Understanding Contact Forces: Normal and Tangential Force Explained

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In summary: This would be the case if the ball were to smoothly roll off the wall. That model is inaccurate in two ways.1. The ball may slide across the wall instead...
  • #1
luis20
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Hi. I have two questions

All contact forces are normal to the surface right? If there was no friction, there wouldn't be any tangential force?

Thanks
 
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  • #2
The normal force is indeed always normal to whatever surface you are talking about. The frictional force is tangential to that surface, and is proportional to the Normal force in elementary analysis.
 
  • #3
jfy4 said:
The frictional force is tangential to that surface, and is proportional to the Normal force in elementary analysis.
Not quite. The maximum static friction force is proportional normal force. The actual tangential frictional force is anything up to that.
 
  • #4
Thanks for the replies

What I want to know is if all tangential forces are due to friction.

In a fast collision, the tangential force can be neglected?
 
  • #5
luis20 said:
What I want to know is if all tangential forces are due to friction.
Not at all. Any kind of force may have a component tangent to the trajectory.
See for example motion on inclined plane. The gravity has a tangential component.

luis20 said:
In a fast collision, the tangential force can be neglected?
For what purpose?

The effect of tangential forces on the final momentum distribution may become important for collisions that are not "head-on". For example, throwing a ball towards a wall at an angle. If the ball-wall friction is large the tangential component of the momentum will decrease and the angle of "reflection" will be smaller than the angle of incidence.
 
  • #6
nasu said:
Not at all. Any kind of force may have a component tangent to the trajectory.
See for example motion on inclined plane. The gravity has a tangential component.For what purpose?

The effect of tangential forces on the final momentum distribution may become important for collisions that are not "head-on". For example, throwing a ball towards a wall at an angle. If the ball-wall friction is large the tangential component of the momentum will decrease and the angle of "reflection" will be smaller than the angle of incidence.

What I meant was all tangential forces on a surface are due to friction. The tangential component of gravity will only affect the surface if there is friction. So I think my statement may be correct.

The thing you said about the ball is what I wanted to read. My teachers neglect the tangential component when we talk about collisions. Maybe that's because they are engineers.

Edit: But if you see collision articles, they talk only about the force which is normal to the contact surface! (tangential neglected) Collision is considered fast so the tangential force might be very little?
 
  • #7
luis20 said:
My teachers neglect the tangential component when we talk about collisions. Maybe that's because they are engineers.

Edit: But if you see collision articles, they talk only about the force which is normal to the contact surface! (tangential neglected) Collision is considered fast so the tangential force might be very little?

Well, what kind of collisions are discussed by your teachers? Are they head-on collisions?
I mean, is there a tangential component of the momentum of the colliding object(s) or not?
 
  • #8
nasu said:
Well, what kind of collisions are discussed by your teachers? Are they head-on collisions?
I mean, is there a tangential component of the momentum of the colliding object(s) or not?

There is a tangential component. It's like your example of the ball.

Collisions which produce torque, where the normal force is not in the direction of the center of mass, those have tangential force for sure right?
 
  • #9
luis20 said:
Collisions which produce torque, where the normal force is not in the direction of the center of mass, those have tangential force for sure right?
It's not a question of whether the normal force passes through the centre of mass. The test is whether the relative motion of the points of contact immediately before collision is normal to the contact plane. Thus, a ball striking a flat surface at an angle will experience a tangential frictional force on impact unless it happened to be spinning in exactly the right way so that it made rolling contact instantaneously.
Also, I don't think one can argue that the contact is too brief for the tangential force to matter. The maximum tangential force is proportional, at each instant, to the actual normal force, just as in normal static arrangements, and the tangential and normal forces operate over the same brief time period.
If a ball radius R rebounds from a surface with speed V at θ to the normal, no sliding, I would think that the ball is now spinning at rate V sin(θ) / R.
 
  • #10
haruspex said:
If a ball radius R rebounds from a surface with speed V at θ to the normal, no sliding, I would think that the ball is now spinning at rate V sin(θ) / R.

This would be the case if the ball were to smoothly roll off the wall. That model is inaccurate in two ways.

1. The ball may slide across the wall instead of rolling without slippage. This would tend to apply in, for instance, the case of a steel ball hitting a steel wall at a glancing angle.

In the limit your spin rate omega(final) = omega(initial).

2. Rotational rebound.

In my youth we used to play with "super balls". These were hard rubber balls that rebounded pretty efficiently (90 to 95% restitution). They were also sticky enough that they would catch immediately and roll without slipping. If you threw one of these at the floor and got a good spin on it you could watch it go into a back and forth bouncing pattern with the direction of rotation reversing at each bounce.

In the limit your spin rate omega(final) = 2 V sin(θ) / R - omega(initial)

The spin rate resulting from any actual collision could be expected to lie somewhere in this range.
 
  • #11
jbriggs444 said:
This would be the case if the ball were to smoothly roll off the wall. That model is inaccurate in two ways.

1. The ball may slide across the wall instead of rolling without slippage. This would tend to apply in, for instance, the case of a steel ball hitting a steel wall at a glancing angle.
I did specify "if no sliding"
2. Rotational rebound.
True - I did forget about that.
 

1. What is a contact force?

A contact force is a type of force that occurs when two objects physically touch or make contact with each other. It can either be a push or a pull force and is responsible for the interactions between objects.

2. What is the difference between normal and tangential force?

Normal force is the force that is exerted perpendicular to the surface of an object when it comes into contact with another object, while tangential force is the force that is exerted parallel to the surface of an object. Normal force is responsible for supporting the weight of an object, while tangential force is responsible for causing objects to slide against each other.

3. How is normal force calculated?

Normal force can be calculated using the formula FN = mg, where m is the mass of the object and g is the acceleration due to gravity. It is important to note that the normal force is equal in magnitude but opposite in direction to the force of gravity acting on an object.

4. What factors affect the magnitude of tangential force?

The magnitude of tangential force is affected by the coefficient of friction between two surfaces, the force of gravity, and the angle at which the objects are in contact. The higher the coefficient of friction, the greater the tangential force required to cause objects to slide against each other.

5. How do contact forces impact the motion of objects?

Contact forces play a crucial role in the motion of objects. Normal force can affect an object's ability to stay stationary or move in a particular direction, while tangential force can either cause objects to slide against each other or keep them in place. These forces are also important in understanding the laws of motion and how objects interact with each other.

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