# Direction of force acting on a body in uniform circular motion?

Direction of force acting on a body in uniform circular motion??

Hello Sir/Ma'am
I'm a a novice. I joined this forum to clear my doubts and improve my physics. Here is my question.. " We generally say, frictional force always acts in a direction, opposite to the direction of motion of a body. Is there any case where it acts in a different direction?? For example, if a car is moving around a pole at a uniform speed (uniform circular motion), the friction is acting towards the centre (also called centrepetal force). And we know that the direction of motion of the car at any point is along the tangent i.e perpendicular to the radius. So,
1) Is it correct to say that here, direction of friction is perpendicular to the direction of motion of the body???
2) And should i conclude that direction of friction force doesn't always need to be opposite to the direction of motion of the body??
3) are there any other cases where direction of friction is not opposite to the direction of motion of the body?? Two cases i know of are walking and riding a bike. Any other cases??
Sir, this is not a homework question. Kindly dont delete it. It is something which i can't understand. Thanks!!

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Simon Bridge
Homework Helper
Hello Sir/Ma'am
Welcome to PF;

I'm a a novice. I joined this forum to clear my doubts and improve my physics. Here is my question.. " We generally say, frictional force always acts in a direction, opposite to the direction of motion of a body. Is there any case where it acts in a different direction?? For example, if a car is moving around a pole at a uniform speed (uniform circular motion), the friction is acting towards the centre (also called centrepetal force). And we know that the direction of motion of the car at any point is along the tangent i.e perpendicular to the radius. So,
1) Is it correct to say that here, direction of friction is perpendicular to the direction of motion of the body???
YOu should be careful about real world objects like cars - they have lots of forces acting on them - many of them friction.
The net force in uniform circular motion points to the center (in an inertial frame).
The only source of this force is the contact between the road and the tires.
This is friction. Therefore...

2) And should i conclude that direction of friction force doesn't always need to be opposite to the direction of motion of the body??
You should conclude whatever you can support with evidence. I won't tell you what to think.

3) are there any other cases where direction of friction is not opposite to the direction of motion of the body?? Two cases i know of are walking and riding a bike. Any other cases??
Sir, this is not a homework question. Kindly dont delete it. It is something which i can't understand. Thanks!!
A block sitting stationary on a slope experiences friction.
Which direction is the friction?
Which direction is the motion?

A car accelerating in a straight line experiences a net frictional force and it is moving.
What direction does the net friction point in?

If you think of friction as a kind of stickyness between surfaces, then you can see that it will oppose the motion that would have been there but for the friction.

Go through each example and see what the motion would have to be without the friction.

Welcome to PF;

You should be careful about real world objects like cars - they have lots of forces acting on them - many of them friction.
The net force in uniform circular motion points to the center (in an inertial frame).
The only source of this force is the contact between the road and the tires.
This is friction. Therefore...

[REPLY] Thank you Sir, I understood this one. Here, the car is being accelerated towards the centre. And the direction of motion of the car is not actually along the tangent. (it is, but only if there is no force acting on it towards the centre). Infact, the car is moving on a circular path, which has no distinct direction. And it is the net force acting on the car (F=ma) towards the centre, not only friction alone. Am i right Sir?? By the way, can you tell me, what are the other force acting on the car (in this case) apart from friction and air resistance??

You should conclude whatever you can support with evidence. I won't tell you what to think.

A block sitting stationary on a slope experiences friction.
Which direction is the friction?
Which direction is the motion?

[REPLY] You said, the box is sitting 'stationary' on a slope. From what i understand, the box would like to slide down the slope. Ofcourse, the direction of friction will be opposite i.e backwards.

A car accelerating in a straight line experiences a net frictional force and it is moving.
What direction does the net friction point in?

[REPLY] The net friction here points backwards i.e., in the opposite direction.

If you think of friction as a kind of stickyness between surfaces, then you can see that it will oppose the motion that would have been there but for the friction.

Go through each example and see what the motion would have to be without the friction.
[REPLY] without the friction, the car will go on and on. It will never de-accelerate. 'I'm sorry Sir.. I haven't got the answer to my last question yet. Or may be, i haven't understood. In the both cases above, the direction of friction is opposite to the direction of motion. (in 1st case, the box is not moving) thats how i comprehend. my question was- ' is there a situation in which, the direction of friction is not opposite to the direction of motion?

Simon Bridge
Homework Helper
Welcome to PF;

You should be careful about real world objects like cars - they have lots of forces acting on them - many of them friction.
The net force in uniform circular motion points to the center (in an inertial frame).
The only source of this force is the contact between the road and the tires.
This is friction. Therefore...

[REPLY] Thank you Sir, I understood this one. Here, the car is being accelerated towards the centre. And the direction of motion of the car is not actually along the tangent. (it is, but only if there is no force acting on it towards the centre).
No - the instantaneous velocity of the car does point along the tangent to it's path, it's acceleration points to the center.

Infact, the car is moving on a circular path, which has no distinct direction. And it is the net force acting on the car (F=ma) towards the centre, not only friction alone. Am i right Sir??
The only forces controlling the car's acceleration are friction forces - that was the point of what I was writing above. By "..." I was inviting you to complete the logic.
By the way, can you tell me, what are the other force acting on the car (in this case) apart from friction and air resistance??
You tell me - the important forces, re the car's acceleration, are at each tire. Notice that you have to turn the front wheels to get the car to go in a circle?

A block sitting stationary on a slope experiences friction.
Which direction is the friction?
Which direction is the motion?

[REPLY] You said, the box is sitting 'stationary' on a slope. From what i understand, the box would like to slide down the slope. Of course, the direction of friction will be opposite i.e backwards.
The direction of friction is "up the slope".
The direction of motion is: no motion - therefore no direction.
Therefore the direction of friction is not related to the direction of motion of the block.
You have noticed that it does oppose the direction the block would have moved in without friction.

A car accelerating in a straight line experiences a net frictional force and it is moving.
What direction does the net friction point in?

[REPLY] The net friction here points backwards i.e., in the opposite direction.
Incorrect.
The car is accelerating forwards.
Therefore the net force is forwards.
The net force comes from the contact between the tires and the road.
Therefore the force is friction.
Therefore the net friction force points in the same direction as the motion of the car.

Welcome to PF;

No - the instantaneous velocity of the car does point along the tangent to it's path, it's acceleration points to the center.

[REPLY] (since, instantaneous velocity points along the tangent) So, at any point, should i assume that force of friction is acting perpendicular to the direction of motion at that point? You have already explained that the 'inward centripetal force' is the net force i.e. sum of all the forces. What i want to know is, what is the direction of the friction force in this situation ?

The only forces controlling the car's acceleration are friction forces - that was the point of what I was writing above. By "..." I was inviting you to complete the logic.You tell me - the important forces, re the car's acceleration, are at each tire. Notice that you have to turn the front wheels to get the car to go in a circle?

The direction of friction is "up the slope".
The direction of motion is: no motion - therefore no direction.
Therefore the direction of friction is not related to the direction of motion of the block.
You have noticed that it does oppose the direction the block would have moved in without friction.
[REPLY] Very nice example. It is clear. Thank you Sir.
Incorrect.
The car is accelerating forwards.
Therefore the net force is forwards.
The net force comes from the contact between the tires and the road.
Therefore the force is friction.
Therefore the net friction force points in the same direction as the motion of the car.
[REPLY] Oh! I made a silly mistake. I think, the wheels slide backwards, and hence, the friction acts forwards i.e. in the same direction as the motion. But i can't understand this sentence of yours- ''net friction force is acting on the car.'' . Ofcourse, the friction is in the same direction as the motion. But the car is moving or accelerating because of the external force by the steerings on the wheels only, right Sir?
I'm sorry Sir, if this is a basic question. I'm a starter and a few basic things are not clear.

Simon Bridge
Homework Helper
Oh! I made a silly mistake. I think, the wheels slide backwards,
The wheels only slide backwards in low-friction situations.
and hence, the friction acts forwards i.e. in the same direction as the motion. But i can't understand this sentence of yours- ''net friction force is acting on the car.'' . Of course, the friction is in the same direction as the motion. But the car is moving or accelerating because of the external force by the steerings on the wheels only, right Sir?
To accelerate, the car must be acted on by an unbalanced force.
That unbalanced force is friction - it acts at the contact between the wheels and the road and points in the same direction as the acceleration.

Don't know what you mean by "external force by the steerings" - steering is not a force.

The wheels only slide backwards in low-friction situations.
[REPLY] Ok Sir! I mean, the wheels try to slide backwards. But they can't do so because their is sufficient amount of friction.

To accelerate, the car must be acted on by an unbalanced force.
That unbalanced force is friction - it acts at the contact between the wheels and the road and points in the same direction as the acceleration.

Don't know what you mean by "external force by the steerings" - steering is not a force.
[REPLY] Sorry Sir..!! I mean, the accelerator (not the steering) applies a force on the wheels. Lets consider this situation- a Car is accelerating in a straight line. And i was trying to draw the free body diagram. Ofcourse, GRAVITY and NORMAL FORCE are the two vertical forces along the Y-axis acting on the car. As for horizontal forces along the X-axis, we have the ACTION force along negative X-axis (when the wheels push the ground backwards). And the REACTION force, along the positive X-axis (the ground pushes the wheels forward) . I think this 'reaction force' propels the car forward because Mass of a body is inversely proportional to its acceleration. Obviously, these action and reaction forces are balanced. Also, we have the friction force in forward direction. This is the net unbalanced force in forward direction, which causes the car to accelerate. Am i right Sir ?? Kindly correct me, if i'm wrong.
Now lets consider a slightly different situation- a Car is moving in a straight line with a constant speed i.e with zero acceleration. What would be the 'free body diagram' like? Ofcourse, GRAVITY and NORMAL FORCE are the two vertical forces along the Y-axis. As for horizontal forces, we have the same ACTION force along negative X-axis (when the wheels push the ground backwards). And the REACTION force, along the positive X-axis (the ground pushes the wheels forward) . This 'reaction force' propels the car forward (Mass of a body is inversely proportional to its acceleration). Obviously, these action and reaction forces are balanced. Also, we have the friction force in forward direction (always). Since, the car is moving with zero acceleration, all the horizontal forces must be balanced. In this case, which force balances out 'friction force' ?? Because, the net force acting on the car must be zero. Is there any other horizontal force apart from the ACTION, REACTION and the FRICTION ??

Simon Bridge
Homework Helper
The net friction in a car going at constant speed is zero.
There are lots of friction forces.

Some of the friction forces, such as between the tires and the road, point forward.
Some of the friction forces, such as at the axles, in the engine, and of the air moving over the body, act to oppose the motion.

The net friction in a car going at constant speed is zero.
There are lots of friction forces.

Some of the friction forces, such as between the tires and the road, point forward.
Some of the friction forces, such as at the axles, in the engine, and of the air moving over the body, act to oppose the motion.
[REPLY] Thank you Sir for very quick help. You said, ''the net friction in this case is zero.'' But these forces (i.e friction at the axles in the engine and the air friction in the opposite direction) must be acting on the car in the first case also (when the car is accelerating). Hence, the net friction must be zero in that case. Then, what causes the car to accelerate ??

Simon Bridge
Homework Helper
But these forces (i.e friction at the axles in the engine and the air friction in the opposite direction) must be acting on the car in the first case also (when the car is accelerating). Hence, the net friction must be zero in that case.
No. Since the different frictions have different causes, there is no reason they must always cancel out.

Generally the retarding forces are speed dependent - bigger for higher speeds.
When you supply more power, the driving friction increases, and the speed increases, and the retarding friction increases with the speed until it is equal to the new driving friction. This is what you experience driving your car right? Take your foot off the gas and the driving friction is now less than the retarding friction, the car slows down, the retarding friction reduces until it is equal to the driving friction and you have a new constant speed.
This is a simplification, true, but it should feel familiar.

The power to drive the car is supplied by the engine - using energy from the fuel - through a complicated arrangement of machinery which we are modelling using the resulting forces. The forces pushing on the car are contact forces which are by nature friction forces.

You should be familiar enough with this line of reasoning to be able to come up with your own answers by now - even if you still don't believe it ;)

The question in front of you is whether friction can ever point in a direction other than the direction of movement. You should now be able to answer that question.

No. Since the different frictions have different causes, there is no reason they must always cancel out.

Generally the retarding forces are speed dependent - bigger for higher speeds.
When you supply more power, the driving friction increases, and the speed increases, and the retarding friction increases with the speed until it is equal to the new driving friction. This is what you experience driving your car right? Take your foot off the gas and the driving friction is now less than the retarding friction, the car slows down, the retarding friction reduces until it is equal to the driving friction and you have a new constant speed.
This is a simplification, true, but it should feel familiar.

The power to drive the car is supplied by the engine - using energy from the fuel - through a complicated arrangement of machinery which we are modelling using the resulting forces. The forces pushing on the car are contact forces which are by nature friction forces.

You should be familiar enough with this line of reasoning to be able to come up with your own answers by now - even if you still don't believe it ;)

The question in front of you is whether friction can ever point in a direction other than the direction of movement. You should now be able to answer that question.
Thanks a lot Sir for all those answers.. That really helped. Now, it is clear to me. If only, i've any doubts, may i post my questions ??

Chestermiller
Mentor
I think what you are talking about here is static friction. When you have static friction, there is no relative sliding motion between the two bodies at their interface. The direction of the static friction force on the body is opposite to the direction that the body would tend to slide if such sliding were possible. In the case of a car going around a track, the car would tend to slide outward, so the static friction force is pointing inward. In the case of a block on an inclined plane, the block would tend to slide down the plane, so the static friction force on the block is up the plane.

I think what you are talking about here is static friction. When you have static friction, there is no relative sliding motion between the two bodies at their interface. The direction of the static friction force on the body is opposite to the direction that the body would tend to slide if such sliding were possible. In the case of a car going around a track, the car would tend to slide outward, so the static friction force is pointing inward. In the case of a block on an inclined plane, the block would tend to slide down the plane, so the static friction force on the block is up the plane.
[REPLY] the instantaneous velocity of the car at a point in along the tangent. Ofcourse, friction is acting inwards i.e along the radius. Is it right to say that at an instant, friction is acting perpendicular to the direction of motion?

Doc Al
Mentor
the instantaneous velocity of the car at a point in along the tangent. Ofcourse, friction is acting inwards i.e along the radius. Is it right to say that at an instant, friction is acting perpendicular to the direction of motion?
Sure.

sophiecentaur
Gold Member
[REPLY] the instantaneous velocity of the car at a point in along the tangent. Ofcourse, friction is acting inwards i.e along the radius. Is it right to say that at an instant, friction is acting perpendicular to the direction of motion?
I can only quote from Simon's first post, replying to your OP
You should be careful about real world objects like cars - they have lots of forces acting on them - many of them friction.
Imo, you should first sort out exactly what is causing circular motion (perfect ball on perfect string) and, where you see it operating in a complicated situation like a car, you will be able to identify the need for a centripetal force and be able to calculate it. This force is, of course, supplied by some of the friction from the tyres - as there's no 'string'. But a real car (unpowered would slow )down due to friction forces acting along a tangent, too. The overall (resultant) friction force direction will be somewhere in between the two and directed in a direction 'behind' the radius of motion. If the car is speeding up, the friction force will aim 'in front' of the radius. Only when the speed is constant will the net force from the tyres be radial.

Doc Al
Mentor
Only when the speed is constant will the net force from the tyres be radial.
Note that the title of this thread specifies uniform circular motion.

(Even then, of course, a real car must overcome air resistance so some component of tire resistance must be in the direction of motion.)

sophiecentaur
Gold Member
Note that the title of this thread specifies uniform circular motion.

(Even then, of course, a real car must overcome air resistance so some component of tire resistance must be in the direction of motion.)
Oh yes, of course. But my point was that the car is a complex system and so the other friction forces are acting all the time, even under uniform motion- when they just happen to balance out in the tangential direction. You need to include air friction as well as tyre friction, natch.

I can only quote from Simon's first post, replying to your OP

Imo, you should first sort out exactly what is causing circular motion (perfect ball on perfect string) and, where you see it operating in a complicated situation like a car, you will be able to identify the need for a centripetal force and be able to calculate it. This force is, of course, supplied by some of the friction from the tyres - as there's no 'string'. But a real car (unpowered would slow )down due to friction forces acting along a tangent, too. The overall (resultant) friction force direction will be somewhere in between the two and directed in a direction 'behind' the radius of motion. If the car is speeding up, the friction force will aim 'in front' of the radius. Only when the speed is constant will the net force from the tyres be radial.
I couldn't understand. Will you make it clear, how the resultant friction will be 'behind the radius' and 'in front of it' when the car slows down and speeds up respectively??

sophiecentaur
Gold Member
I couldn't understand. Will you make it clear, how the resultant friction will be 'behind the radius' and 'in front of it' when the car slows down and speeds up respectively??
There is a radial force keeping the car moving in a circle (this can only be due the friction from the tyres). There is a force slowing it down (or speeding it up), which acts tangentially. These are two vectors which can be added together(as with all vectors) to produce a resultant. This resultant will not point along the radius unless there is no speeding up or slowing down - therefore it will point either forwards or backwards (in front or behind) the radial direction.
All this is very idealised, of course, because tyres are constantly slipping, even when the car is not actually under control and the driver may well need to be pointing the wheels in strange directions in order to keep on a curve - look up Slip Angle of Tyres, for more information.
Does that make sense?

There is a radial force keeping the car moving in a circle (this can only be due the friction from the tyres). There is a force slowing it down (or speeding it up), which acts tangentially. These are two vectors which can be added together(as with all vectors) to produce a resultant. This resultant will not point along the radius unless there is no speeding up or slowing down - therefore it will point either forwards or backwards (in front or behind) the radial direction.
All this is very idealised, of course, because tyres are constantly slipping, even when the car is not actually under control and the driver may well need to be pointing the wheels in strange directions in order to keep on a curve - look up Slip Angle of Tyres, for more information.
Does that make sense?
now this is clear to me for sure. Thanks a lot! I've another question. It is about kinetic friction acting on a body. We all know, kinetic friction of a body is usually smaller than its static friction.
(1) If we increase the speed of the body, what happens to its kinetic friction ? Does it increase, decrease, or remains the same ?
(2) from what i know, the value of the 'coefficient of kinetic friction' generally doesn't change for the speeds up to less than 10m/s. What if we increase the speed? How does it affect the 'coefficient of kinetic friction' ?

Chestermiller
Mentor
In practice, the coefficient of kinetic friction usually decreases with increasing relative tangential velocity between the surfaces.

In practice, the coefficient of kinetic friction usually decreases with increasing relative tangential velocity between the surfaces.
Sir, i'm not talking of the circular motion. The body is travelling in a straight line with some speed. Ofcouse, kinetic friction is opposing the motion of the boday. And this is what i can't understand :-
(1) If we increase the speed of the body, what happens to its kinetic friction ? Does it increase, decrease, or remains the same ?
(2) from what i know, the value of the 'coefficient of kinetic friction' generally doesn't change for the speeds up to less than 10m/s. What if we increase the speed? How does it affect the 'coefficient of kinetic friction' ?

No. Since the different frictions have different causes, there is no reason they must always cancel out.

Generally the retarding forces are speed dependent - bigger for higher speeds.
When you supply more power, the driving friction increases, and the speed increases, and the retarding friction increases with the speed until it is equal to the new driving friction. This is what you experience driving your car right? Take your foot off the gas and the driving friction is now less than the retarding friction, the car slows down, the retarding friction reduces until it is equal to the driving friction and you have a new constant speed.
This is a simplification, true, but it should feel familiar.

The power to drive the car is supplied by the engine - using energy from the fuel - through a complicated arrangement of machinery which we are modelling using the resulting forces. The forces pushing on the car are contact forces which are by nature friction forces.

You should be familiar enough with this line of reasoning to be able to come up with your own answers by now - even if you still don't believe it ;)

The question in front of you is whether friction can ever point in a direction other than the direction of movement. You should now be able to answer that question.
Thanks a lot Sir.!! I've another question. It is about kinetic friction acting on a body. We all know, kinetic friction of a body is usually smaller than its static friction.
(1) If we increase the speed of the body, what happens to its kinetic friction ? Does it increase, decrease, or remains the same ?
(2) from what i know, the value of the 'coefficient of kinetic friction' generally doesn't change for the speeds up to less than 10m/s. What if we increase the speed? How does it affect the 'coefficient of kinetic friction' ?

Hello Sir/Ma'am
I'm a a novice. I joined this forum to clear my doubts and improve my physics. Here is my question.. " We generally say, frictional force always acts in a direction, opposite to the direction of motion of a body. Is there any case where it acts in a different direction?? For example, if a car is moving around a pole at a uniform speed (uniform circular motion), the friction is acting towards the centre (also called centrepetal force). And we know that the direction of motion of the car at any point is along the tangent i.e perpendicular to the radius. So,
1) Is it correct to say that here, direction of friction is perpendicular to the direction of motion of the body???
2) And should i conclude that direction of friction force doesn't always need to be opposite to the direction of motion of the body??
3) are there any other cases where direction of friction is not opposite to the direction of motion of the body?? Two cases i know of are walking and riding a bike. Any other cases??
Sir, this is not a homework question. Kindly dont delete it. It is something which i can't understand. Thanks!!
I know you have got a lot of answers but i don't think they have answered your third question involving other cases.I have made a rough sketch of a problem(in the attachment) in which friction accelerates a block(block-2) from zero velocity to a finite velocity.Please pardon my rough sketch and come back if you have any question on the sketch.
This is called as a forward english problem(by some people)

I know

I know you have got a lot of answers but i don't think they have answered your third question involving other cases.I have made a rough sketch of a problem(in the attachment) in which friction accelerates a block(block-2) from zero velocity to a finite velocity.Please pardon my rough sketch and come back if you have any question on the sketch.
This is called as a forward english problem(by some people)

I know
thanks! But where is that 'rough sketch' ??