Calculating Stopping Force for an Object in Motion

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Discussion Overview

The discussion revolves around calculating the stopping force exerted on a heavy object (20,000 lbs) that comes to an instantaneous stop after moving downward at a constant velocity of 5 ft/s. Participants explore the implications of instantaneous stopping, the necessary calculations, and the factors influencing the stopping force.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants note that stopping an object instantly would require an infinite force, raising questions about the feasibility of such a scenario.
  • One participant suggests estimating the stopping force by considering a scenario where the object creates a divot in the ground, leading to calculations based on average speed and deceleration.
  • Another participant emphasizes that the abruptness of the stop affects the force required, comparing the impact of dropping an object on different surfaces.
  • Concerns are raised about the lack of information regarding the drop height and the condition of the surface the object impacts, suggesting that these factors are critical for accurate calculations.
  • One participant expresses uncertainty about their own understanding of the principles involved, indicating a need for more variables to solve the problem accurately.

Areas of Agreement / Disagreement

Participants generally agree that the concept of instantaneous stopping leads to complications and that various factors must be considered in calculating the stopping force. However, there is no consensus on the exact approach or the necessary variables for a complete solution.

Contextual Notes

Limitations include assumptions about constant deceleration, the specific conditions of the impact surface, and the absence of details regarding the drop height, which all affect the calculations and conclusions drawn.

KelseyG
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Can anyone help me? I'm pulling my hair out on a simple problem that I should know by heart.

I have an object in downward motion that weighs 20,000 lbs moving at a constant velocity of 5 ft/s. It contacts a solid object (say the ground) and is stopped instantly. What force was exerted to stop the object?

Thanks,
Kelsey
 
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Force is the rate of change of momentum with respect to time. If the object stops instantly, you are going to have some problems.
 
KelseyG said:
and is stopped instantly
The ball stopped in zero time, meaning the force is infinite.
 
KelseyG said:
I have an object in downward motion that weighs 20,000 lbs moving at a constant velocity of 5 ft/s. It contacts a solid object (say the ground) and is stopped instantly.
As the previous posts point out, the object didn't really stop instantly - that would require an infinite force. But we can estimate the actual forces involved. For example:

Let's say the object makes a divot 1/4" deep in the surface of the earth. That means the speed object went from 5 ft/sec to 0 ft/sec as it moved 1/4" from the point where the surface of the Earth was to where it ended up. Assuming constant deceleration (an approximation that simplifies the calculation no end) the object moved through that distance at an average speed of 2.5 ft/sec or 30 in/sec. Thus, it covered that distance in 1/120 seconds (1/4" at 30 in/sec). Its speed went from 5 ft/sec to 0 ft/sec in 1/120 seconds, so the acceleration was 600 ft/sec^2.

Now we can use F=ma to calculate the force needed to produce that acceleration. Just remember that if the object weighs 20000 lbs its mass is 20000/32 slugs. However, I'm done doing arithmetic in my head for this problem... There's a reason why scientists and engineers use the metric system these days :)
 
Well, sadly to say, I'm an engineer for forgot this stuff, lol. Thanks for the input. It makes me feel better about it knowing that it's not that simple and that I don't have all the variables I need in order to solve my situation.
 
KelseyG said:
Well, sadly to say, I'm an engineer for forgot this stuff, lol. Thanks for the input. It makes me feel better about it knowing that it's not that simple and that I don't have all the variables I need in order to solve my situation.

The basic principle is that the more abruptly the object is stopped (the shallower the divot in the earth, the less distance it travels as it decelerates) the greater the force. That's why a glass will break if you drop it on a concrete floor but not if you drop it onto a mattress.
 
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There are several things here that need to be addressed... 1) how far did it drop ? 2) There is no way that anyone can comment on the crater impact
component without knowing what state the surface is in... clay, cement, dirt that hasn't seen water for years... At best F = ma
9090.9 Kg x 1.52 m/s squared =13,818.16 N = Kg x m/s squared...
 

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