Help with a physics question! Watch

Timlong365
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#1
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Hello!
I need help with an A-level physics question. The question is:

A body hangs from a spring-balance which is suspended from the ceiling of a lift. What is the mass of the body if the balance registers a reading of 70N when the lift has an upward acceleration of 4.0ms^-2? (g = 10ms^-2)
The answer the book gives is 5kg, but I need help figuring out how you arrive at the answer.
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k4l397
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Best way of approaching this is to just consider the forces on the body that is hanging in the lift. The body itself only has two forces acting on it, the weight and the tension of the spring. We know the tension must be 70N as the question has told you the reading. You can use this information to get the resultant force on the body in terms of m. Using this you can work out the mass of the body with F=ma, as you also know the acceleration.

Hope this is useful
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JD1lla
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Considering the forces acting on the body, you arrive at:

T-mg = ma

Then, just substitute the values you are given into the question

70 - 10m = 4m

70=14m

m = 70/14




Not sure on rules for posting solutions...but erm...yeah...
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username1862217
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Find the total acceleration acting on the object
HINT
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Add the accelerations. It has g=10ms^-2 and an upwards acc of 4ms^-2. If it is upwards then the spring is extended more (like when an elevator is accelerating upwards you feel heavier, as if there is more gravity), so we add the two numbers. Total acc = 14ms^2.
Now use the trusty old formula relating mass and weight.
HINT
Spoiler:
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F = ma where F is the force in newtons, m is the mass in kg, and a is the acceleration in ms^-2.
I'll leave you to see if you get the right answer.

(Original post by Timlong365)
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Timlong365
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(Original post by JD1lla)
Considering the forces acting on the body, you arrive at:

T-mg = ma

Then, just substitute the values you are given into the question

70 - 10m = 4m

70=14m

m = 70/14




Not sure on rules for posting solutions...but erm...yeah...
Great! This is the way we were taught it. I was just a little bit confused because I thought that mg = 70N, but I understand it now.
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Timlong365
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Thanks to all!
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Timlong365
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(Original post by k4l397)
Best way of approaching this is to just consider the forces on the body that is hanging in the lift. The body itself only has two forces acting on it, the weight and the tension of the spring. We know the tension must be 70N as the question has told you the reading. You can use this information to get the resultant force on the body in terms of m. Using this you can work out the mass of the body with F=ma, as you also know the acceleration.

Hope this is useful
Gracias
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