Gravity And Other Forces

GRAVITY and other Forces

Which force matters most? Welcome to the world of forces. I'm a force myself. You'll find out about me on page 27. Before that, though, you're going to meet some of my rivals. Forces are pushes and pulls. Without us, nothing would move at all. Forces make the world go round. 1 Getting up would be impossible (instead of just difficult). 2 Running out of the house couldn't happen. 3 You wouldn't be able to push down on your bike pedals. 4 Your bike brakes could not pull you to a stop. 5 Even just sitting up needs help from forces. The other forces and I work together all the time, but we're not friends exactly. They all think they're too important. In fact, some of them think they're the MOST important forces in the world. They're going to tell you what makes them so special. Then, at the end, we'll decide which actually is the most important force.

Gravity The force that is everywhere I gravity, am a mega-force! I am everywhere and I'm part of every single object. I am a pulling force. Every single object has gravity, but it is stronger in objects containing a lot of matter. A mouse contains a tiny amount of matter. An elephant contains more matter than a mouse. Objects have to be really VERY big before you notice their gravity. Even an elephant is too small. Planets, though, are definitely big enough. LOTS of matter= big gravity Gravity stops you from floating off into the air... ...and keeps AIR from floating off into space. Gravity also makes walking uphill hard work and gymnastics scary. Out in space, some black holes have SO much gravity that they pull together whole galaxies of stars. So, I keep your feet on the ground, bring you air to breathe, and even hold together your galaxy. I MUST be the most important.

Weight Gravity's best friend Gravity and I work together ALL the time. I'm the force gravity makes by pulling on an object. This means I change depending on how strong gravity is. On Earth, the force of gravity is 1g (the 'g' stands for gravity'). Imagine this: when you stand on scales they say 40 kg. Here on Earth you are also rubbish at the high jump. On the Moon, gravity is much weaker: just 0.1654 g up there, the scales would say 6.6 kg. Your legs would still be able to jump with just as much force, though. With only the Moon's weak gravity pulling you down, you would be excellent at the high jump.

The truth about gravity Gravity and weight are such big-heads! Actually, gravity is really weak. I'll explain... Fact about gravity: no.1 Unless an object is massive, you can't even feel its gravity. Fact about gravity: no.2 Even Earth's gravity can easily be beaten. For example, using simple machines is a way to defeat gravity. Ramps make climbing upwards, against gravity, much simpler. Ramps are a kind of simple machine called an inclined plane. Pulleys are another kind of simple machine. Using a pulley can make it easy to lift objects that would otherwise be too heavy. Fact about gravity: no.3 Gravity gets weaker the further away you go. So to make Earth's gravity weaker, all you have to do is... Climb a really, really tall mountain. Go up in a weather balloon. Weather balloons do not actually carry human passengers.) Go 400 km up to the International Space Station, where you won't feel gravity at all.

Friction Grr- WHO says another force is more important than me? I'll fight any other force you can name! I'll push and push at it until it gives up! I'm a pushing force between two objects that are sliding across each other. (Trying to slide, anyway: sometimes I make them stop!) This makes me brilliant at slowing things down. My skills at slowing things down are very useful when going too fast on your bike. I release heat as a side-effect. The more friction there is, the more heat is released. When you have cold hands, the friction of rubbing them together warms them. Experts can even use friction to start a fire. The brake pad now touching wheel causes friction, so the wheel slows down. I do all sorts of slowing down or gripping-on jobs. That definitely makes me the most important force on the street!