External Forces (Gravity Book 3)


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Contact and non-contact forces

But what keeps the Moon from falling down, if all of this gravity is so strong? Well, the answer is that the moon IS falling; all the time, but doesn't get any closer to us! Remember that if there wasn't a force acting, the Moon would be traveling in a straight line. Because there IS a force of attraction toward the Earth, the moon "falls" from a straight line into a curve orbit around the Earth and ends up revolving around us. The Earth's gravity holds it in orbit, so it can't just go off in a straight line.

Think about holding a ball on a string and spinning it in a circle. If you were to cut that string no more gravity , the ball would fly off in a straight line in the direction it was going when you cut the string. That direction, by the way, is not directly away from your hand, but tangent to the circle.

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Tangent is a geometry term used to describe a direction that are related to the slope of a curve. Math stuff. The pull of the string inward toward your hand is like the Earth's gravitational pull inward toward the center of the Earth. Or search the sites for a specific topic. Useful Reference Materials Encyclopedia. Physics4Kids Sections. Go for site help or a list of physics topics at the site map! Current Page: Physics4Kids.

Well, once again, I am only concerned with the block. It's still on earth, we're assuming. So you're going to have the force of gravity acting downwards on the block. But what's keeping it from accelerating downwards? Well, you might say, well, you got the string that's holding it up, that is pulling on it. And that pulling force is known as tension. So what you would have here is an upward force that nets out against the force of gravity.

Introduction to forces and free body diagrams

And sometimes its magnitude is denoted by capital T or it might be a F sub T. Now, let's make things a little bit interesting. Let's try to kind of combine these things, and we'll actually introduce a new force. So let's say that we, this is the ground right over here. I have a block on the ground. And I have a situation where I am pulling on this block using a rope with a force of magnitude, let's just call this the force of tension.

I am pulling on that block. But the block is not moving. What would be the free body diagram for this block? Well, I'll do the same thing again. I will draw the block. Now, in the vertical direction, you have the same thing that you saw in that first scenario. You're going to have the force of gravity or the weight of the block pulling downward on the block. And that's going to be counteracted by the normal force of the ground on the block.

The ground is holding up the block is one way to think about it, keeping it from accelerating downwards. So the normal force is acting upwards. But what about the horizontal direction? I already said that I'm pulling to the right with a force of magnitude F sub T. So let me do that on my free body diagram.

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So this would be F sub T. But I said it's stationary. So there must be something that is counteracting that, that is netting against that, going in that direction.

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What force would that be? Well, that would be the force of friction.

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We've all experienced trying to pull on something, trying to drag something across the ground and it doesn't move, and that's because there's friction between the object and the ground. And friction, fundamentally, it could be because the surfaces of the two objects are rough and you kind of have to grind them pass each other. Or sometimes it can even be due to molecular interactions where they're kind of sticky, where the objects are attracted to each other and you gotta pull passed that. And so in this situation, you have the force of friction counteracting this pulling force, the force of tension, the force of friction.

This right over here is 9.

So you multiply that times 5. You get 49 kilogram meter per second squared, which is the same thing as 49 newtons. So this is a little bit of a conundrum here. Newton's First Law says, an object at rest will stay at rest, or an object in motion will stay in motion, unless there is some unbalanced, or unless there is some net force.

But based on what we've drawn right here, it looks like there's some type of a net force. It looks like I have 49 newtons of force pulling this thing downwards. But you say, no, no no, Sal. Obviously this thing won't start accelerating downwards because there's ice here.

Its resting on a big pool of frozen water. And so my answer to you is, well, if that's your answer, then what is the resulting force that cancels out with gravity to keep these blocks of ice, either one of them, from plummeting down to the core of the Earth? From essentially going into free fall, or accelerating towards the center of the Earth? And you say, well, I guess if these things would be falling, if not for the ice, the ice must be providing the counteracting force. And you are absolutely correct. The ice is providing the counteracting force in the opposite direction.

So the exact magnitude of force, and it is in the opposite direction. And so if the force of gravity on each of these blocks of ice are 49 newtons downwards it is completely netted off by the force of the ice on the block upwards. And that will be a force 49 newtons upwards in either case.

And now, hopefully, it makes sense that Newton's First Law still holds. We have no net force on this in the vertical direction, actually no net force on this in either direction. That's why this guy has a 0 velocity in the horizontal direction. This guy has a constant velocity in the horizontal direction. And neither of them are accelerating in the vertical direction. Because you have the force of the ice on the block, the ice is supporting the block, that's completely counteracting gravity. And this force, in this example, is called the normal force.


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This is the normal force-- it's 49 newtons upwards.

External Forces (Gravity Book 3) External Forces (Gravity Book 3)
External Forces (Gravity Book 3) External Forces (Gravity Book 3)
External Forces (Gravity Book 3) External Forces (Gravity Book 3)
External Forces (Gravity Book 3) External Forces (Gravity Book 3)
External Forces (Gravity Book 3) External Forces (Gravity Book 3)
External Forces (Gravity Book 3) External Forces (Gravity Book 3)
External Forces (Gravity Book 3) External Forces (Gravity Book 3)

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