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this doesnt even make sense

if force = mass x acceleration

then thats like saying a car that weighs 2000 kg traveling at 80 mph and not accelerating will hit you and there will be no force but i would definitely get run over???? (no)

how does this even make sense?

is it only one way, like hitting a car with 50000 m/s^2 of force would make it go 25 mph or whatever? (yes)

note, i understand that pounds is a conversion from newtons (which represents gravitational force = weight), so the numbers aren't exact, but this is more conceptual

thanks, i just need a yes or no, because i can't seem to find one online :)

Saketh Malyala
  • 13,637

1 Answers1

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The acceleration in $F=ma$ is the acceleration caused by the force -- not the acceleration of the object causing the force -- and especially not before it even exerts the force. So if a car hits you, you'd be accelerated backward from whatever your initial speed is to the same speed that the car is going -- that's the acceleration that $a$ represents.

It might be better to remember the formula in the form that Newton originally expressed it, though (updated with modern notation, of course): $$F = \frac{dp}{dt}$$ This tells us that the force is equal in magnitude to the rate of change of momentum of the object the force is exerted upon. Let's say that you have a mass of 70 kg and you're standing still when the car hits you. Then your initial momentum is $0$ kg m/s. The car hits you and in the course of say 0.2 seconds your speed increases to 20 m/s. So your final momentum is $70\cdot 20 = 1400$ kg m/s. Then the force exerted on you is far from 0 N. It is (approximately) $$F\approx \frac{1400-0}{0.2} = 7000\ \text{N}$$