Multiple forces acting on a single point

[rayray]

Hi, first of all I'm new here and ready to learn! So I'm sorry if my questions seem stupid and please correct me if I'm wrong.

 

My question is this: When multiple forces of different speeds that share the same mass act on a single point, how do you calculate the force applied? An example would be a martial artist going in for a punch, and there are three places where he exerts force, the extension of the arm, the jerk on his upper body and using his legs to propel him forward but only the fist connects to the person he is punching.

 

My teacher told me to simply add the forces up and I'm skeptical of that. As F=ma, the mass remains but the speed of these forces are different. So is it possible that the 'sum' of these forces not add up as it should?

[GDG]

They do indeed add up, but you have to consider each force as a vector (not just magnitude, but also direction). Some forces may (partially or fully) cancel each other out.

[iNow]

Momentum of the strike and rigidity/elasticity of the striking object play a role, too, IINM.

[J.C.MacSwell]

Momentum of the strike and rigidity/elasticity of the striking object play a role, too, IINM.

 

They may play a role in affecting what the forces are, but the forces are added as described by GDG.

[rayray]

So if someone uses the same three movements to hit a wall at exactly the same time, does the force just add up normally and not 'compound' or does the speed and timing of forces not matter at all?

[swansont]

"Speed of the force" is a bit of an awkward phrase. A force is a force, but the analysis using force alone may not be appropriate. Applications of conservation of momentum and conservation of energy may tell you more than a force analysis. i.e. the time during which a force acts (momentum), or the distance through which it acts (work/energy) may give you more information than simply looking at the force.

[Kyrisch]

So if someone uses the same three movements to hit a wall at exactly the same time, does the force just add up normally and not 'compound' or does the speed and timing of forces not matter at all?

 

By 'speed' of a force, I'm going to assume you mean magnitude, or 'strength' of a force.

 

In response to your question, if three forces act on a single point on the same exact time, you can use vector maths to find the net force, which will be a single force in a single direction. I'm not sure what you mean about 'timing' because you stipulated that they are acting at exactly the same time, but the "speed" (read: magnitude) of the forces does matter; when you do the maths, the force with the greatest magnitude will affect the direction the most, similar to a weighted average.

[iNow]

This idea of "compounding" the force resonates a bit with me, but I don't know enough about the physics involved, so I'll try to describe it in simpler terms...

 

I'm thinking of a triangle. The force is coming from the lets of that triangle... let's say the arms and the legs of the striker... but would the impact actually be best represented by the hypotenuse of that triangle, and potentially be greater than the sum of the parts (the sum of the force from those two sides)?

 

I'm not sure why I'm having such a hard time expressing this concept. Sorry if I'm not making much sense.

[Kyrisch]

This idea of "compounding" the force resonates a bit with me, but I don't know enough about the physics involved, so I'll try to describe it in simpler terms...

 

I'm thinking of a triangle. The force is coming from the lets of that triangle... let's say the arms and the legs of the striker... but would the impact actually be best represented by the hypotenuse of that triangle, and potentially be greater than the sum of the parts (the sum of the force from those two sides)?

 

I'm not sure why I'm having such a hard time expressing this concept. Sorry if I'm not making much sense.

 

In extension of iNow's post (I seem to do that a lot >.<), a visual might help:

[Royce]

{A bold letter represents a vector}

Multiple Forces on an object: Ft=[math]\sum[/math]F

 

If there is more than one force acting on an object, a single resultant force can be used to express their effect.

 

If you know all the forces and their direction, add them up vectorially.

Adding vectors is not the same as adding magnitudes (or plane old numbers)

 

There is no need to go through an entire lecture on vectors, I'm sure you can find that online.

 

But a simple example of vector addition is:

y

|

| (5N) -> [_] <-(-6N)

|_______________________x

A block has two force exerted on it, a five newton force exerted to the right and a 6 newton force exerted to the left. I represent the force which exerts to the left with a negative sign because in my coordinate system, I defined left as negative. The sign indicates the direction, the 6 indicates the magnitude.

 

 

Now I add them, 5+ -6= -1. This tells me that I have a resultant force of 1N and if this 1N was enough to over come the friction force of the block relative to the surface on which it lays then it would move to the left. Because the math told me so.