Math Is Fun Forum

yonski

Member

Registered: 2005-12-14

Posts: 67

A little motion problem

Hi there,
this is more of a physics question, but maybe someone out there can help.

"A body (not a dead body i hope) 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 4m/s^2? (g=10m/s^2)"

This has been bugging me all day but i can't seem to get the right answer. If anyone could run me through it i'd be very happy!

Thanks, Jon.

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Student: "What's a corollary?"
Lecturer: "What's a corollary? It's like when a theorem has a child. And names it corollary."

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: A little motion problem

As the lift is going upwards, this makes the body "heavier", and it is as if gravity is now 14ms-².

Force = mass x acceleration, and using the values we know this means that 70 = mass x 14.

This in turn means that the body weighs 70/14 = 5kg. As the values work out nicely like that, that's probably right.

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Why did the vector cross the road?
It wanted to be normal.

yonski

Member

Registered: 2005-12-14

Posts: 67

Re: A little motion problem

Okay, yeah, it makes sense when you put it like that. That is the right answer cos it's in the back of the book. I just couldn't understand why.

Thanks.

---

Student: "What's a corollary?"
Lecturer: "What's a corollary? It's like when a theorem has a child. And names it corollary."

George,Y

Member

Registered: 2006-03-12

Posts: 1,379

Re: A little motion problem

Refer to Newton's Laws and you will get a through.

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X'(y-Xβ)=0