S4 Physics Apparent Weight

用戶90882

2010-05-26 6:25 am

Apparent Weight MCQ
Can anyone help me? Thanks

The answer is A. Mainly I don't know why I cannot choose C as the answer? Can anyone explain to me. Thank you.

Jeanie steps onto a weighing scale in a lift and the reading of the scale is 550 N.
If her mass is 45 kg, which of the following statements about the lift is correct?
AThe lift is moving up with acceleration 2.22 m s2.
BThe lift is moving down with acceleration 1.8 m s2.
CThe lift is moving down with deceleration 2.22 m s2.
DThe lift is moving up with deceleration 1.8 m s2.

If you help me, you can get 10 marks
Thank you.

回答 (1)

天同

2010-05-26 7:41 am

✔ 最佳答案

Let a be the acceleration of Jeanie.
Use net-force = mass x acceleration, and taking upward direction as +ve,
550 + (- 45g) = 45a
where g is the acceleration due to gravity, taken to be 10 m/s2
solve for a gives a = 2.22 m/s^2
since a is +ve, the lift is moving up with an acceleration.

Should the lift be moving down, the equation would become,
R + (-45g) = 45 x (-2.22)
this gives R = 350 N
The reading on the scale would be lower than her body weight.

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