# Altitude needed to create an impact of 4kg using a weight of 70g?

1. Jun 13, 2012

### conordee

Would anyone know what altitude would be needed within a vacuum in earths gravity to create an impact of 4kg using a weight of 70g? Thanks

2. Jun 13, 2012

### Staff: Mentor

That depends on the impact. The height (together with gravity and your weight) determines kinetic energy and momentum, but the magnitude of deceleration depends on the length of this deceleration.

In addition, it is not a good practice to talk about "an impact of 4kg". I think you mean the force of 40N (newton), which is the gravitational force a mass of 4k feels on earth.

3. Jun 13, 2012

### conordee

please excuse my bad wording, but I hope you can under stand what im getting at. Say if i was to create a large tube (with a vacuum) 400ft in the air, on earth of course. And i dropped a 70g weight from the top, what hight? would i need the tube so that the force when the weight hit the bottom would be equal to 4kg?

4. Jun 13, 2012

### jbriggs444

As has already been pointed out, the answer is indeterminate. It depends on the rigidity of the object you are dropping and the rigidity of the target onto which it is dropped.

In order for a 70g object to produce a force of 40 Newtons would require a decelleration rate of 40N / .070kg = 571 meters/second/second ~= 57 times the acceleration of gravity.

For sufficiently rigid objects, that acceleration could be attained after a drop of less then 1cm. For sufficiently fluffy objects, it would obviously take much farther.

One crude estimate would be to compare the deflection at impact to the height of the drop. You need the height of the drop to be roughly 57 times the deflection at impact, all things being equal.

[ But in the real world, all things are never equal ]

5. Jun 13, 2012

### K^2

Just to expand on this a little, force of the impact is rarely a parameter you deal with. It can be important, but it's also nearly impossible to predict. Way too many factors go into it. What you can determine for a particular impact are momentum and energy transfer. Momentum is measured in kg*m/s or, equivalently, in N*s. Energy is in Joules. If you know both, you can say a lot about the impact.

6. Jun 13, 2012

### conordee

Ok I get it now, thanks