Isaac Newton published one of the most important books for our understanding of how systems of objects move and interact with each other. The book was published back in 1687 and is called Principia. It is written in Latin. An old language that hardly any of us understand today!
One of the four fundamental laws in this book is Newton's second law, which says that for an object to change velocity there has to be a net force acting on it. You know this from experience. Assume that you are out on your bike. To get it to go faster you have to use more mussel power and to stop it fast you have to use the brake. Just two different ways of forces acting on the cycle and its rider.
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