Category Science

WHAT IS A FORCE?

There are forces acting on us — and everything on our planet — all the time. The push and pull of forces is what keeps things where they are or starts them into motion. Forces enable something to stay the same size and shape or to change size and shape. They can slow down a moving object or speed it up, or change the direction of its motion. Whenever energy is being used, forces are at work.

In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI of newtons and represented by the symbol F.

The original form of Newton’s second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

Concepts related to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in Fluids.

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ARE THERE DIFFERENT KINDS OF FORCE?

There are many different kinds of force. They are affecting everyday objects around us all the time. Here are just some of the many forces that we experience.

An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is applied force acting upon the object. The applied force is the force exerted on the desk by the person.

The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed “downward” towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation:

The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force – sliding and static friction. Though it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.

The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said to obey “Hooke’s Law”), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

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Is wildlife in danger?

Many of us have a favourite wild animal, such as a tiger or bear.

But did you know that many wild animals and plants are now rare? Some may soon die out.

People are causing this problem. We hunt animals and destroy wild places.

Luckily, some groups are working to save rare plants and animals. We can all help.

Bison and many butterflies are rarer than they used to be.

Extinct animals and plants are ones that have died out altogether. Wildlife that is very rare is called endangered.

Endangered animals may be large like a bison, or small creatures like a butterfly. Hunters almost wiped out the bison. Many butterflies are threatened by global warming as well as by people destroying the wild places where they live.

Dinosaurs died out long ago.

A species is a particular type of plant or animal, such as the American bison. Extinction is when all the animal or plants of one type die out, so that none are left.

In the past, many species have died out, often because they couldn’t cope with natural changes. The dinosaurs probably died out after a meteorite struck Earth and made conditions change very quickly. Now global warming is making conditions change quickly, too.

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How we can help the wildlife which is currently in danger?

We must do more to protect nature. Experts say thousands of plants and animals may die out in the next fifty years or so.

Loss of wild habitats, global warming and pollution are all big problems in many places.

People all over the world are now working hard to save rare species. But it can only work if we all help.

Conservation groups help to save rare species such as whales.

Campaign groups such as the WWF (World Wide Fund for Nature) have helped to save many rare species. Greenpeace helped to convince many countries to ban whaling, in order to save whales from extinction.

Everyone can help with conservation work by joining a wildlife organisation. You could take part in a sponsored walk to raise money for your favourite animal.

In China, people are working to save pandas.

In China, giant pandas are in danger of dying out because of loss of their forest habitat. Only 1,000 of these animals remain.

Twelve forest reserves have been set up to save pandas in the wild. Zoos have also managed to breed baby pandas from captive animals. Captive breeding projects like this are very expensive, so they can only help to save a few species.

A ban on hunting protects this beautiful butterfly.

In the 1970s, many governments signed an agreement called the Convention on International Trade in Endangered Species (CITES).

CITES bans trade in most wild animals including this birdwing butterfly. Sadly the profits that can be made from wildlife are big and many people ignore the law.

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Which unwanted visitors are endangering wildlife?

When people bring new species into a habitat, they can threaten local plants and animals.

The new plants and animals can also cause big changes in their new habitat.

Plant-eaters take the food of local animals, while new hunters kill local wildlife.

People brought mongooses to hunt rats, but they became pests.

People have brought new animal hunters to many areas. Mongooses were introduced to Puerto Rico to hunt rats. But they soon threatened local snakes and spread the disease rabies to other animals on the island.

Rabbits brought to Australia nibble pastures bare.

New wildlife brought to Australia caused great changes there. Some of the newcomers took over from local species.

European settlers released rabbits in Australia. The new arrivals bred quickly and became pests, stripping grasslands bare. This left nothing for local species. Rabbits are now killed as pests.

Water hyacinth is now a pest on many waterways.

New plants as well as animals can become pests when they thrive in their new homes.

In the 19th century, a pretty plant called the water hyacinth from South America was released in lakes and ponds in North America, Africa and Asia. It spread quickly and now completely chokes waterways in many areas.

Hedgehogs brought to the Hebrides steal birds’ eggs.

Animals of remote islands are particularly threatened by new predators because they are often not used to being hunted.

On the lonely Hebrides Islands in Scotland, local seabirds are threatened by hedgehogs. These were brought to the islands to eat snails and slugs in people’s gardens, but they now prey on birds’ eggs.

Scientists are trying to reduce the numbers of hedgehogs on the islands. But there is a lot of disagreement between the bird lovers and the hedgehog lovers.

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How can the animal homes be saved?

All over the world, wildlife groups are working to save rare wildlife. This is called conservation.

Creating nature reserves helps to protect rare species. This saves whole areas of habitat.

People also need to take better care of nature everywhere. We should live in ways that don’t harm wildlife.

Tourists visit reserves to see rare animals such as these manatees.

Many people visit reserves to see wild plants and animals in natural settings. The visitors pay a fee which helps to pay for conservation work in the park. This is called eco-tourism.

Reserves also provide jobs and money for local people. Rare mammals called manatees are protected in US coastal reserves.

This coral reef is in a nature reserve.

In many parts of the world, reserves and national parks now protect wild areas. Building and mining is banned in these areas, and no one is allowed to pick plants or harm wild animals.

Marine reserves are just as important as parks on land. The Great Barrier Reef in Australia is a protected coral reef.

Ladders help salmon to swim upriver to lay their eggs.

Many animals make yearly journeys to safe places where they can breed. Fish such as salmon, birds and whales all make these regular journeys, called migrations. Salmon swim upriver to breed. If dams block their way, the fish cannot breed successfully.

Many dams now have ladders. A ladder is a series of steps and pools, one above the other, that help the salmon to swim upriver.

Wildlife workers protected this rare bird by taking it to a tiny new island home.

Endangered animals sometimes have to be moved to places of safety, called sanctuaries, for their own protection. In New Zealand, the introduction of cats and other predators brought the flightless Takahe to the edge of extinction.

Conservationists took some of the last birds and released them on a tiny island offshore, which was first made safe from predators. Cats, weasels, and rats were removed to make the island safe.

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