Block B, which has mask 45 kg, is moving at a constant velocity across a flat surface. As shown in the diagram, it is being pulled forward by a force of 100 N. What is the magnitude of the force represented by the left-pointing arrow in the diagram?

Democritus was a Greek philosopher who lived between 470 - 380 BC. He has been credited with developing the concept of the atom, an idea he got from his mentor Leucippus and adopted it. He conducted a simple experiment by taking a sea shell and breaking it in half. He then took that half and broke it in half over and over again until he was finally left with a fine powder. He then took the smallest piece of the powder and tried to break it but could not, and from this he had discovered that matter was made of indivisible building blocks, atoms.