Topic # 2
FORCES AND ENERGY
1.
FORCES
From the movements of the planets to the energy produced inside atoms, everything that happens in the Universe is ultimately caused by forces. A force is a push or pull that can make an object move or TURN around. The bigger the force, the more movement it can produce. When two or more forces act together on an object, their effects are COMBINED. Sometimes the forces add together to make a larger force, and sometimes they cancel each other out.
NEWTONS
Forces are measured in units called newtons (N), named after English scientist Sir Isaac Newton. The size of a force can be measured using a device called a force meter or newtonmeter. As the load pulls on the hook, it stretches a spring to give a reading on the scale. On Earth, the force of gravity on 1 kg (2.2 lb) is 9.8 newtons.
TURNING FORCES
If an object is fixed at one point and can rotate around it, that point is called a pivot. If a force acts on the object, the object turns around the pivot. The turning force is called a torque and the effect it produces is called a moment. The bigger the force, the greater the moment. The moment also increases if the force acts at a greater distance from the pivot.
A wheelbarrow is free to pivot around the large wheel at the front. When the worker lifts the handles, the force causes the entire wheelbarrow to swing upwards and turn around the wheel. The long body and handles of a wheelbarrow increase the turning effect and make it easier to tip out a heavy load.
INCREASING MOMENTS
It is easier to unscrew a nut with a spanner than with your fingers, because the spanner’s long handle increases the turning effect or moment of the force. The size of a moment is equal to the force used times the distance from the pivot on which it acts. If you use a spanner twice as long, you double the moment, and the nut is twice as easy to turn.
COMBINED FORCES
When forces act in the same direction, they combine to make a bigger force. When they act in opposite directions, they can cancel one another out. If the forces acting on an object balance, the object does not move, but may change shape. If the forces combine to make an overall force in one direction, the object moves in that direction.
SUPPORTING A BRIDGE
A suspension bridge has to support the weight of its own deck, plus the weight of the vehicles that go across it. The deck of the bridge hangs from huge steel cables suspended over giant pillars. The cables and pillars are arranged so that there is no overall force in any direction. A bridge stays up because the forces on it are balanced and cancel one another out.
2.
DYNAMICS
Dynamics is the study of how objects move when forces act on them. Normally objects stay still or move along at a steady pace. They resist changes in their motion because of their INERTIA. Once they start moving, they tend to carry on doing so because of their MOMENTUM. Most types of everyday movement can be explained by just three simple LAWS OF MOTION. These were originally worked out by English physicist Sir Isaac Newton.
LAWS OF MOTION
Newton’s three laws of motion (often called Newton’s laws) explain how forces make objects move. When the forces that are acting on an object are balanced, there is no change in the way it moves. When the forces are unbalanced, there is an overall force in one direction. This changes the object’s speed or the direction in which it is moving. Physicists call a change in speed or direction an acceleration.
NEWTON’S 1ST LAW
An object will stay still or move along at a steady pace unless a force acts on it. For example, a rocket on a launchpad remains in place because there is no force acting on it to make it move.
NEWTON’S 2ND LAW
When a force acts on an object, it makes the object change speed or move in a different direction. When the rocket’s engines fire, the force they produce lifts the rocket up off the launchpad and into the air.
NEWTON’S 3RD LAW
When a force acts on an object, the object pulls or pushes back. This reaction is equal to the original force but in the opposite direction. As the hot gases shoot down from the engines, an equal force pushes the rocket up.
BIOGRAPHY: SIR ISAAC NEWTON English, 1642–1727
Newton’s three laws of motion enabled him to produce a complete theory of gravity, the force that dominates our Universe, and to explain why the Moon circles round Earth. Newton also made major discoveries about optics (the theory of light) and explained how white light is composed of many colours.
INERTIA
Newton’s first law explains that objects remain where they are or move along at a steady speed unless a force acts on them. This idea is known as inertia. The greater the weight (or mass) of an object, the more inertia it has. Heavy objects are harder to move than light ones because they have more inertia. Inertia also makes it harder to stop heavy things once they are moving.
CRASH-TEST DUMMIES
As a car accelerates, passengers are thrown backwards; when a car brakes or crashes, passengers are thrown forwards. In both cases, this is because the inertia caused by their mass resists the change in movement. During crash-tests, dummies that weigh the same as a human body are used to help test safety belts and airbags.
MOMENTUM
Moving objects carry on moving because they have momentum. The momentum of a moving object increases with its mass and its speed. The heavier the object and the faster it is moving, the greater its momentum and the harder it is to stop. If a truck and a car are travelling at the same speed, it takes more force to stop the truck because its greater mass gives it more momentum.
COMPARING MOMENTUM
A foal is smaller and has less mass than a horse. When a foal and a horse gallop along together at the same speed, the horse has more momentum because of its greater mass. This means that it is easier for the foal to start moving, stop moving, and change direction than the horse. The momentum of a moving object is equal to its mass times its velocity.
3.
FRICTION
If you kick a ball across a playground, it bounces and rolls on the ground’s rough surface and soon comes to a halt. What slows it down is friction, which is the force between a moving object and whatever it touches. Cars travel faster if they are STREAMLINED to reduce a type of friction called air resistance. Friction can sometimes be helpful. Without friction between the tyres and the road, cars would not have enough grip to go around corners.
LUBRICATING MACHINERY
Slippery substances such as oil reduce the friction between two surfaces. This is known as lubrication. Machinery has to be lubricated to prevent its moving parts from wearing out due to friction. Most machines are oiled or greased when they are made and are lubricated from time to time as they are used.
STREAMLINING
When objects move, the air around them generates a type of friction called air resistance, or drag, that slows them down. Fast-moving objects such as cars, trains, and aeroplanes are all streamlined – designed with curved and sloping surfaces to cut through the air and reduce drag. This helps them to move faster and use less fuel. Boats can be streamlined too, to reduce water resistance.
4.
ELASTICITY
Forces make things move, but they can also stretch things, squeeze them, and change their shape. A rubber ball changes shape when you use force to squeeze it, but it returns to its original shape when you stop squeezing. Materials that do this have elasticity. They are made up of particles called molecules that can stretch apart. Other materials, such as modelling clay, change shape easily when a force is applied, but they do not return to their original shape when the force is no longer applied. These materials have PLASTICITY.
TRAMPOLINING
A trampoline is made of stretchy rubber fastened to a metal frame by metal springs. When you land on a trampoline, you stretch the rubber and the springs. Both rubber and springs are elastic. As they return to their original shape, they pull back upwards and push you into the air.
PLASTICITY
Materials have plasticity when they are easily moulded into shape and do not return to their original shape when the moulding force is removed. When we talk about plastics, we usually mean various colourful materials that have been made out of chemicals produced from oil. In fact, the word plastic applies to any material that can be easily moulded into different shapes. Even metals can be plastic because, if heated, they soften and can be shaped.
5.
MOTION
Everything in the world is moving. Even things that seem still are in motion, because the atoms inside them are vibrating. An object moves from one place to another when forces act on it and those forces are not balanced. When a force in one direction changes the SPEED or VELOCITY of an object, or the way it moves, this is known as ACCELERATION.
A clock’s pendulum moves back and forth because the forces that act on it are not balanced. The weight on the pendulum and the tightness of the string constantly try to pull the pendulum towards the centre. But its weight and speed swing it past the point of balance (equilibrium point). So the velocity of the pendulum is constantly changing.
SPEEDING ROLLER COASTER
A roller coaster’s carriages accelerate (gather speed) when the force of gravity pulls them down a steep incline. The speed and weight of the carriages then keeps them moving, even when they continue in a straight line or climb upwards.
SPEED
When we think of speed, we think of cars, jet planes, anything that moves quickly. To scientists, however, speed means things moving fast or slow. Speed is defined as the distance an object travels in a certain amount of time. Fast cars travel at higher speed than slow cars, so they can go further in the same time.
MEASURING SPEED
You can calculate the speed of a runner by measuring the time he takes to travel a certain distance. His speed is the distance he travels divided by the time he takes. If the distance is measured in metres and the time in seconds, the speed is measured in metres per second (mps).
VELOCITY
Velocity is the speed of an object moving in a particular direction. Two cars driving at the same speed have different velocities if one of them goes north and the other goes south. Velocity is measured in metres per second (mps), which divides the distance travelled by the time taken, in a specific direction.
ACCELERATION
When we talk of things accelerating, we usually mean they are speeding up. In science, however, acceleration means any change in an object’s velocity, whether it goes faster, slower, or changes direction. According to Newton’s second law of motion, a force is always needed to produce an acceleration. The bigger the force, the faster the change in velocity.
An object that moves in a circle, such as this ball swinging on a string, constantly changes direction. Even when it turns at a steady speed, its velocity is always changing. It takes a force to make it accelerate like this. When an object moves in a circle, the force that constantly pulls it towards the centre and stops it flying off in a straight line is called centripetal force.