Use this tag for questions related to how objects move relative to other objects dependent on your frame of reference, and how this applies to special relativity.
Questions tagged [relative-motion]
722 questions
121
votes
13 answers
If all motion is relative, how does light have a finite speed?
I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds paradoxical to me; I cannot understand how the two…
CircleSquared
- 1,351
103
votes
7 answers
Why does the atmosphere rotate along with the earth?
I was reading somewhere about a really cheap way of travelling: using balloons to get ourselves away from the surface of the earth. The idea held that because the earth rotates, we should be able to land in a different place after some time. As we…
claws
- 7,493
44
votes
12 answers
Is there any physical evidence for motion?
Let's say that we have 2 tennis balls in space, one being in motion (say, pushed by an astronaut), and the other one still.
If we could take a snapshot of both tennis balls, would there be any evidence that could suggest that one is moving and the…
GaelF
- 613
41
votes
3 answers
What are galactic speeds measured against?
The Earth moves through space at 67,000 MPH. The Milky Way travels through a local group at 2,237,000 MPH.
Wouldn't you need a fixed point to be able to measure velocity against? After all, compared to the total speed of our Milky Way, the Earth…
Sean Long
- 513
36
votes
5 answers
How am I able to stand up and walk down the aisle of a flying passenger jet?
The energy of a moving object is $E = mv^2\;.$ That is it increases with velocity squared.
I walk at say 3 miles per hour, or lets round that down to 1 meter per second for a slow walk. I weigh less than $100~\mathrm{kg}\;,$ but lets just round that…
user2800708
- 962
32
votes
7 answers
How can kinetic energy be proportional to the square of velocity, when velocity is relative?
Let's start with kinetic energy (from los Wikipedias)
The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having…
geelen
- 421
28
votes
11 answers
How can any speed be defined as a constant?
We know that the speed of light is a constant, and can therefore be used to calculate many other relative values, but I'm having difficulty understanding how speed can be a constant, seeing as it's dependent on other variables.
For example, take a…
26
votes
11 answers
If absolute velocity does not exist, how can we say a rocket accelerates in empty space?
If a rocket in space fires its thrusters, it is propelled forwards as per the laws of motion. This can be measured by its position relative to other bodies in the universe.
Hypothetically if there was a universe that was completely empty except for…
Leroy
- 403
25
votes
1 answer
Why is Voyager 1 approaching Earth?
When I read JPL's mission status for Voyager 1 and Voyager 2 the distance between Earth and Voyager 1 is decreasing.
Is it right?
Jin Kwon
- 361
22
votes
5 answers
Is it possible to stay up while riding a bike on a moving sidewalk without actually moving?
If I ride a bicycle on a moving sidewalk so that I am not in effect moving at all relative to the ground, will I fall over?
user31402
- 329
20
votes
6 answers
Flying a drone in a moving car/airplane
Why does a drone fly with respect to the air in a moving car/airplane, but not in an elevator as shown in this clip: What Happens If You Fly a Drone In An Elevator?
I understand that the drone will move at the same speed as a moving car/airplane.…
Mac_79
- 415
20
votes
3 answers
Why doesn't the speed of Earth (moving through space) kill us?
I have read different speeds of Earth in different sources. $382\;{\rm km}/{\rm s}$, $12\;{\rm m}/{\rm s}$ and even $108,000\;{\rm km}/{\rm h}$. Basically, it's moving too fast around the Sun. And the Solar System is moving too. So why don't we feel…
Veda
- 319
17
votes
3 answers
Why do we launch rockets during the day?
The clips that I have seen of rockets launching all seem to be carried out during daytime. However, we learnt at school that rockets are fired closer to the equator and towards the east to take maximum advantage of Earth's rotational motion, getting…
QCD_IS_GOOD
- 7,145
16
votes
10 answers
Why do bodies traveling at constant velocity experience the same physics?
I am new to special relativity. I don't have a background in physics.
I was reading Brian Green's The Elegant Universe and got hung up on an example.
In fact, Einstein realized that the principle of relativity makes an even grander claim: the laws…
Dimitar Dimitrov
- 279
16
votes
5 answers
A fly in an accelerating car
A fly is flying around in a car, the fly never touches any surface in the car only fly’s around in the air inside the car. The car accelerates. does the fly slam in to the rear window. or does the fly continue to fly uninterrupted?
Fly
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