# Can someone explain to me what's the thing with surface normal and friction force?

Asked by Christian95 (3258) October 31st, 2009

This is what I study now at physics and my teacher gave me some extra-problems very very difficult and I’m struggling to solve them but I’m stuck at this problems.

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Force normal is the force facing upwards, perpendicular to the object. It is what prevents you from falling down when sitting on a chair. Frictional force can be in any direction depending on context. Imagine a box sliding down a slope, the sliding down direction would be the force of gravity and the force acting towards moving the box towards the top of the slope would be force of friction/resistance.

That’s how I think of the two anyway. It’s been a while since i studied physics. Could you clarify what you are having trouble with? At this point, your Q “what’s the thing” is rather vague.

jlm11f (12350)

Also, friction increases the closer the applied force is to the normal force. That is, if the force being applied is the same direction as the normal, it’s at the highest amount. If the force is perpendicular, it’s lower.

BhacSsylan (9520)

Are you asking for the equations, or just the general definition of the normal and friction forces?

These might help
“http://en.wikipedia.org/wiki/Normal_force”
“http://en.wikipedia.org/wiki/Friction”

@all Sigh, now it’s not linking and I can’t edit it anymore. But you know what to do @Christian95

The normal force is a reactionary force given my Newton’s Third Law. Let’s say you have a block sitting on a table. The force of gravity pulls the block downwards, but similarly, the table pushes back with a normal force upwards. If the block isn’t moving, and the table is level, the normal force is equal in magnitude to the force of gravity and opposite in direction.

The friction force acting on an object is related to the normal force. More precisely, the friction force is equal to the normal force multiplied by a constant, called the coefficient of friction, which depends on the substances used (e.g. rubber on concrete, steel on ice, etc.). A different coefficient of friction is used depending on whether the object is stationary, sliding, or rolling.

If you have any questions, feel free to ask.

Ivan (13429)

Well gosh, I was gonna say exactly what Ivan just said…..

The definition of normal might be of some help as well.

I might add that the normal force also opposes any component of the force applied to the block in the direction of the normal. Also, the friction force always opposes the direction of possible, or actual, motion. A block will have a static coefficient of friction that is usually higher than it’s sliding coefficient of friction. The static friction force prevents the block from moving to the point at which the component force applied to the block in the direction parallel to the surface exceeds the static friction force. After the static friction is overcome the sliding coefficient of friction is used. You might notice this when you’re moving furniture around the house, or something similar. It will be hard to get the object sliding, but much easier to keep it sliding. There are similar analogs for wheel friction coefficients.

I’m not sure if you’ve gone over this but if you haven’t it’s fairly important in solving these “sliding block” problems. Vector Components

I’d guess that you might have already learned vector stuff if you’re in a high school physics class but just in case…..

Shuttle128 (2969)

They both play off of each other. In order to calculate a frictional force you need a normal force which is equal and opposite to gravity. If there were, let’s say, no friction on a hockey puck, it would weigh nothing vertically. Horizontally however it would still feel heavy on a stuck or when it impacts against the side boards.
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By the way, if you need any help with a Physics problem, feel free to PM me and I can give it a shot. (1st year Bachelor of Science)

LeopardGecko (1232)

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