Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I'm sorry if this is a stupid question, but I've never taken a physics class and I was curious about something. But anyway, my question is, is there such a thing as instantly stopping? For example, if I'm traveling at 70MPH and I hit a wall, between the time that I am going at full speed and when I hit the wall, do I slow down until I reach zero, or do I instantly stop? Do I go every speed in between 70 and 0?

I'm sorry if my question is unclear. If it is, please leave a comment and I'll try explaining it more.

share|improve this question

1 Answer 1

up vote 4 down vote accepted

To stop instantly, you would need infinite deceleration. This in turn, requires infinite force, as demonstrable with this equation: $$\vec F=m\vec a$$ So when you hit a wall, you do not instantly stop (e.g. the trunk of the car will still move because the car is getting crushed). In a case of a change in momentum, $m\vec v$, we can use the following equation to calculate force: $$F=\frac{\Delta{p}}{\Delta{t}}$$ However, because the force is nowhere close to infinity, time will never tend to zero either, which means that you cannot come to an instantaneous stop.

share|improve this answer
Thank you very much for that. –  Michael Blake Feb 14 '14 at 0:40
I can't upvote it, but I marked it as an answer. Thank you. –  Michael Blake Feb 14 '14 at 0:45
Could you mention why an infinite force is unreasonable? –  BMS Feb 14 '14 at 6:50

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.