# Mass vs. weight with a falling rock and a spring scale

• physicswhiz
In summary: All Potential energy to be absorbed by the rock...A) All potential energy to be absorbed by the rock.B) All potential energy to be absorbed by the toe.C) The mass of the rock absorbs more potential energy than the mass of the toe.A) All potential energy to be absorbed by the rock.B) All potential energy to be absorbed by the toe.C) The mass of the rock absorbs more potential energy than the mass of the toe.
physicswhiz

## Homework Statement

1. You stand on a spring-loaded bathroom scale in a bathroom. The scale "reads" your mass. What is the scale actually measuring?

2. Similarly, you stand on a spring-loaded bathroom scale in an elevator that is accelerating upward at 2.0 m/s^2. The scale "reads" your mass. What is the scale measuring?

A) Your mass B) Your weight C) The force of the scale pushing up on your feet D) The force of your feet pushing down on the scale
1. A large rock falls on your toe. Which concept is most important in determining how much it hurts?

2. Similarly, if the large rock merely sits on your toe, which concept is most important in determining how much it hurts?

A) The mass of the rock B) The weight of the rock C) Both the mass and the weight are important. D) Either the mass or the weight, as they are related by a single multiplicative constant, g.

## Homework Equations

Weight = mg = dp/dt

Impulse = Δp = mΔv

## The Attempt at a Solution

For 1,2: To me, C seems correct for both of them, but B also seems correct for the first part, as weight is equal to the force of the scale on your feet for that part.

For 3,4: For a falling rock, to me it seems that mass would be most important as the pain level would be proportional to the change in momentum, or the impulse, on your foot. For a stagnant rock, it seems either weight or mass would be the same.

Last edited:
Your questions 1&2 =3&4

You would be correct in reasoning that C is right for the first two questions. While you might be tempted to say that a scale reads your weight, and it is true under normal circumstances that the force of your weight is equal to the upward force of the scale, the scale is calibrated to read the upward force it gives due to your weight. This is why you would appear heavier on a scale accelerating upward in an elevator; the acceleration adds to the "g" term in the equation, which causes a greater upward force to act on the scale.

I believe you may have copied questions 3 and 4 incorrectly; they appear the same as 1 and 2.

Here are the intended questions for 3 and 4:

A large rock falls on your toe. Which concept is most important in determining how much it hurts?

Similarly, if the large rock merely sits on your toe, which concept is most important in determining how much it hurts?

A) The mass of the rock B) The weight of the rock C) Both the mass and the weight are important. D) Either the mass or the weight, as they are related by a single multiplicative constant, g.

My attempt is above.

For question 1...

C) The force of the scale pushing up on your feet
D) The force of your feet pushing down on the scale

Rivercats... Is there a way to show that it's C) and not D) ?

For 3&4 I would have thought the amount of energy your toe had to absorb would be important. That's mgh. I reckon answer D.

CWatters said:
For question 1...

Rivercats... Is there a way to show that it's C) and not D) ?

I should think so. Even though the force of your weight pushing down on the scale is what's being measured, the scale is calibrated to read the upward force that results from that. In other words, the scale attempts to balance the force of your weight by exerting an equal force (normal force) upward, and this normal force is what the scale reads.

physicswhiz said:
Here are the intended questions for 3 and 4:

A large rock falls on your toe. Which concept is most important in determining how much it hurts?

Similarly, if the large rock merely sits on your toe, which concept is most important in determining how much it hurts?

A) The mass of the rock B) The weight of the rock C) Both the mass and the weight are important. D) Either the mass or the weight, as they are related by a single multiplicative constant, g.

My attempt is above.

I would say that D is correct. The amount of pain your foot feels is equivalent to the energy it absorbs, which is mgh. Since mg = weight, using either mg or simply weight means the equation remains the same. So, considering pain, mass and weight are pretty interchangeable.

Consider this also. On the moon, g changes, which means weight changes, not mass. This affects the amount of pain that is felt. This may lead you to answer B, but since D says "either" mass or weight, not both, I would still say D is correct.

Fnet=ma
1a. Fs-mg=0
1b. Fs-mg=ma

Thus the scale shows force acting on the body to produce static or accelerating.
So i think the answer is (C)

2a. All Potential energy to be absorbed by the toe before it stops
ΔPE=mgh
2b. The force acting on the toe equal to mg

So mass is the only constant factor in both 2a and 2b thus the answer is (A)

## 1. What is the difference between mass and weight?

Mass is a measure of the amount of matter in an object, while weight is a measure of the force of gravity acting on an object. Mass is constant and does not change with location, while weight can vary depending on the strength of gravity.

## 2. How is mass measured with a falling rock?

Mass can be measured with a falling rock by using the formula F=ma, where F is the force of gravity, m is the mass of the rock, and a is the acceleration due to gravity. By measuring the acceleration of the rock as it falls, the mass can be calculated.

## 3. What is a spring scale and how does it measure weight?

A spring scale is a device that measures weight by using the force exerted by a spring. The scale stretches the spring when weight is placed on it, and the amount of stretch is measured and converted to a weight measurement.

## 4. How does the mass of the rock affect its weight on a spring scale?

The mass of the rock does not directly affect its weight on a spring scale. The weight measured on a spring scale is affected by the force of gravity, so the weight may vary depending on the location and strength of gravity.

## 5. Can the mass of the rock be measured with a spring scale?

No, the mass of the rock cannot be directly measured with a spring scale. However, the weight measured on a spring scale can be used to calculate the mass using the formula F=ma, where F is the weight measured on the scale, m is the mass of the rock, and a is the acceleration due to gravity.

• Introductory Physics Homework Help
Replies
29
Views
400
• Introductory Physics Homework Help
Replies
5
Views
997
• Introductory Physics Homework Help
Replies
14
Views
3K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Mechanics
Replies
17
Views
1K
• Introductory Physics Homework Help
Replies
8
Views
981
• Introductory Physics Homework Help
Replies
11
Views
2K
• Introductory Physics Homework Help
Replies
17
Views
714
• Introductory Physics Homework Help
Replies
8
Views
6K
• Introductory Physics Homework Help
Replies
4
Views
15K