# Friction Challenge (1 Viewer)

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#### 'AQF

Can anyone think of a way where friction increases the speed of a moving body, instead of decreasing it? Or prove that this is impossible.

#### arildno

Homework Helper
Gold Member
Dearly Missed
Well, in the case of STATIC friction, one might, if you wish, say that the static friction accelerates one of the bodies (think of two boxes on top of each other, the lower box receives an external force, whereas the top box is only influenced by the static friction between itself and the other box).

#### 'AQF

Is there any case where kinetic friction would help accelerate an object? For example, if a wedge is being accelerated at a very high rate, it is possible for the mass to travel up the wedge despite gravity. Would this be considered as speeding up?

#### ZapperZ

Staff Emeritus
2018 Award
[QUOTE='AQF]Is there any case where kinetic friction would help accelerate an object? [/QUOTE]

A sailboat.

Zz.

#### 'AQF

Can you expain this?

#### Tide

Homework Helper
Everytime I drop my luggage on the conveyor at the airport it speeds up! :)

#### russ_watters

Mentor
[QUOTE='AQF]Can you expain this?[/QUOTE] A sailboat going straight downwind essentially uses its sails as a big parachute - and that's kinetic/dynamic friction (drag).

[A sailboat going upwind (at an angle) uses lift.]

#### DaTario

if the ground irregularities have a sawtooth appearance (microscopically speaking, of course) and the object which is on it also has this appearance in the interface, then it is possible in principle for the thermal motion to produce movement, which represents acceleration if you start with the object at rest.

Best Regards,

DaTario

#### Danger

Gold Member
In line with ZZ and Russ' answers, a car wouldn't accelerate very well without friction between the tires and the ground; that, however, sidesteps what I think might be the original question of friction alone causing movement.

#### 'AQF

Can friction alone cause motion, or is this impossible?

Mentor

#### nrqed

Homework Helper
Gold Member
[QUOTE='AQF]Is there any case where kinetic friction would help accelerate an object? For example, if a wedge is being accelerated at a very high rate, it is possible for the mass to travel up the wedge despite gravity. Would this be considered as speeding up?[/QUOTE]

Yes, I would say so. If you have an object on a surface. Let's say a plate on piece of fabric. And you pull violently on the clothing (to the right, say). I think that for a short time, the plate will be moving to the right (and sliding against the piece of fabric) and will be accelerating to the right. The fabric is moving faster than the plate, so in the frame of the piece of fabric, the plate is moving to the left with a kinetic friction acting to the right. But I think it's possible to pull the fabric with sufficient force that the plate will accelerate to the right due to the kinetic friction. But that's not based on an actual proof, just my intuition.

The example of the sailboat I think is a bit a stretch because no kinetic friction in the usual sense (due to two surfaces sliding against each other) is involved. I don't see how a drag force could be considered a kinetic friction force. This is all semantics, but that's my opinion.

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