0303 Friction
0303 Friction
0303 Friction
Friction
The force that opposes the relative motion of two objects in contact is
called friction. Friction is a vector quantity and is always parallel to the
two surfaces in contact. The force of friction F f is directly proportional to
the magnitude of the normal force, F N .
As you already know, the normal force is the force pressing the two
contacting surfaces together. On a horizontal surface, the normal force is equal in magnitude and
opposite in direction to the weight, F g where F g=mg , of the object resting on the surface. The
normal force is always perpendicular to the surface. Therefore, on an incline, the normal force will
not be equal to the weight but rather F g cos θ or mg cos θ . (We’ll deal with hills later).
The force of friction also depends on the coefficient of friction (μ) which is a constant that has no
units. The coefficient of friction is unique to the surface that an object is moving on. The higher the
coefficient of friction, the rougher the surface, and thus the more force of friction. The coefficient
of friction is usually a decimal.
F f =μ F N
For Example:
1.) What will have a higher coefficient of friction. A dry road or a wet road?
(Which will have the rougher surface?)
4.) Julie pushes a 20 kg goat across the highway with a force of 100 N. The coefficient of friction
between the goat’s hooves and the highway is 0.30.
e. Will Julie be able to push the goat? If so, with what acceleration?
5.) Tom wishes to push a basketball hoop to the other side of a street. Tom knows that the force of
friction between the basketball hoop and the street is 100 N. If the basketball hoop as a mass of
30 kg, determine the coefficient of friction between the hoop and the street.
6.) The coefficient of friction between a shopping cart and the parking lot is 0.2. When Jillian is
finished shopping for her family’s holiday feast, the force of friction between the cart and the
parking lot is 70 Newtons. Determine the mass of the shopping cart and its items.
Now that we’ve covered the basics of friction, let’s look at the two types of friction.
Kinetic Friction: the friction between objects in contact when they are in motion.
The force of kinetic friction for two surfaces in contact is less than the force of static friction for the
same two surfaces (meaning the coefficient of kinetic friction is less than the coefficient of static
friction). For example, the coefficient of kinetic friction for copper on steel is 0.36 and the coefficient of
static friction for copper on steel is 0.53.
Note: it is harder to push an object to start moving than it is to push an object already in motion.
For Example:
7.) What is harder to push: a refrigerator at rest or a refrigerator that is already moving?
8.) During a heist, Ray tries to push a 100-kg safe out of a third story window (30 meters high) into
the back of his pick-up truck down below. The static coefficient of friction is 0.80 while the
kinetic coefficient of friction is 0.40.
b. Once the safe begins to move, how much force will Ray
apply to move it at a constant velocity?
c. Once out the window, how much time before the safe hits the pick-up truck?
d. What will be the safe’s velocity right before it hits the pick-up truck?
9.) A 40-kg bookshelf requires a 200 N force to begin its motion. Once moving, it only requires a
force of 50 N.
10.) A 10-kg computer tower is pushed to the right with an acceleration of 5 m/s 2 using a force of
100 N.