Friction Questions
Practice Problems
Friction Relationships
Rank the friction on the 1.0 kg from greatest to least
Phonebook Friction
- Mythbusters
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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) What minimum force is needed to start this block moving (fs)?
fs = Fnus = Fgus = mgus
= 5.0 kg(9.8 m/s2)(.75)
b) What is the frictional force on this object as it moves?
fk = mguk
= 5.0 kg(9.8 m/s2)(.57)
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
Friction Questions
Coefficients of Friction & F = ma
Remember ...
Static Friction fs = UsFn
Kinetic Friction fk = UkFn
On Horizontal Surface
fs = usFg (force to start object moving)
fk = ukW AND W = mg
Example 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 fk = ?
fk = UkFg = .58 x 1000. N
Example 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
fk = 50. N w = ?
uk = .05
fk = ukFg
50. N = .05[Fg]
Example 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 = ?
Uk = .30
fk = Fnuk = Fguk = mguk
= (10. kg)(9.8 m/s2).30
Example 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?
m = 53. kg a = -.10 m/s2
fk = F = ma
fk = F = 53. kg(-.10 m/s2)
Example 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?
Extension: Friction on a Ramp
How does the magnitude of the friction force on a box on a ramp change when the angle of the ramp increases?
Review - Which law of motion best explains this?
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