Forces_Concept_5

Subject Facts

Like magnetism, another force that has an effect at a distance, gravity is very poorly understood We know many things about gravity and its effects, but no one has yet adequately explained why gravity should exist. The force of gravitational attraction operates between air particles of matter; but on the atomic scale, this force is very weak It is the cumulative nature of gravity the more matter, the greater the force which makes it the key force on an astronomical scale

The existence of a force which keeps us bound to the Earth was accepted long ago, though this was argued to be the weight of the air keeping things down. it was not until the late 17th century, with the work of Galileo and Newton, that a mathematical understanding of gravity as a force became possible.

Try to think of gravity putting you towards the centre of the Earth, rather than simply 'down'. When you look at a globe, the idea of 'down' can be misleading: it might be construed as movement across or away from the Earth, rather than towards its centre.

Gravity works between masses, so you attract the Earth with as much force as the Earth attracts you. However, because the Earth's mass is so much greater than yours, its effect on your movement is obvious while your effect on its movement is virtually nil. When masses are particularly large (as planets, moans and stars), the interaction between them becomes more noteworthy. Strict}y speaking, we should acknowledge the gravitational attraction of everything towards everything else; but the effects are usually too small to be observed.

Do heavy things fall faster?

For a long time, it was assumed that heavy things fall faster than fight things: that seemed to be 'common sense'. But Galileo's famous 'thought experiment' demonstrated that it could not be true.

Suppose that two steel balls are dropped from a tall building, and the heavier one hits the ground first What would happen if you tied a cord between the two: would the smaller ball slow down the bigger one, or would the heavier one make the lighter one go faster? Perhaps together they would go faster than the light one on its own, but more slow}y than the heavy one on its own? If you made the cord shorter, would that make a difference? Why? If you made it very short, so short that the balls were touching would that make any difference?

Join the balls together: how fast wilt they go now? They should go faster than the big bail on its own, if the theory is correct.At what point will the cord be short enough for the pair to go faster than the bigger ball on its own?

The situation doesn't make sense because the initial assumption is incorrect. it may often be the case that lighter objects fall more slowly: leaves and feathers take a long time and marbles don't. But this isn't a fair comparison, since the effect of the object's shape on the alt" resistance must also be taken into account .

We really need to test two objects that are identical except for their masses. For example, we could try a cricket ball and a hollow rubber one (the fur on tennis balls might have some friction effect). Drop them together; and they will hit the ground at the same time. On one occasion, caught a child inadvertently dropping the more massive of two balls slightly earlier through a desire to prove that the heavier ball would land first.)

The principle that 'mass doesn't matter' in determining speed of fall can be related to other scenarios, such as a toy car rolling down a ramp.The idea that a heavier toy car will go faster down the ramp than a fighter one, all other things being equal, is an appealing one. But try it! (See Figure 8.) The two trucks will go down the ramp side by side (more or less), and be travelling at the same speed when they hit the bottom of the ramp. At this point, the situation changes, The frictional forces (air resistance and axle friction) slowing both trucks should be the same, but the more massive truck will need a greater force to make it stop within the same distance or a longer distance to stop with the same retarding force. That is why the heavier vehicle will probably go further, but not faster.

The difference between weight and mass

Mass is the amount of 'stuff' that an object contains. It is measured in kilograms (kg). Denser materials contain more 'stuff' for a given volume than less dense ones. Weight is the force that a mass exerts on the ground due to gravity, and is measured in newtons (N). What we refer to as 'weight' in everyday life is really mass. The more mass an object has, the more it will weigh a given gravity. The Moon's gravity is one sixth that of the Earth. If you visit the Moon, it will not change your mass (you will still contain the same amount of 'stuff'; but your weight (the force that you exert on the Moon's surface) will be only one sixth what it was on the Earth.

If you are floating on water; you wilt feet 'weightless'. Your mass is unchanged but your weight is countered by the upthrust of the water: If you were floating and a swimmer swam into you, the swimmer's mass would make a significant difference to the impact: slowing down a mass of 80kg would take four times as much force (for the same change in speed) as slowing down a mass of 20kg. Give yourself a I N prod with a force meter; think of the difference between that and a 4N prod.

In orbit

If you throw a stone horizontally from where you are standing it will go some distance before being pulled to the ground by gravity. If you throw it harder (with more force), it will go further before it is puled down, Harder still, and it will go further still.

Now consider this thought experiment*. If we take these stones to the top of a very tall mountain and start throwing again (see Figure 9), the first throw (l) will go further because we are standing higher up. The second (2), more forceful throw will go further still. With even greater force, the stone may keep falling towards the Earth but, because of the curvature of the planet, keep missing. It will go on falling and missing until it makes the way around the Earth. If we ignore the effects of air resistance (3), it should keep going in orbit. An orbit is a continual 'falling and missing',

In one -final effort (4), you throw with even more force.This time, the stone is going further and falling less, so it doesn't go into orbit but spirals out and away from the planet. It has reached 'escape velocity'.

Tides

The gravitational attraction between the Moon and the Earth affects both bodies. Not only are the Moon and Earth held in orbit by their combined gravitational fields about a central point (within the Earth), but the Moon also exerts a significant tug on our oceans (see Figure 10).As the Earth rotates on its axis, -the 'ocean budge' due to the Moon moves around its surface.

It does not quite move around every 24 hours, because the Moon is moving as well On the opposite side of the Earth there is another 'ocean bulge' caused by the off-centre swing of the Earth/Moon system.

Twice each month, the tides are particularly high, as the gravitational forces of the Moon and the Sun are in line and reinforce each other. Twice each month the tides are particularly low, as the forces of the Moon and the Sun are at right angles and partly cancel each other out.