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Old Mar 12th 2014, 02:48 PM   #1
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Newton's 2nd Law Problem

Hey everyone. I am new and I am trying to figure out this physics problem.

Here is the question

A 12,000-kg bus slows from 30 m/s to 10 m/s in 10 seconds. What is the net force acting on the bus?

I do not understand what it means to find the net force.

This is what I have discovered so far: (my teacher makes us use PIPES)

P: Fnet = ?

I: Mass of bus = 12,000 kg ; Vi = 30 m/s ; Vf = 10 m/s ; change in time = 10 seconds

P/E: Fg=mg ; Fg = Fn

S: Fg = 12000 kg (9.8 N/kg) Fg = 117600 N
Fn = Fg Fn = 117600 N

I know that Fg is my gravitational force and Fn is my normal force, but I don't know what it means to find the net force acting on the bus.

Any advice?
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Old Mar 12th 2014, 02:49 PM   #2
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I also attached my force diagram to the thread, but I'm not sure if it is right or not.
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Old Mar 13th 2014, 06:55 AM   #3
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Force = Mass times Acceleration

Don't forget the deceleration of 20m/s over 10 seconds (20/10 m/s/s)

Then F=ma

Not sure what the precise implication of "Net" is in this question.
Not sure if the gravitational acceleration is required for this answer...
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Old Mar 13th 2014, 09:51 AM   #4
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What you have done is calculate the weight of the bus in Newtons, but that force is cancelled by the normal force acting on the bus from the road. Hence the net vertical force acting on the bus is zero. Consider this: from F=ma you know that if the net force is not zero then there must be acceleration; in this case since the bus is travelling horizontally (not vertically) there is no net vertical force. So the net force acting on the bus is in the horizontal direction only, and it serves to slow the bus. So: first determine the bus's rate of deceleration (acceleration = change in velocity divided by change in time) then apply F=ma to determine the net force.
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Old Mar 14th 2014, 10:50 AM   #5
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Another Perspective

The term "net" (as in net profit) used with force is a bad ideal. It is better to put the idea "net force" directly into the 2nd Law. Write ΣF = ma. The Greek character Sigma means "discrete sum." Don't use "net." A net is a tool for catching butterflies.
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