Mass vs weight

  • #1
rudransh verma
Gold Member
1,067
91
https://www.feynmanlectures.caltech.edu/I_09.html
“Weight and inertia are proportional, and on the earth’s surface are often taken to be numerically equal, which causes a certain confusion to the student. On Mars, weights would be different but the amount of force needed to overcome inertia would be the same.”

What does it mean?
The statement seems confusing!

If we try to throw a ball up on Mars then it would require less force than on Earth because the resisting force of gravity is less. Weight is less!
 
Last edited:

Answers and Replies

  • #2
Doc Al
Mentor
45,461
1,949
Weight is a measure of the gravitational force on an object. Sure, a ball on Mars would weigh less than on Earth.

But does that change the amount of force needed to produce a given acceleration? What does Newton's 2nd law say about that?
 
  • Like
Likes russ_watters
  • #3
rudransh verma
Gold Member
1,067
91
Weight is a measure of the gravitational force on an object. Sure, a ball on Mars would weigh less than on Earth.

But does that change the amount of force needed to produce a given acceleration? What does Newton's 2nd law say about that?
If a ball is at rest on Earth and one on mars, there will be less force needed to overcome inertia of rest on mars.
 
  • #4
phinds
Science Advisor
Insights Author
Gold Member
2022 Award
18,140
10,969
If a ball is at rest on Earth and one on mars, there will be less force needed to overcome inertia of rest.
How on Earth (or Mars) do you come to such a nonsensical conclusion? Have you really THOUGHT about this. Do you have ANY sense, physically, of what inertia IS?
 
  • Like
Likes russ_watters
  • #5
Doc Al
Mentor
45,461
1,949
If a ball is at rest on Earth and one on mars, there will be less force needed to overcome inertia of rest on mars.
Oh really? How about calculating the force needed to give a 1 kg ball an acceleration of 1 m/s^2. Do it for both Mars and for Earth.
 
  • #6
berkeman
Mentor
64,131
15,340
Oh really? How about calculating the force needed to give a 1 kg ball an acceleration of 1 m/s^2. Do it for both Mars and for Earth.

Specifically a horizontal acceleration... :wink:
 
  • #7
Doc Al
Mentor
45,461
1,949
Specifically a horizontal acceleration... :wink:
That would perhaps eliminate some confusion, but the net force required will be the same regardless of direction. :wink:
 
  • Like
Likes russ_watters and berkeman
  • #8
rudransh verma
Gold Member
1,067
91
Specifically a horizontal acceleration...
No not horizontal but vertical. Horizontal will be same.
That would perhaps eliminate some confusion, but the net force required will be the same regardless of direction. :wink:
The force required to lift a 1kg mass on Earth is 9.8N but on Mars it’s only 3.7 N. So weight as well as the force required to break the state of inertia will vary from planet to planet specially for vertical lifts.
 
  • #9
Doc Al
Mentor
45,461
1,949
So weight as well as the force required to break the state of inertia will vary from planet to planet specially for vertical lifts.
The net force required to accelerate a mass will be the same on any planet. Of course, the amount of force you have to provide to produce a given vertical acceleration will vary.

The (net) force required to accelerate a mass ("overcome inertia") is the same everywhere. That's Feynman's point.
 
  • #10
rudransh verma
Gold Member
1,067
91
The (net) force required to accelerate a mass ("overcome inertia") is the same everywhere. That's Feynman's point.
Ok! But it’s quite fascinating that a person throwing a ball up can throw it even further on planets with less gravity like on mars. Astronauts can jump even higher on moon.
But what if he races against time. Will he beat his record on moon? Will he run faster or slower than on earth?
 
  • #11
phinds
Science Advisor
Insights Author
Gold Member
2022 Award
18,140
10,969
No not horizontal but vertical. Horizontal will be same.
Continuing to spout the same nonsense will not make it true. You would be better off listening to what people are saying to you rather than ignoring it.
 
  • #13
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
29,562
15,015
This thread is turning into another dumpster fire.

@rudransh verma , it's very clear you are ttempting more advanced physics than you are ready for. You will make more progress by backing up and getting a solid foundation.
 
  • Like
Likes DaveE, phinds and Doc Al
  • #14
ergospherical
881
1,208
But what if he races against time. Will he beat his record on moon? Will he run faster or slower than on earth?

This would be quite an interesting question! I don't think there's any simple answer, because there's so many factors at play. The most obvious one is the drag-reduction because of the lack of atmosphere. The athlete also doesn't need to work as hard to overcome his weight (vertically), but since he/she spends longer in the air it's probably also harder to put a lot of power down (just look at the stride rate of professional sprinters!).
 
  • #15
rudransh verma
Gold Member
1,067
91
The most obvious one is the drag-reduction because of the lack of atmosphere.
Let’s take atmosphere out of the question. Then I think if a person jumps on moon with some force making an angle of 45 with horizontal then he will surely go higher on moon but the horizontal distance covered will be same. In other words he cannot run faster on moon but surely he can run longer. What do you say?
 
  • #16
ergospherical
881
1,208
Say, the astronaut can leave the ground at a speed ##v## in a direction of his choosing. (In other words, he can exert an impulse of fixed magnitude on the ground). How do the parameters of the resulting parabolic trajectory change as you vary the gravitational acceleration ##g##?
 
  • #17
russ_watters
Mentor
22,054
9,151
Let’s take atmosphere out of the question. Then I think if a person jumps on moon with some force making an angle of 45 with horizontal then he will surely go higher on moon but the horizontal distance covered will be same. In other words he cannot run faster on moon but surely he can run longer. What do you say?
That's still wrong/a mess. How can he go higher if he doesn't also go further? And you should go reread the posts about net force.
 
  • #18
phinds
Science Advisor
Insights Author
Gold Member
2022 Award
18,140
10,969
@rudransh verma it is very annoying that you rarely seem to answer questions that people ask you, you just ignore us and keep posting nonsense. You would be better off trying to figure out why we ask what we ask rather than ignoring our questions.
 
  • Like
Likes vela and Vanadium 50
  • #19
Arjan82
409
303
Let’s take atmosphere out of the question. Then I think if a person jumps on moon with some force making an angle of 45 with horizontal then he will surely go higher on moon but the horizontal distance covered will be same. In other words he cannot run faster on moon but surely he can run longer. What do you say?

So, I hope it's clear by now that the gravitational force on Mars is lower than on earth. Gravitational force, or mass times the acceleration of gravity, is what we call weight. Weight is a force.

Mass on the other hand is a measure for the amount of 'stuff' (let's keep it classical to not further confuse things). So, that's the difference between mass and weight. Mass is what resists acceleration. A more massive thing needs a higher force for the same acceleration. This is the same on Mars, the Moon, Earth or Jupiter for that matter.

Let's say you have a car and it can do 0 to 60 in 4 seconds in a vacuum on Earth (just to exclude aerodynamic drag, which just further confuses things). How would it perform on Mars? Exactly the same! (assuming enough grip of course). And on Jupiter? Again: the same!*)

The reason why you can jump higher on the moon is actually more complicated than you might think:
First, getting off the ground: the gravitational force (weight) keeping you on the surface is lower, this lower force is what you have to work against with your legs when jumping. But a lower force means you have higher 'excess force' on your body, since the sum of all forces on your body, so its weight and the force generated by your legs, is what is determining your acceleration. The higher net force means a higher acceleration and therefore a higher velocity the moment you leave the ground.
Then, returning to the ground: when you are airborne there is only one force acting on you: the gravitational force, or your weight. This is lower on the moon, but your mass is the same, therefore your acceleration back to the surface is lower on the moon, giving you more hang-time.


*) you can throw in all kinds of other secondary order effects like the resistance of the tires due to its weight and all that, but let's keep it simple, we are talking about the fundamentals.
 
  • #20
phinds
Science Advisor
Insights Author
Gold Member
2022 Award
18,140
10,969
Then I think if a person jumps on moon with some force making an angle of 45 with horizontal then he will surely go higher on moon but the horizontal distance covered will be same.
Once again you do not seem to have actually thought about the physical meaning of what you say. This seems to be a persistent problem that you have. For some reason you can't seem to connect concepts to the physical world. You should give this some thought.
 
  • Like
  • Sad
Likes russ_watters, weirdoguy, rudransh verma and 1 other person
  • #21
rudransh verma
Gold Member
1,067
91
it is very annoying that you rarely seem to answer questions that people ask you, you just ignore us and keep posting nonsense.
For some reason you can't seem to connect concepts to the physical world.
I am not smart as you guys. That was foolish.:headbang: Instead of thinking in velocity I was thinking in terms of force but the force stops acting as soon as we lift of the ground.
 
Last edited:
  • #22
rudransh verma
Gold Member
1,067
91
How do the parameters of the resulting parabolic trajectory change as you vary the gravitational acceleration g?
You are right. I should be watching for the parabola. That will be bigger as we lower the value of g. But do we actually make parabolas when running? Is running actually repeated jumping?
 
  • #23
russ_watters
Mentor
22,054
9,151
I am not smart as you guys.
It's not your intelligence, it's your approach/attitude (this has been pointed out to you before...). These concepts are successfully taught to schoolchildren.
But do we actually make parabolas when running? Is running actually repeated jumping?
When we're in the air, yes.
 
  • #24
phinds
Science Advisor
Insights Author
Gold Member
2022 Award
18,140
10,969
I am not smart as you guys.
That may or may not be but it has nothing to do with anything. As @russ_watters just said, your intelligence is not the problem. The problems, as I see it, are that
(1) you don't seem to make any attempt to connect concepts to the physical world even to the application of trivial every-day actions.
(2) you ignore good advice when it is given
(3) you don't seem to believe that we know what we are talking about
(4) you don't answer direct questions that are put to you.
 
  • Like
Likes weirdoguy, Vanadium 50 and russ_watters
  • #25
ergospherical
881
1,208
I can see why some advisors are frustrated, but making mistakes (and crucially learning from them) is one of, if not the most, important part of the learning process. I think there's some good evidence of that in this thread, e.g. #10, #15 and #22. And our job is to be that helping hand guiding you along the path. That said, I do agree you should try really hard to focus your enquiries.

Back to the point...
You are right. I should be watching for the parabola. That will be bigger as we lower the value of g. But do we actually make parabolas when running? Is running actually repeated jumping?
... you tell me! What does the velocity profile of the runner look like? Since gravity is vertical, their horizontal speed is unchanged whilst airborne in the absence of drag. The horizontal acceleration is confined to those times where there's a foot on the ground.
 
  • #26
rudransh verma
Gold Member
1,067
91
Since gravity is vertical, their horizontal speed is unchanged whilst airborne in the absence of drag. The horizontal acceleration is confined to those times where there's a foot on the ground.
It’s horizontal velocity will be same. It looks like there will be no difference. But there is a drag on earth. So I think the person will move faster on moon.
 
  • #27
russ_watters
Mentor
22,054
9,151
It’s horizontal velocity will be same. It looks like there will be no difference. But there is a drag on earth. So I think the person will move faster on moon.
You're talking in half-completed thoughts. What is it that determines how fast someone can run? Think/talk it through.
 
  • #29
russ_watters
Mentor
22,054
9,151
Friction and your strength!
Those are forces. What are they doing? What does your strength actually do? Does less friction (between what and what?) mean you can run faster? If so, why? Put more thought/effort into this.
 
  • #30
rudransh verma
Gold Member
1,067
91
What are they doing? What does your strength actually do? Does less friction (between what and what?) mean you can run faster?
They are giving you forward momentum. My strength helps me to push the ground even harder. Without friction we can’t run.
 
  • #31
russ_watters
Mentor
22,054
9,151
They are giving you forward momentum.
Momentum? Why would that matter to how fast you can run? (hint: it really doesn't).
My strength helps me to push the ground even harder.
What direction are you pushing on the ground?
Without friction we can’t run.
You mean friction between your feet and the ground? Does that mean you run slower if there is less friction?

What does Newton's 2nd Law tell us about the net force required to run at constant speed?

Speak clearly and finish thoughts completely.
 
  • #32
russ_watters
Mentor
22,054
9,151
You're really not engaging your brain here to think this through, you're just doing one-line thoughts.

Newon's 2nd (and 1st, really) law tells us that running at constant speed doesn't require net force. So, what are our legs primarily doing when we run? They're bouncing us up and down. That's what we use most of our strength for. Lower weight means less strength required to bounce up and down, which means people would be able to run faster/for longer.

So, what effect does lower friction have? Since net force is only required for acceleration, lower friction means slower acceleration.
 
  • Like
Likes Vanadium 50 and jbriggs444
  • #33
jbriggs444
Science Advisor
Homework Helper
11,573
6,221
So, what are our legs primarily doing when we run? They're bouncing us up and down.
It might be helpful to think about how a kangaroo moves.

Edit: Imagine my surprise when trying to look up some experimental detail to find:
https://www.smithsonianmag.com/air-space-magazine/mr-moonwalk-180967001/ said:
Kuehnegger maintains the “kangaroo hop” was best. “By performing a kangaroo-type leaping and jumping, you require the least amount of energy, and consume the least amount of oxygen.”
 
  • Like
Likes russ_watters
  • #34
rudransh verma
Gold Member
1,067
91
Then, returning to the ground: when you are airborne there is only one force acting on you: the gravitational force, or your weight. This is lower on the moon, but your mass is the same, therefore your acceleration back to the surface is lower on the moon, giving you more hang-time.
Lower weight means less strength required to bounce up and down, which means people would be able to run faster
Don’t you think in a single jump when the person is going up he is moving faster than on Earth because there is no drag and less gravity but when coming down he slows down because of low gravity. So net result is that the person is not actually gaining any speed over on earth.
 

Attachments

  • ADB8561A-C68F-4562-BF87-F8174BD10A70.jpeg
    ADB8561A-C68F-4562-BF87-F8174BD10A70.jpeg
    13.1 KB · Views: 33
  • #35
sysprog
2,613
1,783
Arjan78 said:
Then, returning to the ground: when you are airborne there is only one force acting on you: the gravitational force, or your weight. This is lower on the moon, but your mass is the same, therefore your acceleration back to the surface is lower on the moon, giving you more hang-time.
Your movement still has its horizontal component.
rudransh verma said:
Don’t you think in a single jump when the person is going up he is moving faster than on Earth because there is no drag and less gravity but when coming down he slows down because of low gravity. So net result is that the person is not actually gaining any speed over on earth.
The vertical component of a leap off the ground must overcome both inertia and gravity; the horizontal component must overcome inertia alone.

1647687256929.png
 

Suggested for: Mass vs weight

  • Last Post
6
Replies
202
Views
6K
Replies
2
Views
342
Replies
14
Views
930
Replies
19
Views
401
Replies
9
Views
457
Replies
2
Views
309
Replies
12
Views
442
Replies
38
Views
985
  • Last Post
Replies
3
Views
457
Top