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# Forces A-Z

Acceleration   The measure of how quickly velocity increases. A decrease in velocity is called deceleration.
Centrifugal force   The force that seems to push outwards on an object moving in a circle. When we sit on a spinning fairground ride such as a Chair-o-Plane we feel centrifugal force pushing us outwards. At the same time there is another force at work, keeping us moving in a circle. This is called the centripetal force.
Centripetal force   The force that pulls inwards on an object moving in a circle. It balances centrifugal force. Gravity is a type of centripetal force that keeps the planets moving around the Sun.
Drag   A force that slows an object down as it travels through a gas or liquid. Drag occurs because of friction between the object and the molecules around it. Unlike friction, drag is affected by an object’s speed. Drag that takes place in the air is called air resistance. Drag that takes place in a liquid is called water resistance.
Force   Something that changes the movement or shape of an object. Once an object is set in motion, it will carry on moving at the same speed and in the same direction until another force speeds it up, changes its direction or slows it down. Forces always act in pairs. When both forces are equal, an object will stay still or will move at a constant speed in one direction. When one force is greater than the other, the forces are unbalanced and the object will change direction or speed. When several forces act on an object, the overall effect is called the resultant force. For example, lift, thrust, gravity and drag all act on an aeroplane, while its resultant force is a forward motion.
Friction   A force that tries to stop things moving. Friction is produced when two surfaces rub together. Rough surfaces produce more friction than smooth ones. Friction can make machines wear out, but it can also be helpful. For example, a vehicle stops because of friction provided by its brakes.
Gravity   The force that attracts all objects to each other. The larger an object’s mass, or the greater its density, the greater its gravitational pull. The greater the distance between objects, the smaller the force of gravity between them. When we jump in the air, gravity is the force that pulls us back to the Earth.
Inertia   The tendency for objects to resist changes to their motion; that is, the tendency for objects that are moving to carry on moving, and for objects that are still to stay still. All objects always have inertia, whether they are moving or still.
Lever   A stiff bar that pivots at a fulcrum. If the fulcrum is near one end, a small force at the other end will move a heavy weight.
Lift   An upward force that holds aircraft in the air. As an aeroplane moves, the shape of its wings forces air to flow faster over the top of the wing than the air flowing beneath it. This means that there is less pressure on the upper side of the wing than on the lower side. The result is lift.
Machine   A device that changes the direction or size of a force, making it easier to perform certain tasks. The most simple machines are: a wheel, a lever, a wedge, a screw, a pulley and an inclined plane.
Momentum   The tendency for objects that are moving to carry on moving. An object’s momentum is a result of its mass and its velocity.
Motion   The word given to any kind of movement. All movement is caused by forces. An object going in a straight line has linear motion. An object that circles a central point has circular motion. An object that twists around on the spot has rotary motion. An object moving to and fro has reciprocating motion.
Newton, Isaac   (1642–1727) The English scientist who discovered the laws of gravity and motion. Isaac Newton proposed that the Earth’s gravity attracted the Moon and kept it circling the Earth.
Newton   The unit in which force is measured. One newton is the force of the Earth’s gravity on an object with a mass of 102g (about the size of a small apple).
Nuclear force   The force that binds neutrons and protons together inside the nucleus of an atom.
Pressure   The amount of force acting on a certain area.
Reaction   A force that opposes another force of equal strength. Forces always act in pairs, where one force is called the action and the other is called the reaction.
Thrust   The force from a vehicle’s engine that results in forward motion.
Velocity   In science, speed measures how quickly an object moves, but it does not specify its direction. Velocity is a measure of both how quickly an object moves and the direction in which it travels, i.e. how quickly an object changes its position. Velocity is equal to distance divided by time.
Weight   A measure of the gravitational force pulling on an object. A big book is weighty because it is pulled downwards by the Earth’s gravity. In space, away from the Earth’s gravity, objects and people are not pulled down, so they become weightless and float about freely. Objects in space still have mass, however.

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