# 5.1 Friction  (Page 3/12)

 Page 3 / 12

## Take-home experiment

Find a small plastic object (such as a food container) and slide it on a kitchen table by giving it a gentle tap. Now spray water on the table, simulating a light shower of rain. What happens now when you give the object the same-sized tap? Now add a few drops of (vegetable or olive) oil on the surface of the water and give the same tap. What happens now? This latter situation is particularly important for drivers to note, especially after a light rain shower. Why?

Many people have experienced the slipperiness of walking on ice. However, many parts of the body, especially the joints, have much smaller coefficients of friction—often three or four times less than ice. A joint is formed by the ends of two bones, which are connected by thick tissues. The knee joint is formed by the lower leg bone (the tibia) and the thighbone (the femur). The hip is a ball (at the end of the femur) and socket (part of the pelvis) joint. The ends of the bones in the joint are covered by cartilage, which provides a smooth, almost glassy surface. The joints also produce a fluid (synovial fluid) that reduces friction and wear. A damaged or arthritic joint can be replaced by an artificial joint ( [link] ). These replacements can be made of metals (stainless steel or titanium) or plastic (polyethylene), also with very small coefficients of friction.

Other natural lubricants include saliva produced in our mouths to aid in the swallowing process, and the slippery mucus found between organs in the body, allowing them to move freely past each other during heartbeats, during breathing, and when a person moves. Artificial lubricants are also common in hospitals and doctor’s clinics. For example, when ultrasonic imaging is carried out, the gel that couples the transducer to the skin also serves to to lubricate the surface between the transducer and the skin—thereby reducing the coefficient of friction between the two surfaces. This allows the transducer to mover freely over the skin.

## Skiing exercise

A skier with a mass of 62 kg is sliding down a snowy slope. Find the coefficient of kinetic friction for the skier if friction is known to be 45.0 N.

Strategy

The magnitude of kinetic friction was given in to be 45.0 N. Kinetic friction is related to the normal force $N$ as ${f}_{\text{k}}={\mu }_{\text{k}}N$ ; thus, the coefficient of kinetic friction can be found if we can find the normal force of the skier on a slope. The normal force is always perpendicular to the surface, and since there is no motion perpendicular to the surface, the normal force should equal the component of the skier’s weight perpendicular to the slope. (See the skier and free-body diagram in [link] .)

is there a summary for this whole physics book
what the difference between laws and principles
principle are like rules... it can change BUT LAW IS LAW... confirmed theory
Fortune
Hello want to study physics but it seems so difficult
Yes
Aifuwa
I read it but when I try to solve questions it seems difficult
Aifuwa
Jerry where are you from
Joshua
can i study some problem on motion and projection motion
Weamie
hello pls am new here so i want someone to highlight me on this "application of expansion/contraction
Awoke
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rebecca
yess
rebecca
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Awoke
yess
rebecca
ok
Awoke
iam new here
Mohamed
u are welcome
Awoke
hello am new. am I welcome
Miracle
yes you are😁
clifford
we can all learn and help each other
Osei
yeah you are right
Neyaz
so we can all share our ideas on a certain topic
Osei
sure
clifford
in beginning all subjects are difficult need to work hard
Mohammed
hi guyz
Lamin
How can I understand physics
Thankgod
Where will I start from
Thankgod
(fundamentals of physics) start there
clifford
OK thanks
Thankgod
Hello, im college now, engineering, 1 years, what book of physics i must use, to be understand easily?
Megryan
try physics for scientist and engineers/ cutnell and Johnson
clifford
Okey po!
Megryan
Is not difficult if u really want to learn
Gabriel
explanations on harmonic motion
example ofchange of state of the body in the effectof heat
solid ice = liquid water due to radical changes in temperature which are controlled by the capacity of heat an object can withstand.
Shii
what is the upper fix point of the temperature scale
Mohamed
why is the thermal expansion of thermometric liquid in glass increased in volume
Mohamed
what's heat?
sununu
heat is energy, it called heat energy. Energy moves from a hot body to a cool body. Fire transfers energy to a pot of water and the more energy the water has the hotter it becomes.
clifford
Heat is the transfer of energy from one place to another
Gabriel
what is normal force?
the force that pushes upward on us. the force that opposes gravity
clifford
upthrust of air
Newton's 3rd law. the force of the ground (earth) that pushes back on gravity, keeping us on the ground instead of sinking into it.
clifford
I really need lots of questions on frictional force
Shii
I can help answering what I can
Shii
does friction also need some force to perform?
Mohit
no friction is a force just like the gravitational force
clifford
yeah but u can't apply friction anywhere else like other forces
Mohit
I don't understand that question. friction does work alongside other forces based on the situation.
clifford
eg. when walking there are two forces acting on us gravitational and frictional force. friction helps us move forward and gravity keeps us on the ground
clifford
friction is a contact force. Two surfaces are necessary for the force to work.
clifford
hope this helped
clifford
the friction force which oppose while it contact with surrounding. there are two kind of friction. slidding and rolling friction.
Neyaz
Two unequal masses M1 and M2 are connected by a string of tension T on a plane,find the acceleration and tension in the string
Ogboru
derive the equation
Ogboru
Hi
Olamide
What is physics?
physics is a branch of science in which we are dealing with the knowledge of our physical things. macroscopic as well as microscopic. we are going look inside the univers with the help of physics. you can learn nature with the help of physics. so many branches of physics you have to learn physics.
vijay
What are quarks?
6 type of quarks
Neyaz
what is candela
Candela is the unit for the measurement of light intensity.
Osei
any one can prove that 1hrpower= 746 watt
Newton second is the unit of ...............?
Neyaz
Impulse and momentum
Fauzia
force×time and mass× velocity
vijay
Good
Neyaz
What is the simple harmonic motion?
oscillatory motion under a retarding force proportional to the amount of displacement from an equilibrium position
Yuri
Straight out of google, you could do that to, I suppose.
Yuri
*too
Yuri
ok
Fauzia
Oscillatory motion under a regarding force proportional to the amount of displacement from an equilibrium position
Neyaz
examples of work done by load of gravity
What is ehrenfest theorem?
You can look it up, faster and more reliable answer.
Yuri
That isn't a question to ask on a forum and I also have no idea what that is.
Yuri
what is the work done by gravity on the load 87kj,11.684m,mass xkg[g=19m/s
Maureen
What is law of mass action?
rate of chemical reactions is proportional to concentration of reactants ...
ok thanks
Fauzia
what the difference between laws and principles
Mary