Yr 11 moving about qs help (1 Viewer)

[princesssuku99]

Hey can someone please help me with this qs:
A physics teacher decides to use bathroom scales (calibrated in newtons) in an elevator. The scales provide a measure of the force with which they push up on the teacher. When the lift is stationary the reading on the scales is 823 N. What will be the reading on the scales when the elevator is:

a) moving upwards at a constant speed of 2.0m/s?
b) accelerating downwards at 2.0m/s/s?
c) accelerating upwards at 2.0m/s/s?

Thanks

 

[Silly Sausage]

You should draw a diagram and identify which forces are acting upon him/her.

 

[InteGrand]

Hey can someone please help me with this qs:
A physics teacher decides to use bathroom scales (calibrated in newtons) in an elevator. The scales provide a measure of the force with which they push up on the teacher. When the lift is stationary the reading on the scales is 823 N. What will be the reading on the scales when the elevator is:

a) moving upwards at a constant speed of 2.0m/s?
b) accelerating downwards at 2.0m/s/s?
c) accelerating upwards at 2.0m/s/s?

Thanks

The formula we use here is that:



where a is the acceleration of the elevator (with upwards taken as positive), m is the mass of the physics teacher, and the 'actual weight' is the weight of the physics teacher when there is no acceleration. ('perceived weight' refers to the reading on the scales IF the scales showed results in newtons.) (I don't know if you'll be required to derive this formula.)

Then the scales reading (in kg) is:

so:

a) Acceleration here is 0, so the reading WOULD be 823 N. However, scales typically show kg instead, so the reading on the scale would be . (Since acceleration on earth is 9.8 m s^-2, so a 823 N on earth corresponds to mass reading of 823/9.8 kg).

b) From part a), we know the mass of the physics teacher to be m = 823/9.8 kg. Also, the acceleration is a= -2.0 m s^-2 Hence the perceived weight is equal to: m(g+a) = 823/9.8 *(9.8 -2.0) N = 655.0408.... N, which gives the mass reading as 66.840.... kg (dividing by 9.8 m s^-2), or 67 kg to 2 sig. fig.

c) This time, a = +2.0 m s^-2, and m is same as above.

Hence the perceived weight is equal to: m(g+a) = 823/9.8 *(9.8 + 2.0) N = 990.959.... N, which gives the mass reading as 101.118.... kg (dividing by 9.8 m s^-2), or 100 kg to 2 sig. fig.

 

[princesssuku99]

Thank you