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student34
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no change in velocity? How can a rock that isn't moving have acceleration?
It cannot have a (net) acceleration in a frame where it stays at rest.How can a rock that isn't moving have acceleration?
mfb said:It cannot have a (net) acceleration in a frame where it stays at rest.
It can have forces acting on it (like gravity, and a force from the floor), but they have to cancel.
JimJam said:Acceleration is the rate of change of velocity, isn't it? So it is speeding up in whatever direction 9.81m/s every second, therefore there must be a change in V?
student34 said:Sorry, I meant that it does not make sense to me that still object has acceleration g. There is no change in velocity, yet Einstein says that it has the same acceleration as something that has a changing velocity.
Drakkith said:First, are we talking about simple classical acceleration, or about General Relativity?
In classical physics the answer is that an object that isn't changing velocity has no acceleration. Gravity provides a force on an object, but for those on the ground there is no acceleration. The force on an object is equal to the force that would accelerate that object at 9.8 m/s2.
WannabeNewton said:No it is always true that [itex]\sum F_{i} = m\sum a_{i} = m\sum \frac{\mathrm{d} v_{i}}{\mathrm{d} t}[/itex] for a single particle constant mass system (the indexes label the motion coupled to the respective force). The point is that when you place a pebble on the ground, there is an upwards reaction force from the ground on the pebble (loosely put it arises out of the electrostatic repulsions between the pebble and the ground) and this reaction force just happens to cancel out the weight of the pebble so that the net acceleration given above vanishes.
Because it is; What has been said thus far that has contradicted that?student34 said:Then why do they tell us that acceleration is a change in velocity all thoughout our textbook?
student34 said:Then why do they tell us that acceleration is a change in velocity all thoughout our textbook?
There are two kinds of acceleration.student34 said:There is no change in velocity, yet Einstein says that it has the same acceleration as something that has a changing velocity.
DrGreg said:Acceleration and velocity are always relative to something. You need to know what the "something" is, or is assumed to be, before you can say whether an object is accelerating or not.
In Newtonian physics we measure acceleration relative to the Earth's surface.
In General Relativity we measure (proper) acceleration relative to falling objects.
WannabeNewton said:Because it is; What has been said thus far that has contradicted that?
student34 said:So a rock with zero velocity and zero speed has velocity!?
You may as well type Chineese instead of those formulas because it would mean the exact same to me.
student34 said:So a rock with zero velocity and zero speed has velocity!?
You may as well type Chineese instead of those formulas because it would mean the exact same to me.
Yes. If a rock falls, there's 2 ways to look at it.student34 said:Oh, so are you saying that to a rock falling, a different still-object will seem to be accelerating - hence relativity?
DrGreg said:Yes. If a rock falls, there's 2 ways to look at it.The revolutionary aspect of General Relativity was to base it on (2) instead of (1).
- The rock is accelerating downwards relative to the ground.
- The ground is accelerating upwards relative to the rock.
student34 said:no change in velocity? How can a rock that isn't moving have acceleration?
DrGreg said:Yes. If a rock falls, there's 2 ways to look at it.The revolutionary aspect of General Relativity was to base it on (2) instead of (1).
- The rock is accelerating downwards relative to the ground.
- The ground is accelerating upwards relative to the rock.
I don't think so. In post #5 he clarified that he actually asks about Einstein's General Relativity where objects at rest on the Earth’s surface have a proper acceleration upwards of 1g.mikemeg said:I could be wrong, but I think we're going wayyyyy beyond what the original poster was talking about.
He was specificaly asking about Einstein's view on acceleration in post #5soothsayer said:He also clarified that he is in first year physics, so I don't think GR is going to help him understand much.
That is coordinate acceleration. But the OP is confused about statements regarding proper acceleration.soothsayer said:Acceleration is change in velocity.
A.T. said:He was specificaly asking about Einstein's view on acceleration in post #5
A.T. said:That is coordinate acceleration. But the OP is confused about statements regarding proper acceleration.
It can't. As phrased the query is meaningless.student34 said:no change in velocity? How can a rock that isn't moving have acceleration?
An object can have an acceleration of 9.81m/s^2 even when there is no force acting on it because of the force of gravity. Gravity is a natural phenomenon that causes objects with mass to be attracted to each other. On Earth, the acceleration due to gravity is approximately 9.81m/s^2. This means that all objects near the Earth's surface will experience an acceleration of 9.81m/s^2 due to the force of gravity, regardless of whether there are any other forces acting on them.
The mass of an object does not affect its acceleration of 9.81m/s^2 due to gravity. According to Newton's Second Law of Motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. However, in the case of gravity, the force of gravity is directly proportional to the mass of the object, canceling out the effect of mass on acceleration. Therefore, all objects, regardless of their mass, will experience the same acceleration of 9.81m/s^2 due to gravity.
The acceleration of 9.81m/s^2 is equal to the Earth's gravitational field strength. Gravitational field strength is a measure of the force of gravity per unit mass at a specific point in space. On Earth, the gravitational field strength is approximately 9.81N/kg, which is equivalent to an acceleration of 9.81m/s^2. This means that the acceleration due to gravity is a direct result of the Earth's gravitational field strength.
No, an object cannot have an acceleration greater than 9.81m/s^2 due to gravity. The acceleration due to gravity is a constant value, and it is the maximum acceleration an object can experience due to gravity. This is because the force of gravity is directly proportional to the mass of the object, and as an object's acceleration increases, its mass would also need to increase to maintain the constant acceleration of 9.81m/s^2.
Air resistance can affect an object's acceleration of 9.81m/s^2 by opposing the force of gravity. When an object is falling, air resistance acts in the opposite direction of its motion, slowing it down. This means that the object's acceleration due to gravity will decrease as the air resistance increases. However, in most cases, the effect of air resistance on an object's acceleration of 9.81m/s^2 is relatively small and can be ignored.