how to understand relativity? and how fast can an electron move when there is an opposing force acting on it?

Here is an attempt to explain the greatest after thought in human history with simple terms…

Suppose you and your friend are travelling in 2 cars on a highway in the same direction. You are travelling at 60mph and your friend is traveling at 90mph. From your perspective your friend is traveling at 30mph since he is moving relative to you. But to the cop who is gunning the cars' speed sees your friend going at 90mph. In this scenario speed is not constant. It's relative.

Now let's make some changes to the scenario. Let's replace your friend and his car with a photon that obviously travels at speed of light, which we'll call ‘C’. And let's increase your speed to 0.5C. Let's leave the cop and his speed gun untouched.

The cop now measures the photon at C and you at 0.5C. From your perspective you would expect to clock the speed of light relatively at 0.5C (C - your speed) right? Wrong… You will still clock speed of light as C. Even if you increase your speed to 0.99C, you will see that photon zooming past you at C. Light doesn't seem to know if you are traveling or not. It's speed with respect to you or anyone else always remains constant. (Yeah… Initially, I couldn't get it around my head either…)

So if the cop standing in the side of the road and you who is moving at the near speed of light experience the speed of light the same way, something else has to be very different for you and the cop right? That something is Time (and distance). Why? Because speed is just a measure of distance over time. If one side of the equation (speed) is adamant and refusing to change, the other side has to. This means you and the cop perceive time very differently. From the cops perspective the clock in your should be running very very slowly. But you wouldn't see any difference in time. You will still feel everything as normal…

For better understanding imagine this. If you travel at your original speed of 0.5C and decide to leave the earth for a couple of hours, when you come back you would have spent only a couple of hours on your clock, but it would be a month or two (maybe more) in the future on earth.

A similar kind of a time dilation was used in Interstellar movie