# Force applied by a constant stream of particles

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1. Dec 20, 2015

### LaszloNagy

1. The problem statement, all variables and given/known data
I'm making a program that draws a graph of the weight of an hourglass. Obviously at first we know the weight, and as the grains begin to fall it decreases by the amount of mass in free fall. However I ran into a problem when the grains begin to hit the bottom. It's impossible to know the weight of a single grain, all we know is the total volume, the height which the grains travel, and the density of the sand.

2. Relevant equations
How could I calculate the change this series of tiny impacts causes in the hourglass's weight?

3. The attempt at a solution
I tried F = m*g*h, but I don't think this would work since we are talking about a continuous stream, not a single object.

2. Dec 20, 2015

### Staff: Mentor

Consider the amount of mass that hits the bottom over some (short) time interval T, and its momentum. It does not matter how many grains of sand are in, as long as the approximation of a continuous mass flow works you don't have to care about object numbers.

Alternatively, think what the center of mass is doing and treat the hourglass as black box for this step.