Physics

Force Concept Explained



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Force like any physical concept is best explained by use of familiar examples. Technically speaking, force represents the change per time of momentum, i.e., a combination of mass [the mount of "stuff"], speed or direction, as that time interval becomes small. In high school physics, most people learn that force is mass times acceleration, which includes a direction. That famous formula F=ma is in fact only a special case which applies only when mass is constant, but in general mass can also change. Force represents the instantaneous change in momentum- whether via mass, speed or direction of motion. One may imagine a bucket of sand falling upright from, say, a workman's scaffolding. [One may imagine the bucket was being hoisted on a rope which broke.] At the instant of the rope breaking, that bucket has stopped going upward and so is for a split second suspended in the air moving neither up nor down. Of course, immediately gravity takes hold and the bucket starts falling; gravity is a force due to the pull of massive bodies- in this case from the Earth itself. Of course that force keeps acting on the bucket of sand and so as it falls, it picks up speed. The acceleration, i.e., the change in speed of the bucket falling, is the same whether the bucket is completely full or mostly empty, big or small. Nevertheless, the wallop when the bucket of sand hits the ground, i.e., the force with which it hits the ground, very much depends on how much stuff is in the bucket. At the moment of impact, the ground exerts a force on the bucket in our example as well because the bucket stops falling. That force will cause the noise one hears and the spilling of sand out of the bucket, possibly even damage to the bucket itself. Of course, forces can balance each other and often do. In the example, initially, the tension in the rope was acting against the force of gravity pulling the bucket down. In other words, the force of someone pulling on the rope has to be more than the weight of the bucket of sand for it to move upwards. If the bucket has a hole in it so that sand was leaking out, one would need less and less force to pull the bucket upward because the mass, i.e., the amount of stuff in the bucket along with that of the bucket itself, would be changing. Finally, if one supposes the falling bucket of sand lands on a very sturdy trampoline [strong enough not to tear] instead of the ground, the bucket would bounce back. In reality the speed the bucket bounces upward at would be a bit smaller than the speed it was going when it hit the trampoline, but the main thing to change would be the direction because the bucket would suddenly be moving upwards. The concept of force is a handy why of describing these types of changes.

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