Another homework of Newton's law of motion

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  • #1
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Homework Statement


A proud angler hangs her catch from a spring balance, which is supported from the roof of an elevator.
a.) if the elevator has an upward acceleration of 2.45m/s2 and the balance reads 50.0N, what is the true weight of the fish?
b.) Under what circumstances will the balance read 30.0N?
c.) What will the balance read if the cable breaks?

given:
a = 2.45m/s2
g = 9.8 m/s2
wbalance = 50.0N
wtrue = ?

what formulas will I use? Please help
 

Answers and Replies

  • #2
56
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Just look for the forces acting on the fish. You know it's acceleration and scale reading is actually the force between the fish and the scale (spring).

You can also solve this by using fictitious force (elevator is non-inertial reference frame).
 
  • #3
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Just look for the forces acting on the fish. You know it's acceleration and scale reading is actually the force between the fish and the scale (spring).

You can also solve this by using fictitious force (elevator is non-inertial reference frame).

Sorry, Im so dumb when it comes to physics.
Does this mean I'll use F = ma then solve weight from there?
 
  • #4
56
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Gravity pulls the fish down, and the spring pulls it up. Force from the spring is greater, so the fish (along with the elevator and everything inside) accelerates upwards. So, you have

[tex]m\vec{a}=m\vec{g}+\vec{F},[/tex]

where [itex]\vec{F}[/itex] is elastic force of the spring. When you get rid of the vectors, you have

[tex]ma=-mg+F.[/tex]

I assumed that upwards is the positive direction. Everything else (that points downwards) is negative - gravity in this case.
 
  • #5
17
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Gravity pulls the fish down, and the spring pulls it up. Force from the spring is greater, so the fish (along with the elevator and everything inside) accelerates upwards. So, you have

[tex]m\vec{a}=m\vec{g}+\vec{F},[/tex]

where [itex]\vec{F}[/itex] is elastic force of the spring. When you get rid of the vectors, you have

[tex]ma=-mg+F.[/tex]

I assumed that upwards is the positive direction. Everything else (that points downwards) is negative - gravity in this case.

Im so confused.... is the 50.0N a weight or a force?
 
Last edited:
  • #6
156
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Homework Statement


A proud angler hangs her catch from a spring balance, which is supported from the roof of an elevator.
a.) if the elevator has an upward acceleration of 2.45m/s2 and the balance reads 50.0N, what is the true weight of the fish?
b.) Under what circumstances will the balance read 30.0N?
c.) What will the balance read if the cable breaks?

given:
a = 2.45m/s2
g = 9.8 m/s2
wbalance = 50.0N
wtrue = ?

what formulas will I use? Please help

Take a look at http://answers.yahoo.com/question/i...VzdWx0BHNlYwNzcg--?qid=20061106061139AAPqm7P"

So Neutons being weight and g being the acceleration due to gravity, the 50N = m (g + 2.45). Find the mass in kg, then solve for the new weight.

Then go back and find out what acceleration the elevator should have to produce 30N.
 
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  • #7
HallsofIvy
Science Advisor
Homework Helper
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Im so confused.... is the 50.0N a weight or a force?
Both. A weight is a force. While the elevator is going upward, the net force on the spring (so the reading on the scale) is the weight of the fish, mg, plus the "ma" where m is the mass of the fish (not weight, weight is mg) and a is the acceleration of the elevator.

While the elevator is going upward, the net force is the weight of the fish minus ma.
 

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