# Conveyor belt problem

i have a test tomorow and our teacher gave us an example of a problem that might be coming tomorow. the question is
Sand falls on a conveyor belt at 60 kg per second and the belt moves at 2 meters per second.
a) what is the force moving the sand?
b) what is the magnitude of the force?
c) force exerted on belt by the engine.
d) the engine has 40% efficiency. how much is the input power?

well i am guessing that the first three parts will be the same based on some of the other questions i looked at https://www.physicsforums.com/archive.../t-150868.html [Broken]
do we multiply the rate at which the mass is changing times the velocity to find the force on the belt? so would the force on the belt be 120 N since 60 x 2?? and the magnitude would be the same im guessing. and the force exerted on the belt by the engine would also be 120 N right? but im a bit lost on how to convert it to power. can anyone help me please? i have the test tomorow and i need to know if i am correct or not. thank you in advance

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rl.bhat
Homework Helper
It is the problem of varying mass.
At any instant let the moment of the system be
P = m*v
External force = dP/dt = v*dm/dt, because dv/dt = 0
Power supplied by the external force = F*v.

It is the problem of varying mass.
At any instant let the moment of the system be
P = m*v
External force = dP/dt = v*dm/dt, because dv/dt = 0
Power supplied by the external force = F*v.

yea but these are just the formulas. i dont get the answer. like are my answers right? thank

rl.bhat
Homework Helper