# Homework Help: Impluse / momentum

1. Feb 18, 2007

### ludakrishna

1. A dump truck is being filled with sand. The sand falls straight
downward from rest from a height of 2.00 m above the truck bed, and the
mass of sand that hits the truck per second is 55.0 kg/s. The truck is
parked on the platform of a weight scale by how much does the scale
reading exceed the weight of the truck and sand?

2. J = Ft p=mv mv(f) - mv(0)

3. i am completely lost.

2. Feb 18, 2007

### AlephZero

The relevant equation is "force = rate of change of momentum".

Start by finding the momentum of the sand just before it hits the truck.

3. Feb 18, 2007

### ludakrishna

Mass per second = 55kg , h= 2m

because mgh = ½ mv2

Velocity of sand v=sqrt(2gh) , = 6.2 m/s

I am going round and round.

When sand hits the top of the truck it hits it with a force which is equal to change in momentum. Final velocity of sand is 6.2 m/s which becomes zero upon striking the truck.

This change in momentum gives the force that cause apparent increase in weight.

V1= 6.2 , V2= 0

F = m a = (m V1- mV2) = 55 x 6.2 = 341 Newton

Mass = weight/g = 341/9.81 = 34.7 kg

i'm stuck from here on

4. Feb 18, 2007

### hage567

Right. So why are you setting it equal to ma?? Weight is measured in newtons, not kilograms.

5. Feb 18, 2007

### AlephZero

You got it. The answer to the question is 341 Newton, or 34.7 kgF.

Most likely the truck will be on a scale calibrated in "Kilograms" which means kilograms force not kilograms mass.

1 kgF = the force exerted by the earth's gravity on a mass of 1 kg = 9.81 Newtons.