# Force due to velocity

1. May 30, 2008

### shamrock5585

ok this has been kind of annoying to me cuz i cant find a good way of calculating this... is there a way to calculate the force that an object will exert when it is moving at a certain velocity... force equals mass times acceleration, but if an object is travelling at a high rate of speed it will exert a greater force than if it were at rest even if it is not accelerating. The only way i can see to find the force is to find how long the impact takes which would be a very small amount of time and then divide your velocity by that to get your deceleration... so then your deceleration/acceleration times the mass of the object is the force it will exert when it strikes something... is there a way to calculate the force without knowing how long the impact takes?

2. May 30, 2008

### Feldoh

Well force is a change in momentum, so you could measure the change on a second object by the first object. That would be the force of the first object on the second wouldn't it?

3. May 30, 2008

### evouga

Yeah, that would give you the impulse - the force integrated over the entire collision. To compute the force at any given time you need the duration of collision and a lot more information - colliding objects tend to compress, then expand, so the force isn't constant.

Last edited: May 30, 2008
4. May 30, 2008

### shamrock5585

im not sure i follow what you're saying exactly... im saying basically if you put something on a scale it will say what the weight of it is, which is the force it is applying on the scale (mass times acceleration due to gravity)... now if you drop it from 5 feet it will hit the scale at a velocity of 12.7 ft/s and will apply a greater force when it strikes the scale.

5. May 30, 2008

### shamrock5585

ok so the force integrated over the entire collision would be the average force applied... so theres no way to figure out what the average force would be without the time duration of the impact?

6. May 30, 2008

### D H

Staff Emeritus
No. That's why it hurts when you are hit with a 5 ounce baseball but not with a small 5 ounce pillow.

7. May 30, 2008

### evouga

That's not quite true. The impulse is independent of the duration, the average force is not. To get the average force you can divide the impulse by the duration.

8. Jun 3, 2008

### rockerdoctor

You have to have either the time or the acceleration to find the force. Absent of acceleration the way to find force would be to use the momentum equations.
p=mv
p=(Fnet)*(Δt)

therefore...

(Fnet)*(Δt)=mv
Fnet=(mv)/(Δt)

9. Jun 4, 2008

### shamrock5585

yes the pressure that is applied is different but that is still dependant on the force... im not really even sure whos response your refering to anyway

10. Jun 4, 2008

### HallsofIvy

Staff Emeritus
The crucial point is that there is NO "force due to velocity" to begin with. The average force will be depend upon the average deceleration of the object which, in turn, depends on time required for the object (striking another object) to change from its original velocity to its final velocity.

The average force a ball moving at velocity v, exerts striking a steel plate will be quite different from the average force exerted by an identical ball, also moving at velocity v, striking a soft pillow.

A ball moving at velocity v has a specific energy, not force. Energy is conserved, not force.

11. Jun 4, 2008

### shamrock5585

ok if you want to pick apart my wording... if the ball didnt have a velocity in the first place then it wouldnt exert a force on the object it is striking other than its own weight (if it is being dropped). so the force is due to its change in velocity, hows that for wording.

The force that the ball exerts on the steel and the pillow is different because the duration of the impact is different.

Last edited: Jun 4, 2008
12. Jun 4, 2008

### Ed Aboud

Yes you can, if you know it power output:

W = work
F= force
P = power

$$W = Fs$$

$$\frac{dW}{dt} = \frac{d(Fs)}{dt}$$

$$\frac{dW}{dt} = F \frac{ds}{dt}$$ ..............assuming the force is constant

$$P = Fv$$

$$F = \frac{P}{v}$$

13. Jun 4, 2008

### HallsofIvy

Staff Emeritus
Still wrong. Force= mass*acceleration. Force is due to acceleration which is rate of change of velocity, not velocity or change of velocity.

Exactly. So you cannot calculate the force given only the velocity. My point is that you should not worry about your inability to find a formula relating force to velocity because there is no such formula.

14. Jun 4, 2008

### Staff: Mentor

It's more than just wording, shamrock, and we're just trying to be helpful. Some of the issues:
No, an object only exerts a force when it is accelerating.
The first part is correct, which should tell you clearly that the answer to the question at the end is no.
Same question as above, same answer: no.

15. Jun 5, 2008

### shamrock5585

ok you guys are picking apart my wording way too much... im sorry if it is not perfectly clear to you...

sorry halls of ivy... rate of change of velocity not change in velocity. I was pretty sure you knew what i was talking about but i guess i have to be extra clear.

Russ i was saying if it was not accelerating because i was clarifying that the object has constant velocity. it would decelerate at impact but i was clarifying that it was not accelerating initially in motion.

i know that the force distribution is different on impact for different materials because of compression. what i was suggesting was that if you knew a material constant for impact on a material, couldnt it be possible to predict the force it would exert if it struck at a certain velocity.

Last edited: Jun 5, 2008
16. Jun 5, 2008

### Staff: Mentor

Yes, but it can be complicated. For metal objects it isn't too bad because they deform elastically for quite a bit of energy absorption. So you can model the impact as a spring-mass system, with the kinetic energy before the collision being equal to the potential energy after (but before any rebound). In that case, the impact force increases linearly with distance and parabolically (I think - you may want to try to derive that...) with duration of the impact.

17. Jun 6, 2008

### Ed Aboud

I'm sure there is something wrong about my last quote, you guys seem to have a better knowledge of this, but can anyone tell me why it is wrong? I don't see why it is.

18. Mar 11, 2009

### srfriggen

This all seems to tie in with Relativity and the Twin Paradox. The twin who experiences a force, in that case acceleration, would be the one who ages relatively more slowly. If they were both moving at a constant velocity they would age the same. That illustration, to me, seems to show, without any formulas, that force is not dependent on velocity.

19. Mar 11, 2009

### redargon

F=ma (accepted Newton's Law)
an object with constant velocity has no acceleration
therefore F=m*0=0 and therfefore, no force.
When it hits an object, it decelerates (negative acceleration). This acceleration is used to calculate the force of the impact. The higher the velocity if the object before impact, the larger it's deceleration would have to be to bring it to a stop during impact and therefore a larger force would be generated at impact.
No need for the twin paradox, it is basic classical mechanics (with velocities well below c, of course )

20. Mar 11, 2009

### srfriggen

yeah, i understand all that. i was just trying to point out that the twin paradox scenario is only resolved once you accept that only acceleration causes force, not velocity. just pointing out a different example of how that applies. i think it's good sometimes to look at things from different perspectives to really hammer down the point.