? Why does F=ma instead of F=mv ?

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    F=ma
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Discussion Overview

This discussion centers around the question of why the equation F=ma is used instead of F=mv, particularly in the context of a hypothetical scenario involving a car hitting a stationary person. Participants explore concepts of force, acceleration, and momentum, while addressing misunderstandings related to these principles.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants clarify that velocity is the current speed and acceleration is the change in speed, but emphasize that acceleration can still occur even at constant speed if direction changes.
  • One participant argues that if a car traveling at a constant speed hits a stationary person, the force exerted should not be zero, as the person would experience a significant acceleration upon impact.
  • Another participant introduces the concept of kinetic energy (KE) and suggests that while F=ma determines the force required to accelerate a body, KE describes the energy possessed by the body at constant speed.
  • Some participants challenge the original premise that a force would be zero, pointing out logical fallacies and emphasizing the need to apply Newton's second law to individual bodies rather than the system as a whole.
  • One participant introduces the concept of momentum (p=mv) to describe the effect of the car on the person, suggesting that momentum is a relevant factor in understanding the impact.
  • Another participant notes that while the net force on the car may be zero when moving at constant velocity, this does not imply that no forces are acting on it or on the person being hit.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of force, acceleration, and the implications of a car hitting a stationary person. There is no consensus on the original premises or the conclusions drawn from them, indicating ongoing debate and exploration of the concepts involved.

Contextual Notes

Some participants highlight limitations in the original premises, such as the assumption that force can be determined solely by mass and acceleration without considering the context of the collision. The discussion also touches on the importance of distinguishing between net force and individual forces acting on bodies in motion.

Mental Gridlock
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I know this is probably a complete n00b question, but I have tried all over the www and couldn't find it explained.

Correct any of these premises if they are wrong, maybe that's my problem, but...

velocity is current speed and acceleration is change in speed.

So, if something is moving at a constant speed, it's acceleration is zero.

So if a car (say 2000kg) is traveling at a constant speed of say 70mph on the motorway, and I like to have a picnic in the center lane, and that car hits me, the force it exerts should be zero because F=2000 x 0 which is zero so no force exerted on me?

Yet I'm still thinking I'd be in trouble.. So what would cause me trouble if not a force exerted on me..

I assume my misunderstanding is something very basic and obvious.

Thanks.
 
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Mental Gridlock said:
I know this is probably a complete n00b question, but I have tried all over the www and couldn't find it explained.

Correct any of these premises if they are wrong, maybe that's my problem, but...

velocity is current speed and acceleration is change in speed.

So, if something is moving at a constant speed, it's acceleration is zero.

So far so good.

Mental Gridlock said:
So if a car (say 2000kg) is traveling at a constant speed of say 70mph on the motorway, and I like to have a picnic in the center lane, and that car hits me, the force it exerts should be zero because F=2000 x 0 which is zero so no force exerted on me?

Yet I'm still thinking I'd be in trouble.. So what would cause me trouble if not a force exerted on me..

Assuming no friction or air resistance, then sure, the car doesn't have a force on it. In reality, it does. Still, you are sitting in the middle of the highway moving at zero velocity. After you are hit, do you think you will still be moving at zero velocity? Certainly not. You would be accelerated to effectively the same speed as the car and over a very short time, meaning a large acceleration and a large force to go with it.
 
Mental Gridlock said:
So, if something is moving at a constant speed, it's acceleration is zero.

So if a car (say 2000kg) is traveling at a constant speed of say 70mph on the motorway, and I like to have a picnic in the center lane, and that car hits me, the force it exerts should be zero because F=2000 x 0 which is zero so no force exerted on me?

Yet I'm still thinking I'd be in trouble.. So what would cause me trouble if not a force exerted on me..
For determining the force exerted on you the important thing is your acceleration. If you are sitting at your highway picnic and no car is hitting you then you are at 0 mph and remain at 0 mph so you are not accelerating and there is no force. When the car hits you go from 0 mph to 70 mph almost instantaneously so you accelerate dramatically due to the very large force acting on you.

The car, meanwhile, has also accelerated. It has much more mass than you do, say 20 times as much. So while you accelerate by 70 mph the car decelerates by 3 or 4 mph. The force required to do so leaves a big dent in the fender, plus all of the mess from your picnic.
 
Probably the more important question is ... why would someone be having a picnic in the middle of the motorway?
 
Whovian said:
Probably the more important question is ... why would someone be having a picnic in the middle of the motorway?

If a car hitting you would exert 0 force...then why not? Hence OP's question.
 
It should be

F = ma

vs

KE = 1/2mv^2

For accelerating systems, F=ma, determines the force required to ACCELERATE the body. This is not the energy the body possesses.

The situation you are describing, constant speed of car, the KE formula does what you want, determines the kinetic energy that the body possesses. i.e. how hard it "hits" something.
 
Mental Gridlock said:
I know this is probably a complete n00b question,
Correct any of these premises if they are wrong, maybe that's my problem, but...

Mental Gridlock said:
So if a car (say 2000kg) is traveling at a constant speed of say 70mph on the motorway, and I like to have a picnic in the center lane, and that car hits me, the force it exerts should be zero because F=2000 x 0 which is zero so no force exerted on me?

Yet I'm still thinking I'd be in trouble.. So what would cause me trouble if not a force exerted on me..

So, is this a premise or a conclusion? If it's a premise, where did you get it from, because it certainly looks wrong. If it's a conclusion, you made a non-sequitur logical fallacy, since it does not follow from your previous premises.

Do you know that 2nd Newton's law should be applied to one body at a time? Your premises are about the car, and it seems to me you are wondering if a force would be exerted on an unlucky object in the middle of the road.
 
I think you should use this relationship; p = mv (which is Momentum).
p; the affect on you. So to calculate that:
p = 2000 (kg) x 70 (mph) = 140,000 kg.mi/h
----
 
Mental Gridlock said:
velocity is current speed and acceleration is change in speed.
No,Acceleration is change in velocity

Mental Gridlock said:
So, if something is moving at a constant speed, it's acceleration is zero.
Not necessarily.If the object is changing direction.(Think of centripetal motion.)

OP said:
So if a car (say 2000kg) is traveling at a constant speed of say 70mph on the motorway, and I like to have a picnic in the center lane, and that car hits me, the force it exerts should be zero because F=2000 x 0 which is zero so no force exerted on me?

Force=Change in momentum/time taken
Say you have 50kg and after the car hit you,your velocity increases to 50m/s(Common sense) in 5 seconds
Force exerted on you=##\frac{mv-mu}{t}=\frac{50*50-50*0}{5}=500N##
So the car exerted 500N of force on you.

(That's the net force acting on the car
Using F=ma,
The net force on car is 0N yes.But it's moving at constant velocity then how can there be no force?
Ans:The net force is zero(That does not mean there are no forces)

Force exerted on you,
F=ma
F=50*a(You will surely accelerate,the car won't go through you.)
 
  • #10
the force it exerts should be zero because F=2000 x 0 which is zero so no force exerted on me?

When the car hits you the car will slow down (slightly) and you will be accelerated (significantly). So the acceleration isn't zero.
 
  • #11
Two-year old thread that did not need to be resurrected. Locked.
 

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