
Do 5.2 Section Review p. 130 # 28-32
(SKIP 27 and 31)
Physics Principals and Problems © 2005 Started 2006-2007 School Year