Force due to velocity

  • #1
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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?
 

Answers and Replies

  • #2
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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
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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.
 
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  • #4
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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
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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
D H
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No. That's why it hurts when you are hit with a 5 ounce baseball but not with a small 5 ounce pillow.
 
  • #7
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ok so the force integrated over the entire collision would be the average force applied...
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
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
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No. That's why it hurts when you are hit with a 5 ounce baseball but not with a small 5 ounce pillow.
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
HallsofIvy
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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
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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.
 
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  • #12
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Yes you can, if you know it power output:

W = work
F= force
P = power

[tex]W = Fs[/tex]

[tex] \frac{dW}{dt} = \frac{d(Fs)}{dt} [/tex]

[tex] \frac{dW}{dt} = F \frac{ds}{dt} [/tex] ..............assuming the force is constant

[tex] P = Fv [/tex]

[tex] F = \frac{P}{v} [/tex]
 
  • #13
HallsofIvy
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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.
Still wrong. Force= mass*acceleration. Force is due to acceleration which is rate of change of velocity, not velocity or change of velocity.

The force that the ball exerts on the steel and the pillow is different because the duration of the impact is different.
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
russ_watters
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It's more than just wording, shamrock, and we're just trying to be helpful. Some of the issues:
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. [emphasis added]
No, an object only exerts a force when it is 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?
The first part is correct, which should tell you clearly that the answer to the question at the end is no.
so theres no way to figure out what the average force would be without the time duration of the impact?
Same question as above, same answer: no.
 
  • #15
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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.
 
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  • #16
russ_watters
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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.
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
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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
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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
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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 :wink:)
 
  • #20
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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.
 
  • #21
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.
I'm new and thought your conversation fasinating - Force due to mass and velocity: 2 students were jogging in the gym one male 6'4 170lbs and the other female 5'2 120lbs. Both were jogging 6m/s then increased their velocity to 10m/s. They maintained that pace for 5s when both unfortunately ran into another student. At what force was that impact?
 
  • #22
I'm new and thought your conversation fasinating - Force due to mass and velocity: 2 students were jogging in the gym one male 6'4 170lbs and the other female 5'2 120lbs. Both were jogging 6m/s then increased their velocity to 10m/s. They maintained that pace for 5s when both unfortunately ran into another student standing at rest. At what force did they impact that student?
 
  • #23
cjl
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That isn't an answerable question. You need the duration of the impact or the acceleration of the person during impact to figure out the force during impact.
 
  • #24
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Interesting semantics.
F=mA
F=m * dv/dt
Velocity is equally responsible for force as acceleration is! One could argue that there IS force due to velocity. The rate of change of velocity. lol

If you think of a macroscopic object moving in space, it could be said to have a velocity with respect to another object but it can never be said to not be accelerating because it is forever being acted on by all the other particles forces in the universe... and as such is acting back on all other particles in the universe.

So by virtue of the fact that the object is moving from one point to another at a given fixed velocity with respect to another object, it is imparting forces on all other objects in the universe.

There is no velocity without acceleration.
 
  • #25
russ_watters
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Interesting semantics.
F=mA
F=m * dv/dt
Velocity is equally responsible for force as acceleration is! One could argue that there IS force due to velocity. The rate of change of velocity. lol
No, velocity is not equally responsible as acceleration. You might say velocity change and time elapsed are equally responsibe, since they are both part of the formula for acceleration, but even that isn't very useful.

Ie, if you have acceleration and mass, you can find force. If you have dv and mass, you can't find force. That makes one more useful than the other for finding the force.
If you think of a macroscopic object moving in space, it could be said to have a velocity with respect to another object but it can never be said to not be accelerating because it is forever being acted on by all the other particles forces in the universe... and as such is acting back on all other particles in the universe.

So by virtue of the fact that the object is moving from one point to another at a given fixed velocity with respect to another object, it is imparting forces on all other objects in the universe.
No. That's gravity you are talking about and it acts just fine with or without motion.
 

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