Click here to purchase the entire book in PDF format. The newton Take a 1-kilogram block of anything and put it on a surface that is completely frictionless - and I mean completely frictionless. No friction between it and the table it's sitting on, no air friction... nothing. If you push the block, it will start to accelerate to a new velocity. If you stop pushing, it will stay at that velocity and keep going forever. The harder you push, the faster the block will accelerate. In other words, if you push harder, you'll get to a higher velocity in the same amount of time. Let's look at an example. The block is stopped, sitting there, waiting to be pushed (so its velocity equals 0 m/s). You push it for exactly one second and then stop pushing. The block has some new velocity that you can now measure. Let's say that you pushed the block hard enough that it is now travelling at 1 m/s. Therefore, the acceleration of the block was 1 m/s per second (because you only pushed for exactly 1 second). This can also be stated as an acceleration of $1 m/s^2$. The amount that you push is called force - you can say that you are applying force to the block instead of saying that you are pushing it. Force is usually measured in pounds, but that is actually for a different system of measurement as we'll see later. If we're think in the SI, then we measure force in newtons, abbreviated N. One additional thing about force - if you drop something on the Earth, it will accelerate as it falls at a rate of about 9.8 m/s each second. So, after one second it's falling 9.8 m/s, after 2 seconds, it's falling 19.6 m/s and so on. There's a nice law that says that Force (in newtons) is equal to mass times acceleration or $$F = ma$$ (2.1) Where $F$ is force in newtons, $m$ is mass in kilograms and $a$ is acceleration in m/s. We already know that 1 newton is equal to 1 kg accelerated at 1 m/s (1 N = 1 kg * 1 m/s) but we can also think of it the other way - 1 kg on the surface of the Earth weighs 9.8 N (because 1 kg * 9.8 m/s = 9.8 N).