Confused about what direction friction really acts in.

[Denu]

If an object is moving forwards on a rough plane, we assume that there is a force opposing its motion, namely friction or resistance. This friction includes the friction because of the contact between the moving object and the ground. From Newton's third law, we can conclude that because the object exerts a force of let's say A Newtons on the ground, the ground exerts a force of -A Newtons on the object (magnitude = A, but in the opposite direction). This is when I get totally confused. When we are solving problems involving these laws, and we form equations according to Newton's second law (Fnet = ma), why do we take the direction of friction to be backwards or opposite to the direction of the driving force? It sure sounds stupid of doing otherwise because we know that friction opposes motion, and in this case the direction of the frictional force would be opposite to the direction of the driving force of the object. BUT! Isn't the frictional force which is exerted ON the object BY the ground in the same direction as the driving force of the object? Aren't we only concerned with forces acting ON an object when we use Fnet = ma.

I know the question above seems mind boggling, but I need some answers. Please, I don't care if you call me stupid for asking such questions, but could I get some answers over here? I can't seem to progress anywhere...

 

[Naty1]

"..why do we take the direction of friction to be backwards or opposite to the direction of the driving force?"


Yes, the forces of acceleration and friction each act on a body in question. Friction is "smart": it always opposes other forces, and so the sign is opposite.

For static friction the frictional force is exactly what it must be in order to prevent motion between the surfaces; it balances (opposes) the net force tending to cause such motion.

If an accelerating force is causing a body to move to the right, you can be sure friction is opposing it by dragging to the left. Similar with rotational motion: if a fan or an engine, say, is spinning clockwise, that darn friction of air or bearings, respectively, is dragging counterwise. If a piston is moving up, friction is dragging it down; when the piston falls, friction reverses direction and pulls up! Very "smart" that friction!...

More details here:
http://en.wikipedia.org/wiki/Friction

 

[AlephZero]

BUT! Isn't the frictional force which is exerted ON the object BY the ground in the same direction as the driving force of the object?

No, you have got it the wrong way round for some reason.

I read your OP sevral times, but I can't see any clues WHY you have got it back to front. You understand the idea of action and reaction being equal and opposite, and you said "we know that friction opposes motion" ...

Another way to look at it is the work done by the fricton force equals force x distance moved. The friction force is taking mechanical energy out of the system (and converting it into heat), not putting more mechanical energy in. So the friction force must be in the opposite direction to the relative motion.

 

[Denu]

I read your OP sevral times, but I can't see any clues WHY you have got it back to front.

That's the problem; I don't know WHY I've got it all wrong. It all started when I was doing some Mechanics problems at home today with Newton's third law. Everything seemed fine yesterday, but I suddenly started getting confused.

 

[chrisbaird]

I think you are confusing the different forces involved by not keeping them distinct. The best way to avoid confusion is to be methodical and draw it all out. Start with a stationary block sitting on a flat table. Earth's gravity is pulling the block down. The equal and opposite force to this is the block pulling the Earth up. There is also an upward force supplied by the table which keeps the block from falling. Its opposite and equal force is the block pushing down on the table. When applying F_tot = ma you must do it for one object at a time. We only care about the block, so we can forget the force that the Earth or the table feels in return. The block only feels a downward force due to gravity and an upward force due to the table. These cancel out, so the total force is zero, therefore the acceleration is zero and the block remains stationary.

Now if we give the block a push along the table, we are supplying another force in the forwards direction. A force due to friction arises in the backwards direction to oppose our push. If friction is strong enough, its force cancels my push and the block goes nowhere.

 

[sophiecentaur]

The main argument is, surely, that, if friction acted the other way round, things would speed up rather than slow down. Once that has been accepted, the direction of the friction force is obvious.

 

[ashishsinghal]

friction acts opposite to the direction in which motion tends to happen.

 

[Doc Al]

Friction acts to oppose slipping between surfaces.

 

[Libohove90]

Now if we give the block a push along the table, we are supplying another force in the forwards direction. A force due to friction arises in the backwards direction to oppose our push. If friction is strong enough, its force cancels my push and the block goes nowhere.

What do you mean the block goes nowhere? A cancellation of force means there's no acceleration. I can push a box along a table at a constant velocity...forces still cancel out at that situation.

 

[Vikrant94]

Okay, you were right in your initial post, you have to analyse forces on a body. I'll illustrate by example. Let's say we're pushing a block on a table, which creates friction.

Now choose a body. Let's start with the block. Let's say we're pushing the block from East to West, relative to the (stationary) table. We know the friction must oppose this relative motion (remember, friction opposes relative motion, everywhere!), so it must act from West to East.

Now look at the table. Relative to the block, it is moving West to East (I hope you understand this, otherwise you'll have to refresh your ideas about relative motion). So friction acts opposite to this relative motion on the table, that is it acts from East to West on the table. This also satisfies Newton's third law.

Remember in analyzing forces... Choose your body! Then list the forces. Then their direction. Remember to note the directions of relative motion.

 

[Denu]

Okay, you were right in your initial post, you have to analyse forces on a body. I'll illustrate by example. Let's say we're pushing a block on a table, which creates friction.

Now choose a body. Let's start with the block. Let's say we're pushing the block from East to West, relative to the (stationary) table. We know the friction must oppose this relative motion (remember, friction opposes relative motion, everywhere!), so it must act from West to East.

Now look at the table. Relative to the block, it is moving West to East (I hope you understand this, otherwise you'll have to refresh your ideas about relative motion). So friction acts opposite to this relative motion on the table, that is it acts from East to West on the table. This also satisfies Newton's third law.

Remember in analyzing forces... Choose your body! Then list the forces. Then their direction. Remember to note the directions of relative motion.

I think I understand what the problem was: I wasn't sure about which "system" to consider in my calculations. Take the table and object example for instance. There's a force opposing the motion of the block on the table, because of the contact of the block with the table. Now in this case we're not concerned about the frictional forces between the table and the ground itself. We can't start concluding that because the table exerts a force from west to east to oppose the east to west motion of the block, the ground must be exerting a force from east to west, right? You have to be careful with internal, and external forces, am I correct? I think I understand now. Thank you everyone for helping me do so.

 

[Himal kharel]

friction acts opposite to direction of velocity not applied force actually

 

[sophiecentaur]

friction acts opposite to direction of velocity not applied force actually

So, no velocity no friction?

 

[rcgldr]

It sure sounds stupid of doing otherwise because we know that friction opposes motion.

Kinetic (sliding) friction opposes relative motion between surfaces. Static (non-sliding) friction opposes what would be relative motion if the friction was non-existant.

If an object is moving forwards on a rough plane, we assume that there is a force opposing its motion, namely friction or resistance. This friction includes the friction because of the contact between the moving object and the ground. From Newton's third law, we can conclude that because the object exerts a force of let's say A Newtons on the ground, the ground exerts a force of -A Newtons on the object (magnitude = A, but in the opposite direction). This is when I get totally confused. When we are solving problems involving these laws, and we form equations according to Newton's second law (Fnet = ma), why do we take the direction of friction to be backwards or opposite to the direction of the driving force?

Newtons third laws states that forces only exist in equal and opposing pairs. If there is a driving force, say the tires of a car being driven "forwards" by an engine, the tires exert a backwards force onto the pavement, which results in a tiny amount of backwards acceleration (and rotation) of the earth, and the pavement exerts an equal and opposing forwards force onto the tires, resulting in forwards acceleration of the car. To simplify things, the effect on the much more massive Earth is usually ignored because the motion and acceleration are tiny compared to the motion and acceleration of the car.