# Stopping an object in motion

• KelseyG
In summary, a person is seeking help with a problem involving an object in downward motion that weighs 20,000 lbs and is moving at a constant velocity of 5 ft/s. The object contacts a solid object and stops instantly, leading to a discussion about the force exerted to stop the object. The conversation involves estimating the forces involved and the variables needed to solve the problem. It is concluded that the more abruptly the object is stopped, the greater the force, as demonstrated by the example of a glass breaking when dropped on a concrete floor. There are also factors such as the distance the object dropped and the state of the surface that need to be considered in calculating the force.

#### KelseyG

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

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.

cnh1995
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...

## What is inertia?

Inertia is a property of matter that describes the tendency of an object to resist changes in its state of motion. An object in motion will continue moving at a constant speed and direction unless acted upon by an external force.

## What is the relationship between mass and inertia?

The greater an object's mass, the greater its inertia. This means that a more massive object will require a greater force to change its state of motion compared to a less massive object. In other words, mass and inertia are directly proportional.

## What is the difference between stopping an object and slowing down an object?

Stopping an object involves bringing it to a complete halt, while slowing down an object involves reducing its speed. Stopping an object requires a greater force than slowing it down, as it requires overcoming the object's inertia.

## What are the factors that affect the stopping distance of an object in motion?

The stopping distance of an object in motion is affected by its initial speed, mass, and the force applied to stop it. Other factors such as friction and air resistance may also play a role in determining the stopping distance.

## How can we calculate the force needed to stop an object in motion?

The force needed to stop an object in motion can be calculated by using Newton's second law of motion, which states that force is equal to mass multiplied by acceleration. So, the force needed to stop an object can be found by dividing its mass by the time it takes to come to a complete stop.

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