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Phonebook Friction - Mythbusters

School Blocks YouTube? Download the file below.

Mythbusters_-_Phone_Book_Friction.mov

Ranking Task

Rank the friction on the 1.0 kg from greatest to least

rampfriction

 

 

 

 

 

FRICTION PROBLEMS

 

 

 

*Remember
Fn = Weight = mg
on horizontal surface

 

 

Ex) A 5.0 kg Steel block is resting on a *horizontal table*.  The coefficient of static friction (us) is 0.75 and the uk is 0.57

a) Minimum force is needed to start this block moving (fs)? 

 

Ex) A 5.0 kg Steel block is resting on a *horizontal table*.  The coefficient of static friction (us) is 0.75 and the uk is 0.57.

a) What minimum force is needed to start this block moving?

 

fs= Fnus = Fgus = mgus

 

fs = mgus

 

= 5.0 kg(9.8 m/s2)(.75)

 

= 37. N

 

 

 

 

 

 

 

b) What is the frictional force on this object as it moves?

 

fk = mguk

= 5.0 kg(9.8 m/s2)(.57)

 

= 28 N

 

 

 

 

 

 

c) What force must be applied to the object to keep it moving at constant velocity? (Hint: Fnet = 0)

At Constant velocity

 

F applied =  friction force

 

= 28 N

 

 

 

 

 

 

Friction Questions

 

Coefficients of Friction & F = ma

 

 

 

 

 

 

 

Remember ...

Static Friction    fs = UsFn

Kinetic Friction  fk = UkFn

 

 

On Horizontal Surface

fs = usW

(force to start object moving)

fk = ukW       AND   W = mg

 

 

Ex 1) A 1000. N car skids on a wet concrete road. If the road is horizontal, what is the friction force on the car?

Ex 1) A 1000. N car skids on a wet concrete road. If the road is horizontal, what is the friction force on the car?

 

w = 1000. N

Uk  = .58

f =

fk = UkFg = .58 x 1000. N

 

= 5.8 x 102 N

 

 

Ex 2) A skier is being pulled along a horizontal surface at constant speed with a force of 50. N.

 What is the weight of the skier?

 

Ex 2) A skier is being pulled along a horizontal surface at constant speed with a force of 50. N.

 What is the weight of the skier?

 

At constant speed:
 
fk = Applied Force

 

f = 50. N

w = ?

uk = .05

fk =ukFg

50. N = .05[Fg]  

       

w = 1000 N

  

 

 

Ex 3) A 10. kg block of wood sliding on a horizontal wooden table is brought to rest.
What is the force on the block of wood that caused it to stop?

 

 

Ex 3) A 10. kg block of wood sliding on a horizontal wooden table is brought to rest. What is the force on the block of wood that caused it to stop?
 

The force of kinetic friction
stops the block.

fk = ?

 

U= .30

 

fk = Fnuk = Fguk = mguk

 

fk = mguk

 

fk = (10. kg)(9.8 m/s2).30 = 29. N

 

 

 

Ex 4) A 53. kg block, slowed by friction, has an acceleration of -0.10 m/s2.

What is the force of friction on the block?

 

 

Ex 4) A 53. kg block, slowed by friction, has an acceleration of -0.1 m/s2. What is the force of friction on the block?

 

m = 53. kg

a = -.10 m/s2

 

fk = F = ma

 

fk=F = 53. kg(-.10 m/s2)

 

= -5.3 N

 

Ex 5) A force of 8.0 N gives a 3.0 kg mass an acceleration of 2.0 m/s2  to the right.

 

What is the friction on the block?

 

Ex 5) A force of 8.0 N gives a 3.0 kg mass an acceleration of 2.0 m/s2  to the right.

What is the friction on the block?

Fapplied = 8.0 N Right

 

m = 3.0 kg

 

a = 2.0 m/s2  Right

 

 

Fnet = ma = 3.0 kg(2.0 m/s2 )

 

= 6.0 N


What friction force will produce a net force of 6.0 N?

 

 

 

friction = 2 N left

 

 

 

 

How does the magnitude of the friction force
on a box on a ramp change when the angle
of the ramp increases?

Free Body Diagram

 

[ HTML5 ]

 

 

 

 

Friction decreases because the
normal force decreases

 

 

 

Review - Which law of motion best explains this?

School Blocks YouTube? Download the file below.

Liquid_Nitrogen_ping_pong_ball.mp4

 

 

 

 

 

3rd Law of Motion - Action/Reaction

 

Review

Friction Video - Georgia Pacific Broadcasting

 

 

 

***AP Pulleys***

 

 

 

Momentum

 

 

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