- #1

- 739

- 3

I hope you guys can clear out any misconceptions and help me understand thus. Thanks so much for all the help!

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter sgstudent
- Start date

- #1

- 739

- 3

I hope you guys can clear out any misconceptions and help me understand thus. Thanks so much for all the help!

- #2

Ken G

Gold Member

- 4,462

- 333

- #3

- 739

- 3

Oh, so there is acceleration when dealing with inertia? Then for a car case, when we brake and get lashed forward how do we apply inertia? Thanks for the help!

- #4

- 37,211

- 7,297

Note that acceleration doesn't just mean going faster. That's only when some of the acceleration is in the same direction as the movement. Acceleration in the reverse direction is what is more commonly called deceleration, while acceleration at right angles will make the object change course.

- #5

- 22

- 0

Oh, so there is acceleration when dealing with inertia? Then for a car case, when we brake and get lashed forward how do we apply inertia? Thanks for the help!

A useful way of thinking about it is breaking up that process into two phases, the first being after you brake and then get lashed forward until immediately before the seat belt becomes taut, and the second beginning where the first ends.

In the first phase, you're still going forward because you have mass, and hence inertia. The fact that you pressed the brakes, at least directly, does nothing to slow you down personally, as the brake force acts on the car, and not you. So some other force must slow you down, preferably before you hit the windshield.

But during the second phase, where your belt becomes taut and the "force" of its tightness "pulls" you back into your seat so that both you and the car that you're in come to a stop, the reason that the pulling force is even necessary to stop you from continuing to move forward is because you have mass, which is inertia.

So inertia is "responsible" for both the fact that you lurch forward after you press the brakes, and the fact that you are pulled back when and only when your seat belt becomes taut.

The former is an example of inertia as a property of matter

In the latter case, when you are pulled back by the seat belt, a force

So in essence, if the force exerted on an object is nonexistent, then all masses, no matter how small or large, will respond in the same way - nothing changes. For that reason, it is not necessary to know what the mass of that object happens to be, although it certainly doesn't hurt (it would just happen to be extraneous information). If the force exerted on an object is nonzero, then knowing the mass of that object becomes absolutely essential to "predict" its future - that is, to know the instantaneous acceleration it will undergo.

In a world without forces, it doesn't matter what your mass is. Everything either moves in a straight line at the same speed or is perpetually stationary. But a world without forces would have to be a world without masses (because of gravity).

Hope this helps.

Last edited:

- #6

- 125

- 1

Inertia is the resistance to force. The more mass an object has, the more inertia. The more inertia an object has, the more force is needed to produce acceleration.

One example is when skydiving. Lets say there is skydiver A and skydiver B. Skydiver A weighs 80 kg and Skydiver B weighs 90 kg. Skydiver B will fall a greater distance before reaching terminal velocity than Skydiver A. (Terminal velocity is reached when an object in free fall stops accelerating. This occurs when the acceleration due to gravity is cancelled out by air resistance.) Because Skydiver B weighs 10KG more than Skydiver A, he has a greater resistance to air resistance and is able to fall farther because he has more mass.

There are two formulas for finding acceleration.

1) A = ΔV/T, where A is Acceleration, ΔV is change in velocity, and T is the time interval

2) A = F/M, where A is acceleration, F is the net force, and M is the mass.

For A = F/M, when you increase the value of M, the acceleration produced decreases. When you decrease the value of M, the acceleration produced increases.

One example is when skydiving. Lets say there is skydiver A and skydiver B. Skydiver A weighs 80 kg and Skydiver B weighs 90 kg. Skydiver B will fall a greater distance before reaching terminal velocity than Skydiver A. (Terminal velocity is reached when an object in free fall stops accelerating. This occurs when the acceleration due to gravity is cancelled out by air resistance.) Because Skydiver B weighs 10KG more than Skydiver A, he has a greater resistance to air resistance and is able to fall farther because he has more mass.

There are two formulas for finding acceleration.

1) A = ΔV/T, where A is Acceleration, ΔV is change in velocity, and T is the time interval

2) A = F/M, where A is acceleration, F is the net force, and M is the mass.

For A = F/M, when you increase the value of M, the acceleration produced decreases. When you decrease the value of M, the acceleration produced increases.

Last edited:

- #7

Ken G

Gold Member

- 4,462

- 333

Be careful-- it is important to say this correctly: the more inertia (mass) an object has, the more force is needed to get a givenInertia is the resistance to force. The more inertia an object has, the more force is needed to move it.

That is potentially confusing. In Einstein's model of gravity (general relativity), your statementFor example, it is much easier to pickup a 5lb rock than it is to pick up a 50lb boulder.

No, acceleration isImportant Note: The difference between velocity and acceleration is that velocity is vector while acceleration is scalar.

- #8

- 4,662

- 6

This is essentially Newton's definition of inertia, and my college textbook (Becker,Inertia is the tendency of objects to keep moving at the same speed in the same straight line. It requires a force to change this, and the more mass the higher the force needed.................................

However, photons have momentum, and a force is required to change the direction of a photon beam. So if

- #9

- 125

- 1

Be careful-- it is important to say this correctly: the more inertia (mass) an object has, the more force is needed to get a givenacceleration(not to "move" it).

That is potentially confusing. In Einstein's model of gravity (general relativity), your statementisindeed a reflection of inertia, but most people know Newton's model of gravity instead, and in that model, it isnot.In Newton's model, gravity is a force, so an object with more mass experiences a larger gravitational force. Thus, to lift the object at all (even with very tiny acceleration), you need more force on the more massive object, to balance gravity. But these are balanced forces, so have nothing to do with inertia (which relates to how much acceleration you get when the forces areunbalanced). So if the OPer is thinking in terms of Newton's model of gravity, then your answer will get them very confused about what inertia is. Of course, Einstein's model includes the "equivalence principle", and in that situation, your description is indeed equivalent to inertia, so it's not formally wrong, but I fear it will confuse the OPer.

No, acceleration isalsoa vector (as are ΔV/t and F/m), seeharuspex's post above.

All right, thanks mate.

It looks like i'll have to do a bit of studying as well.

So with Newton's model, the more inertia an object has the more gravity acts upon it? And when the force of gravity is greater, the force needed to balance gravity is also greater?

Thanks,

- #10

Ken G

Gold Member

- 4,462

- 333

Exactly. The connection between inertia and the force of gravity is just a coincidence in Newton's picture, which was the big weakness that Einstein corrected (when he came up with the idea that having a force of gravity is equivalent to being in an accelerating reference frame and not noticing it).So with Newton's model, the more inertia an object has the more gravity acts upon it? And when the force of gravity is greater, the force needed to balance gravity is also greater?

- #11

- 739

- 3

- #12

- 3

- 0

Note that acceleration doesn't just mean going faster. That's only when some of the acceleration is in the same direction as the movement. Acceleration in the reverse direction is what is more commonly called deceleration, while acceleration at right angles will make the object change course.

I agree, great answer

Share: