Wikipremed Friction Problem

[shffl]

http://www.wikipremed.com/01physicscards.php?card=95

Can anyone explain how 5m/s^2 was obtained?

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[TwoPaddles]

http://www.wikipremed.com/01physicscards.php?card=95

Can anyone explain how 5m/s^2 was obtained?

f = µN
µ = 0.5
f = 0.5N ---> frictional force is equal to half of the normal force
The man is on a horizontal surface, so Normal force = gravity
f = 0.5*gravity
gravity = 10m/s2
f = 5m/s2
This value is responsible to accelerate the man while he slides backward.

 

[shffl]

f = µN
µ = 0.5
f = 0.5N ---> frictional force is equal to half of the normal force
The man is on a horizontal surface, so Normal force = gravity
f = 0.5*gravity
gravity = 10m/s2
f = 5m/s2
This value is responsible to accelerate the man while he slides backward.

thanks for the help. i was wondering what happened to mass since normal force = mg. how did you get normal force = gravity instead? thanks

 

[TwoPaddles]

thanks for the help. i was wondering what happened to mass since normal force = mg. how did you get normal force = gravity instead? thanks

Yeah well they didn't give the mass. On a horizontal surface, force and gravity cancel each other out.
That's why when you push an object across a horizontal surface, only the applied force and friction (opposing that applied force) is taken into account (and not the normal force or gravity). But when you are on an incline, the situation is different.

 

[shffl]

Yeah well they didn't give the mass. On a horizontal surface, force and gravity cancel each other out.
That's why when you push an object across a horizontal surface, only the applied force and friction (opposing that applied force) is taken into account (and not the normal force or gravity). But when you are on an incline, the situation is different.

I see. Thanks! So am I to assume that normal force = gravity on horizontal surfaces? Even if mass was included in the problem, I don't have to use it in my calculations?

 

[TwoPaddles]

I see. Thanks! So am I to assume that normal force = gravity on horizontal surfaces? Even if mass was included in the problem, I don't have to use it in my calculations?

Well those kind of problems usually involve friction or work. So I guess it depends on the problem that you are asked for. For example, if you were asked a "work" problem, you use either conservation of energy or the work-friction equation: W=-fx ; where x is the displacement.
and you already know the formula for friction: f = µN
On a horizontal surface N = mg.. so the equation becomes f = µmg = W
So here the mass is taken into account.
In the problem that you did, friction was what caused the man to accelerate.

Maybe this explanation didn't make sense but it just really depends on what you are being asked. I got used to it through practice.

 

[shffl]

Ah I think I'm slowly getting it. If F = ma and friction is the force that's causing the man to accelerate, then f = ma --> f = µmg --> µmg = ma --> µg = a --> (0.5)x10 = 5m/s^2

Thanks!!