Edexcel...Physics Unit 1?

can you explain this please

if you throw an object from an aeroplane it will decelerate at 9.8 m/s2
just before hitting the ground its the acceleration is zero-due to air resistance.

what happens in the absence of air resistance?
for how long will the object accelerate? I'm guessing forever if that's true than
an object traveling in space should have an infinite speed

Reply 1

dknt

12

Original post by reb0xx

can you explain this please

if you throw an object from an aeroplane it will decelerate at 9.8 m/s2
just before hitting the ground its the acceleration is zero-due to air resistance.

what happens in the absence of air resistance?
for how long will the object accelerate? I'm guessing forever if that's true than
an object traveling in space should have an infinite speed

So we throw something out an aeroplane, it has an acceleration vertically downwards due to gravity. As this object speeds up the drag force increases, causing the resultant force on the body to decrease and hence a lower acceleration, until it reaches its terminal velocity.

Now without a drag force, assuming you keep the force on it constant, it has a constant acceleration. So from what you said, yes, it seems like a body in space could reach infinite speeds, given infinite time. However, this is assuming we follow Newtonian mechanics all the way. Once we get to a significant fraction of the speed of light theories of relativity start to kick in, with things like Lorentz factor and relativity awesomeness, which ultimately mean you will never reach the speed of light. I'm afraid we can't just keep firing our rockets to reach speeds greater than light speed :frown:

(edited 13 years ago)

Reply 2

jaroc

6

Original post by reb0xx

can you explain this please

if you throw an object from an aeroplane it will decelerate at 9.8 m/s2
just before hitting the ground its the acceleration is zero-due to air resistance.

what happens in the absence of air resistance?
for how long will the object accelerate? I'm guessing forever if that's true than
an object traveling in space should have an infinite speed

In the absence of air resistance the object would accelerate constantly at 9.8m/s/s (assuming that all the time the object is not too far from the surface of the Earth, so that gravitational acceleration is more or less constant).

However, an object cannot have an infinite speed. It's maximum speed is limited by the speed of light, c=3x10^m/s. As an object's speed increases, its mass increases, too. For small speeds this effect is negligible but for greater you need to use relativistic mechanics. Theoretically mass of an object at the speed of light tends to infinity, which means that to give an object such a kinetic energy you would have to do infinite work. This is why only massless particles reach this speed. Other objects can at most get very close to that but no further.

So, to conclude - according to Newtonian mechanics, an object would accelerate infinitely in a constant gravitational field. But Newtonian mechanics is not true as a law of nature - it's only a very good approximation for speeds much smaller than the speed of light. When the speeds are large, you have to use theory of relativity.

Reply 3

Vulpes

15

Original post by reb0xx

can you explain this please

if you throw an object from an aeroplane it will decelerate at 9.8 m/s2
just before hitting the ground its the acceleration is zero-due to air resistance.

what happens in the absence of air resistance?
for how long will the object accelerate? I'm guessing forever if that's true than
an object traveling in space should have an infinite speed

Unit 4 - Particle Physics. Just saying :tongue:

Edexcel...Physics Unit 1?

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