Easy Peasy Lemon Squeezy
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Hi all. I have some questions about torque I can't figure out. If this is not the right place to put my question, I apologise cause it's a GAMSAT question and not a HSC question.
Sorry that this is a long post. Not allowed to distribute the exam papers so I've reworded the question slightly.
There are two similar versions of this questions. I haven't done physics so some concepts are confusing for me, but I can do the maths when I get the concept right.
Version 1:
Snow White is bed-ridden. Doc wants to find her mass without moving her off the bed (see diagram). The nurse has a weighing scale that can support the pair of legs of the bed at one end or the pair at the other end, but not both pairs at the same time.
To determine the mass of the patient the nurse first places the pair of legs at X on the scale and notes a reading of 700 N. He then places the pair of legs at Y on the scale and notes a reading of 800 N.
The nurse knows that the mass of the bed and bedding is 90 kg.
With this information, he is able to find how much Snow White weighs.
Assume that the bed is stationary during weighing and the acceleration due to gravity is 10 m s–2 .
Questions:
1. The total force exerted by the floor on the bed is:
Correct Answer: 1500N
I get this, this is just 700N + 800N.
2. What is the mass of the patient?
Correct Answer: 60kg. I get this too, we know the mass of the bed is 90kg, 1500N/10 = 150, 150-90=60kg
3. Suppose the length of the bed, the horizontal distance between X and Y in Figure 1, is L.
At what horizontal distance from Y is the centre of gravity of the patient, bed and bedding, considered as a single body, located?
How do you approach this question without just guestimating? I know the working, other people have asked this online too, 700 = 1500X and I can solve for X algebraically, but I don't understand how they got to this equation.
I know torque is force x distance the force is applied at from the pivot point. 700 is the force applied at L distance from Y. But how is 1500N the force applied at x distance? The 1500N of force is spread out all along L, not at a single contact point. Even if it were at a single point, why is the center of gravity at the 7/15L mark from Y?
Since this is a torque question, and the bed is not rotating, isn't it saying 700N = 800N?
Version 2:
Same as above, but this time, you know the bed is two metres long and has an unknown mass of M.
At one end, the scales read 232kg, at the other end, it reads 220kg. The centre of mass of the patient is 0.8m from the foot of the bed. What is the mass of the patient?
Again, I know this is a question about torque but I don't see how to do it.
Thank you so much for reading.
Sorry that this is a long post. Not allowed to distribute the exam papers so I've reworded the question slightly.
There are two similar versions of this questions. I haven't done physics so some concepts are confusing for me, but I can do the maths when I get the concept right.
Version 1:
Snow White is bed-ridden. Doc wants to find her mass without moving her off the bed (see diagram). The nurse has a weighing scale that can support the pair of legs of the bed at one end or the pair at the other end, but not both pairs at the same time.
To determine the mass of the patient the nurse first places the pair of legs at X on the scale and notes a reading of 700 N. He then places the pair of legs at Y on the scale and notes a reading of 800 N.
The nurse knows that the mass of the bed and bedding is 90 kg.
With this information, he is able to find how much Snow White weighs.
Assume that the bed is stationary during weighing and the acceleration due to gravity is 10 m s–2 .
Questions:
1. The total force exerted by the floor on the bed is:
Correct Answer: 1500N
I get this, this is just 700N + 800N.
2. What is the mass of the patient?
Correct Answer: 60kg. I get this too, we know the mass of the bed is 90kg, 1500N/10 = 150, 150-90=60kg
3. Suppose the length of the bed, the horizontal distance between X and Y in Figure 1, is L.
At what horizontal distance from Y is the centre of gravity of the patient, bed and bedding, considered as a single body, located?
How do you approach this question without just guestimating? I know the working, other people have asked this online too, 700 = 1500X and I can solve for X algebraically, but I don't understand how they got to this equation.
I know torque is force x distance the force is applied at from the pivot point. 700 is the force applied at L distance from Y. But how is 1500N the force applied at x distance? The 1500N of force is spread out all along L, not at a single contact point. Even if it were at a single point, why is the center of gravity at the 7/15L mark from Y?
Since this is a torque question, and the bed is not rotating, isn't it saying 700N = 800N?
Version 2:
Same as above, but this time, you know the bed is two metres long and has an unknown mass of M.
At one end, the scales read 232kg, at the other end, it reads 220kg. The centre of mass of the patient is 0.8m from the foot of the bed. What is the mass of the patient?
Again, I know this is a question about torque but I don't see how to do it.
Thank you so much for reading.
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