**. Well I wouldn’t say that’s wrong, but that’s only truly correct in a classical sense. That is, the following equation only holds for**

*the net external force on an object is the product of the object’s mass and its acceleration***:**

*non-relativistic speeds*The more accurate way to quote Newton’s Second Law, which holds for all situations, is this:

**. And we express this mathematically as:**

*the net external force on an object is the time rate of change of momentum of the object itself*And I’ll illustrate the generality of this expression by using this to derive a relativistic expression for the acceleration of a body. Now, to start off, let us recall the relativistic momentum expression:

And therefore the rate of change of momentum of a body must be given by a direct differentiation with respect to time:

And if you simplify this, it becomes:

And we recognize two things, that is:

And therefore:

Notice that Newton’s Second Law no longer just involves the product of the mass and acceleration – this means that the expression

**is not a generally valid expression! And hence my conviction that Newton’s Second Law should be reformulated in terms of rate of momentum change instead.**

*F = ma*Now, let us rearrange what we have above:

So let’s say you want to accelerate a body to the speed of light (

*c**ms^-1*), notice that as the speed

**tends towards**

*v***, the acceleration tends to**

*c***for a finite force:**

*zero*So how do you accelerate something to a speed of

**given that the acceleration fades away to zero when you near the speed of light? Easy, you use an**

*c***! But is there such a thing as an infinite force?**

*infinite force***However, is there another way to go about it? Well, yes! Notice that as the mass goes towards zero:**

*No!*Therefore, if the mass of a body is zero, then it is possible to accelerate the body all the way to the speed of light!

Ask yourself, what is the mass of a photon? :p It’s simply zero, which is expected! Otherwise it can’t go at the speed of light! :)

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