Phonebook Friction - Mythbusters

Mythbusters_-_Phone_Book_Friction.mov

Rank the friction on the 1.0 kg from greatest to least

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?

Friction decreases because the
normal force decreases

Review - Which law of motion best explains this?

3rd Law of Motion - Action/Reaction

Review

Friction Video - Georgia Pacific Broadcasting

***AP Pulleys***