Kinds of Friction
Physical Science
Name: ________________________________________________ Period: _______ Date: ___________
Essential Question: How do I classify frictional forces into one of four types: static, sliding, rolling, and fluid ?
Fric Friction is a force that objects have which makes them resist motion or movement across or against one
another. If one object with a flat surface is placed on top of another, the top object can be lifted without any act
of resisting except that of gravity. If one object is pushed or pulled over another, there is an act of resistance
that is caused by friction. If one object moves over a stationary object, the stationary object will slow down the
moving object. If a force tries to move an object at rest on a surface, the surface resists this motion with an
opposing force of friction. The size or amount of the frictional force depends on the kind of surfaces which are
rubbing together.
Friction can be used for many important things such as helping the wheels of a train stay on the rails of the
track. It also allows a mechanical device that carries things from one place to another also known as a conveyer;
it helps the belt of a conveyor stay on pulleys without slipping. We need friction to walk; if we did not have any
friction, everything would be slipping and sliding away. That is why we can't walk on ice very well; the ice is so
smooth and flat that it produces less friction than a sidewalk, making it hard to walk on ice.
Friction occurs any time when two objects or surfaces are rubbed together. The more the pressure on the
objects when they are rubbed together, the more friction. If you are trying to slow down, friction works with
you, but when you try to speed up, friction works against you.
Different methods can be used to reduce the amount of friction between the surfaces of the objects in contact.
One way to reduce friction is to apply a lubricant to the surfaces, another is to use casters, rollers, or ball
bearings between the surfaces, and another is to smooth the surfaces of the objects in contact.
Different kinds of motion give rise to different types of friction between objects. Static friction occurs between
stationary objects, while sliding friction occurs between objects as they slide against each other. Other types of
friction include rolling friction and fluid friction. Static friction prevents an object from moving against a surface.
It is the force that keeps a book from sliding off a desk, even when the desk is slightly tilted, and that allows you
to pick up an object without the object slipping through your fingers. In order to move something, you must first
overcome the force of static friction between the object and the surface on which it is resting. This force
depends on the coefficient of static friction (µs) between the object and the surface and the normal force (N) of
the object.
A book sliding off a desk or brakes slowing down a wheel are both examples of sliding friction, also called kinetic
friction. Sliding friction acts in the direction opposite the direction of motion. It prevents the book or wheel
from moving as fast as it would without friction. When sliding friction is acting, another force must be present to
keep an object moving. In the case of a book sliding off a desk, this force is gravity.
Rolling friction hinders the motion of an object rolling along a surface. Rolling friction slows down a ball rolling
on a basketball court or softball field, and it slows down the motion of a tire rolling along the ground. Another
force must be present to keep an object rolling. For example, a pedaling bicyclist provides the force necessary to
the keep a bike in motion.
Objects moving through a fluid experience fluid friction, or drag. Drag acts between the object and the fluid and
hinders the motion of the object. The force of drag depends upon the object’s shape, material, and speed, as
well as the fluid’s viscosity. Viscosity is a measure of a fluid’s resistance to flow. It results from the friction that
occurs between the fluid’s molecules, and it differs depending on the type of fluid. Drag slows down airplanes
flying through the air and fish swimming through water. An airplane’s engines help it overcome drag and travel
forward, while a fish uses its muscles to overcome drag and swim. Calculating the force of drag is much more
complicated than calculating other types of friction. (see Aerodynamics)
http://www.edu.pe.ca/southernkings/friction.htm
Clarifying Questions:
1. What is the force that makes object resist motion?
_______________________________________________________________________________________
2. What is the force that resist motion when lifting an object? Explain.
___________________________________________________________________________________________
_______________________________________________________________________________________
3. what does a stationary object do to a moving object?
_______________________________________________________________________________________
4. The size or amount of frictional force depends on ____________________________________________
_______________________________________________________________________________________
5. How does friction help trains work?
___________________________________________________________________________________________
_______________________________________________________________________________________
6. Why do we need friction to walk?
___________________________________________________________________________________________
_______________________________________________________________________________________
7. When does friction occur or happen?
_______________________________________________________________________________________
_______________________________________________________________________________________
8. How is friction increased? Read and Think!
_______________________________________________________________________________________
_______________________________________________________________________________________
9. How is friction reduced? Read and Think!
___________________________________________________________________________________________
_______________________________________________________________________________________
10. What are the four kinds of friction?
___________________________________________________________________________________________
_______________________________________________________________________________________
11. What must you overcome first before you can move an object?
___________________________________________________________________________________________
_______________________________________________________________________________________
12. What is another name for sliding friction?
_______________________________________________________________________________________
_______________________________________________________________________________________
13. What is the direction of a sliding friction?
_______________________________________________________________________________________
_______________________________________________________________________________________
14. What does rolling friction do to the motion of a rolling ball?
_______________________________________________________________________________________
_______________________________________________________________________________________
15. What hinders the motion of an object on liquid and gas ( fluid )?
___________________________________________________________________________________________
_______________________________________________________________________________________
16. The amount of fluid friction or drag depends on _____________________________________________
_______________________________________________________________________________________
_______________________________________________________________________________________
17. What is aerodynamics? ( Research) think! You see this in cars, airplanes, helmets etc.
___________________________________________________________________________________________
_______________________________________________________________________________________