SOUTHERNSIDE MONTESSORI SCHOOL
Camella Homes 4, Poblacion, Muntinlupa City
First Quarter
Grade 8 - Science
SY 2018-2019
Notes #3: Friction and Uniform Circular Motion
Friction
it refers to the force that resists motion whenever two materials are in contact with each
other, and are sliding or rolling over each other
static, sliding and rolling occur between solid surfaces
Fluid friction occurs in liquid and gas
Types of Friction
Static (for bodies at rest) – acts on objects when they are resting on a surface.
- Include all case in which the frictional force is enough to prevent relative motion
between surfaces contact.
Sliding/Kinetic (for bodies in sliding motion) – occurs when there is a relative (sliding) motion
at the interface of the surfaces in contact.
- It is weaker than static friction.
Rolling (for bodies in rolling motion) – takes place when one surface rotates as it moves
over another surface but does not slip nor slide at the area of contact.
Fluid – it is a friction that acts on objects that are moving through a fluid (air or liquid)
Factors that influence the total amount of friction
Table 1: Coefficient of Friction (µ)
Materials Coefficient of Static Friction Coefficient of Kinetic Friction
µs µk
Glass on glass 0.94 0.40
Steel on steel 0.58 0.3
Wood on wood 0.4 0.2
Rubber tire on dry concrete 1.0 0.7
Rubber tire on wet concrete 0.7 0.5
Human Joints 0.01 0.01
Frictional Force Equation:
f=µFN or µ=f/FN
Where: f = frictional force
µ= coefficient of friction; its value depends on the
nature of 2 surfaces in contact
FN= Normal force
Advantages and Disadvantages of Friction
Advantages Disadvantages
It makes walking and writing possible It makes moving heavy pieces of
It prevents vehicles from skidding furniture difficult
It holds screws and nails and places It causes wear and tear of machines
Some Ways of Reducing Friction
Lubrication
Use wheels, rollers or ball bearings
Some Ways of Increasing Friction
Patterns on the surface of tires and soles of the shoes,
Usage of powder by different athletes to keep their palms dry
Roads are made up of surfaces to provide grip for vehicle tires
CIRCULAR MOTION
Circular motion -is the movement of an object in a circular path or curved path.
Example:
an artificial satellite orbiting the earth at constant height;
a vehicle travelling around a highway curve or rotunda
Uniform Circular Motion – can be described as motion of an object travelling along a
circular path at a constant speed but with changing velocity
Formula:
𝟐𝛑𝐑
𝑽=
𝐓
Where:
V= velocity T= time
2πr= distance it covers
Centripetal Force –is needed for an object to move in circular motion.
- A force that cause centripetal acceleration.
Using the Newton’s 2nd Law of Motion: Fc= ma, we can come up with the expression for
centripetal force.
𝐯²
Fc = mv2/r ac =
𝐫
Where:
m = mass r = radius where:
v = speed/velocity in a circle
v = velocity
r = radius
Terminologies:
1. Acceleration – the rate of change of velocity per unit time
2. Velocity – a vector quantity whose magnitude is a body’s speed and whose
direction is the body’s direction of motion
3. Center seeking – always toward the center of the circle
4. Period – the time it takes the object to complete one cycle or revolution.
(symbol = “T)
5. Centripetal Acceleration – does not change speed, only changes direction,
also called radial acceleration. (Symbol =” ar” or ” ac”)
Application:
The carousel and Ferris wheel are two amusement rides that employ circular motion. As the
carousel rotates, the swings hang out from the vertical line, thereby enabling chains to
provide the force giving the required centripetal acceleration, while still counteracting the
force of gravity. The motion of the Ferris wheel is quite different. At the top, the centripetal
force is the difference between the weight and the normal force of the seat on the
passenger, with the weight of the passenger greater than the normal force. At the lowest
position, it is still the difference between the forces with the normal force greater than the
weights.
