# Homework Help: What force is exerted on the windscreen?

1. May 14, 2008

### looi76

1. The problem statement, all variables and given/known data
A car is driving through a rainstorm. Each raindrop of mass $$0.00005kg$$ hits the windscreen with velocity of $$16 ms^{-1}$$. If $$500$$ raindrops hit the glass and run down the windscreen each second without bouncing, what force is exerted on the windscreen?

2. Relevant equations

Force = Mass x Acceleration

Momentum = Mass x Velocity

Inelastic Collisions:
$$m_1u_1 + m_2u_2 = (m_1+m_2)v$$

3. The attempt at a solution

$$m = 5.0 \times 10^{-5}kg \ , \ u = 16ms^{-1}$$

don't know how to start, need help!

Last edited: May 14, 2008
2. May 14, 2008

### lzkelley

The equations you gave are correct; but also remember that F = change in momentum / change in time.
Make sure you keep track of your units, and a force needs to end up being kg*m/s^2.

3. May 14, 2008

### looi76

Can someone please explain. Still, I don't know how to start =(

4. May 14, 2008

### mgb_phys

As lzkelley said, force is change in momentum / time and momentum is mass * velocity.
The rain drop is accelerated to the speed of the car by the windscreen and so pushes on the widnscreen with a force equal to the raindrops mass * change in velocity.

Assuming the drop is falling vertically, you have the change in velocity, you know the total mass of raindrops falling per second so you have the force.

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