Elementary Q: how to calculate speed of Mass B, upon impacting Mass A

[Lifeboatboy]

Homework Statement

Mass A is stationary, weighing 1500kg and is impacted (horizontally) by Mass B, which weighs 1400kg. The impact is a direct line at the centre of both masses (i.e. for the sake of me keeping up with you experts, please assume no rotational consideration, ignore friction, and assume constant velocity without deceleration or acceleration - which I realise will irritate the purists, but you'll lose me otherwise).

Mass A is displaced 36m after impact in a straight line. Mass B stops at the point of impact with negligible momentum and rotation.

Is there a way for me to calculate the speed/velocity at the time of impact?

Relevant Equations

Unsure

 

[haruspex]

Problem Statement: Mass A is stationary, weighing 1500kg and is impacted (horizontally) by Mass B, which weighs 1400kg. The impact is a direct line at the centre of both masses (i.e. for the sake of me keeping up with you experts, please assume no rotational consideration, ignore friction, and assume constant velocity without deceleration or acceleration - which I realize will irritate the purists, but you'll lose me otherwise).

Mass A is displaced 36m after impact in a straight line. Mass B stops at the point of impact with negligible momentum and rotation.

Is there a way for me to calculate the speed/velocity at the time of impact?
Relevant Equations: Unsure

View attachment 244715

You need to know the speed of mass A just after the collision. The distance it traveled is not enough - you need to know the magnitude of the force that brought it to a stop over that distance.

 

[PeroK]

Problem Statement: Mass A is stationary, weighing 1500kg and is impacted (horizontally) by Mass B, which weighs 1400kg. The impact is a direct line at the centre of both masses (i.e. for the sake of me keeping up with you experts, please assume no rotational consideration, ignore friction, and assume constant velocity without deceleration or acceleration - which I realize will irritate the purists, but you'll lose me otherwise).

Mass A is displaced 36m after impact in a straight line. Mass B stops at the point of impact with negligible momentum and rotation.

Is there a way for me to calculate the speed/velocity at the time of impact?
Relevant Equations: Unsure

View attachment 244715

This looks like a crude model of a car crash.

Accident investigation is a specialist subject which uses a lot of specialized physics and modeling- in addition to basic collision theory.

We're not able to help you prove that someone was speeding or not if that's the objective.

 

[Lifeboatboy]

You need to know the speed of mass A just after the collision. The distance it traveled is not enough - you need to know the magnitude of the force that brought it to a stop over that distance.

Thank you for the response! Massively appreciated. For the sake of arguments (and my simple maths), can I say the speed of Mass A is 6 metres per second at constant speed until it comes to a stop (again, to keep it simple), thus taking 6 seconds at constant speed to travel it's 36 metres.

 

[Lifeboatboy]

This looks like a crude model of a car crash.

Accident investigation is a specialist subject which uses a lot of specialized physics and modeling- in addition to basic collision theory.

We're not able to help you prove that someone was speeding or not if that's the objective.

@PeroK - This is a hypothetical scenario that occurred to me whilst having a shave in the shower this morning. I'm asking myself: if 2 items strike one another (removing complexities such as friction, air displacement, energy transfer and all the stuff I remember from school), is there a simple formula I should have remembered, and if so, what else would I need.

 

[berkeman]

Thread is closed temporarily for Moderation...

 

[berkeman]

Okay, now that we have established that this is a hypothetical scenario and not for an accident investigation, the thread has been cleaned up a bit and is now re-opened. Thanks for your patience.

 

[haruspex]

Thank you for the response! Massively appreciated. For the sake of arguments (and my simple maths), can I say the speed of Mass A is 6 metres per second at constant speed until it comes to a stop (again, to keep it simple), thus taking 6 seconds at constant speed to travel it's 36 metres.

What conservation law(s) do you know that could be relevant?

 

[DaveE]

I'm not sure I understand your question.
"ignore friction, and assume constant velocity without deceleration or acceleration"
Then what makes everything stop moving?

 

[haruspex]

I'm not sure I understand your question.
"ignore friction, and assume constant velocity without deceleration or acceleration"
Then what makes everything stop moving?

Maybe it hit a wall. I take post #4 as replacing the stopping distance info with post-collision speed info.

 

[Lifeboatboy]

@haruspex @DaveE Thank you so much for taking the time.
@DaveE - Yours is a good analogy, possibly better than mine! And alas I don't know what conservation laws are applicable ( ).
I thought this would be one of those things where Newton or some other latter-day genius had a formula that I should have remembered.
Maybe this isn't something I should lose sleep over. I genuinely thought there was a formula that would apply.

 

[haruspex]

@haruspex @DaveE Thank you so much for taking the time.
@DaveE - Yours is a good analogy, possibly better than mine! And alas I don't know what conservation laws are applicable ( ).
I thought this would be one of those things where Newton or some other latter-day genius had a formula that I should have remembered.
Maybe this isn't something I should lose sleep over. I genuinely thought there was a formula that would apply.

If you knew the coefficient of kinetic friction between mass A and the floor you could find the acceleration after impact (a=-μ_kg). With the stopping distance s, that would tell you A's velocity just after impact (v²=-2as).
Then knowing the two masses you can use conservation of momentum to find B's velocity before impact.

 

[Lifeboatboy]

LOL! @haruspex : Sir/Madam - you're way out of my league, and I doff my cap very, very sincerely. It was just a question that occurred to me in the shower, and I thought I ought to know the answer. Now though, I feel considerably better knowing that a) I haven't had to worry about this since school, b) I haven't needed it since school, and c) I probably didn't even know how to do it when I was *at* school!

Sincerely - thank you.

PS - smart ass ;-) x