Who Applies the Larger Force on Earth and Moon?

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Discussion Overview

The discussion revolves around the forces applied by two individuals, Fred on Earth and Jane on the Moon, as they lift and throw a 2.0 kg rock. The scope includes conceptual understanding of force in relation to gravity and the mechanics of throwing an object horizontally.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that Fred must apply a larger force to lift the rock due to the greater gravity on Earth compared to the Moon.
  • Others argue that when throwing the rock horizontally, the force required may depend on the time taken to accelerate the rock, suggesting that both could apply equal force if the same conditions are met.
  • One participant points out that to achieve horizontal velocity, a vertical force must also be applied, complicating the assessment of total force required.
  • Another participant notes that the question lacks clarity regarding whether it refers specifically to horizontal force, which could lead to different interpretations of the required force.
  • Concerns are raised about the possibility of having the same answer for both questions, with some participants indicating that there is no rule against this.

Areas of Agreement / Disagreement

Participants generally agree that lifting the rock requires different forces due to gravity, but there is disagreement regarding the forces involved in throwing the rock horizontally, with multiple competing views on how to interpret the problem.

Contextual Notes

The discussion reveals limitations in the clarity of the questions posed, particularly regarding the components of force involved in the horizontal throw and the assumptions about time and acceleration.

Amelina Yoo
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Q: Fred is on Earth, and Jane is on the Moon.

a) Each lifts a 2.0 kg rock. Who has to apply the larger force?
b) Each throws the rock horizontally with initial speed of 6.0 metres per second. Who applies the larger force?

For both questions, I wrote: Fred, because the gravity on Earth is greater than that of the Moon, and so there is a larger force acting upon the rock. But, seeing as it is my answer to BOTH question, I am feeling doubtful, because how can there be the same answer to two questions?
 
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Amelina Yoo said:
a) Each lifts a 2.0 kg rock. Who has to apply the larger force?
I believe you're right, it's Fred.
Amelina Yoo said:
b) Each throws the rock horizontally with initial speed of 6.0 metres per second. Who applies the larger force?
If atmospheric resistance is to be taken into account, I believe it should be Fred. If not, I think both will apply equal force. The rock will take longer to land on moon.
 
The second question does not have a good answer, as it is asked.
Even without leaving our planet you can achieve the 6 m/s speed of the rock with any force. It just depends on how long it takes to accelerate the rock.
Of course, besides the horizontal force, you need to apply a vertical force equal to the weight of the rock, in every case.
So assuming that both use the same horizontal force, for the same amount of time, the total force will be larger on Earth.

Regarding your concern, there is no rule to rule out same answer to two different questions.
 
Amelina Yoo said:
Q: Fred is on Earth, and Jane is on the Moon.

a) Each lifts a 2.0 kg rock. Who has to apply the larger force?
b) Each throws the rock horizontally with initial speed of 6.0 metres per second. Who applies the larger force?

For both questions, I wrote: Fred, because the gravity on Earth is greater than that of the Moon, and so there is a larger force acting upon the rock. But, seeing as it is my answer to BOTH question, I am feeling doubtful, because how can there be the same answer to two questions?
To throw a rock requires applying force over a distance, or work, on the rock. Work is the dot product of force and displacement. There is no extra force due to gravity horizontally therefore the dot product does not vary with gravity. Therefore, both use the same force (assuming it takes the same amount of time to throw).
 
Last edited:
Lifting a rock in lower gravity requires less force. Accelerating a given mass in a direction perpendicular to any gravitational field requires the same force.
 
If you want to have horizontal velocity at the end of acceleration you need to exert a vertical force as well. Otherwise the rock falls during the acceleration period and the final velocity won't be horizontal. The question in the OP does not specify that is about the horizontal component of the force only.
 

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