# Weight is measure of ineria.

You have cement block placed on earth and moon with same mass. When you try to push the block it would be easier to push the block on moon than on earth i.e. the inertial property shown by block on earth is more than it shows on moon. But the mass is same on earth and moon. When you have inertia different and mass same how can you tell mass is measure of inertia? Weight should be the measure of inertia. Right?

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russ_watters
Mentor
Sorry, you are not correct: f=ma

You aren't confusing inertia with friction, are you?

Inertia is an ambiguous term and really isn't used much. Depending on the context, it can refer to mass, momentum, or shorthand for "The Principle of Inertia" as embodied by Newton's first law of motion.

The reason an object is easier to push on the moon, given the object is sliding against a similar surface, is because the frictional force is less. See Normal Force.

Then there's Moment of Inertia.

Sorry, you are not correct: f=ma

You aren't confusing inertia with friction, are you?
Ok,i got your point. You got confused as my cement block example isn't that perfect.
Now, replace cement block with a football and kick it. You observe it flying more distance on moon than on earth. Here, there is no friction. Mass is same but inertial property is different.
Tell, How can you tell mass is measure of inertia?

Inertia is an ambiguous term and really isn't used much. Depending on the context, it can refer to mass, momentum, or shorthand for "The Principle of Inertia" as embodied by Newton's first law of motion.

The reason an object is easier to push on the moon, given the object is sliding against a similar surface, is because the frictional force is less. See Normal Force.

Then there's Moment of Inertia.
I got it. But, give me a single situation where you can tell weight is the measure of inertia is wrong statement.

Inertia has been defined as an intrinsic property of matter and does not depend on external forces such as gravity, friction, drag, etc. Stating that inertia depends on weight is wrong by definition. You can redefine inertia but your definition won't be in agreement with the 'standard definition', which as I said before is ambiguous.

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CWatters
Homework Helper
Gold Member
Now, replace cement block with a football and kick it. You observe it flying more distance on moon than on earth. Here, there is no friction. Mass is same but inertial property is different.
No that's not correct. The "inertial property" is the same because the mass is the same.

You see a different result on the moon because...

a) Lower gravity on moon means less friction with the ground.
b) No air resistance on the moon to slow the ball down.
c) Lower gravity means you can kick ball higher with same so time of flight and distance also longer.

CWatters
Homework Helper
Gold Member
Consider two spacecraft trying to dock in orbit. Crew and spacecraft are weightless but they still have mass and inertia. A 20mph collision between spacecraft would do just as much damage up there as if it occured on the ground.

No that's not correct. The "inertial property" is the same because the mass is the same.

You see a different result on the moon because...

a) Lower gravity on moon means less friction with the ground.
b) No air resistance on the moon to slow the ball down.
c) Lower gravity means you can kick ball higher with same so time of flight and distance also longer.
Please don't bring friction in to play(a). I m speaking about only the vertical distance and there is no air friction also(b). Inertia is property of a body to oppose its state of rest or motion. You only told in point (c) that lower gravity so you can kick the ball higher, Which tells that inertia is depending on gravity. On the other hand you see mass not depending on gravity. Then how can you tell mass is the measure of inertia when it doesn't depend on gravity. Also give me an example where you can contradict the statement weight is measure of inertia.

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Drakkith
Staff Emeritus
Please don't bring friction in to play(a). I m speaking about only the vertical distance and there is no air friction also(b). Inertia is property of a body to oppose its state of rest or motion. You only told in point (c) that lower gravity so you can kick the ball higher, Which tells that inertia is depending on gravity. On the other hand you see mass not depending on gravity. Then how can you tell mass is the measure of inertia when it doesn't depend on gravity. Also give me an example where you can contradict the statement weight is measure of inertia.
As you said, inertia is the property of a body to oppose it's state of rest or motion. Thus whether you are on the Moon or on Earth and you kick a football it will attain the same vertical velocity because the force applied and the mass is the same. The reason it flies higher is because the Moon exerts less Force on the football to pull it back down, which causes it to decelerate at a slower rate while rising and accelerate at a slower rate while falling than it would on Earth. Inertia depends only on the Mass, not anything else like gravity.

Please don't bring friction in to play(a). I m speaking about only the vertical distance and there is no air friction also(b). Inertia is property of a body to oppose its state of rest or motion. You only told in point (c) that lower gravity so you can kick the ball higher, Which tells that inertia is depending on gravity. On the other hand you see mass not depending on gravity. Then how can you tell mass is the measure of inertia when it doesn't depend on gravity. Also give me an example where you can contradict the statement weight is measure of inertia.
I think you're getting confused. If you ignore the reduced friction caused by weaker gravity on the moon then a block is just as hard to accelerate (has the same inertia and mass) on the moon as on the Earth so you're first post is inaccurate.

A football would fly further and higher on the moon because of the weaker gravity and lack of atmosphere NOT because the football has 'less mass'. Assuming the same force is applied and ignoring air resistance the acceleration on the ball is the same on Earth or on the Moon. Once it's in the air however the vertical component of it's velocity is reduced faster on Earth than on the Moon because of the stronger force of weight acting on it's mass (which is still the same) - this results in a shorter flight time and lower maximum height. This means even if the horizontal component was the same it wouldn't go as far but of course on Earth you have to consider air resistance and this compounds the reduction on how far the ball flies because it not only is in the air for less time but it travels slower horizontally. Sorry for the wall of text.

As you said, inertia is the property of a body to oppose it's state of rest or motion. Thus whether you are on the Moon or on Earth and you kick a football it will attain the same vertical velocity because the force applied and the mass is the same. The reason it flies higher is because the Moon exerts less Force on the football to pull it back down, which causes it to decelerate at a slower rate while rising and accelerate at a slower rate while falling than it would on Earth. Inertia depends only on the Mass, not anything else like gravity.
Thank you sir. I understood the exact point. Thank you all for your kind response.

CWatters