## Sunday 8 June 2008

### Science note unit 9 - force

IS note unit 9 – force

Three effect of force

- Change the speed (accelerate for same direction and slow down when opposes)

- Change the direction of motion

- Change the shape (when the force is larger than the force that hold the shape)

We Use Newton balance to measure the force and the unit is Newton (N).

A contract force opposing your force is called friction. They happens when two surfaces are in contact, it always opposes the motion of object. They come from the tiny bumps that the bumps on two surfaces catch each other. So that the rougher surface cause bigger friction. Also, when the object move through water or air, there’s friction too, the friction of air is called air friction or air resistance. Moving against friction can produce heat.

Three ways to reduce friction:

- Changing from sliding motion to rolling motion

- Moving object on a layer of air

- Streamlined object towards air friction

- Smoothen the surface (e.g. use lubricants)

Friction sometimes is useful too, e.g. pick up something.

Force of gravity is a non- contract force. It pulls thing direct downwards to the core of earth (or other object which known as universal gravitation, they attracts other objects toward it by their mass, but the force is too small that we can’t feel, force of gravity is only about the planet). The force on an object is known as weight, a weight of 1kg cause 10N force on gravity on Earth, force of gravity on Moon is 1/6 of Earth.

Action and Reaction

- They occurs in pairs

- The magnitude of force is exactly equal

- The force react in opposite direction

Rocket use that reaction by burning fuels to fly, the fuels are liquid oxygen and liquid hydrogen. They give a clean flame, burning reaction of this fuel (2H2+O2=>2H2O) and a high temperature, so that this is a big propulsion for rocket. There solid grain fuel also. In the middle of journey, the astronauts is weightless because the friction between the planets are offset.

Function of space suit:

maintain living environment of astronaut (e.g. body temperature, pressure, resistance radiation, treat body waste, supply oxygen and remove carbon dioxide, etc.

When the rocket backs to the atmosphere, it’s rise in temperature by heating effect. It can be reduces by insulating tiles and parachutes.