Материал: Андрианова English Thesaurus of Student-Physicist 2013

The magnitude of the gravitational force acting on an object of mass m near Earth's surface is called the weight, w, of the object, given by w = mg, where g is the acceleration of gravity. SI unit: newton (N). Unlike mass, weight is not an inherent property of an object because it can take different values, depending on the value of g in a given location.

Newton’s third law

According to Newton, as the nail is driven into the block by the force exerted by the hammer, the hammer is slowed down and stopped by the force exerted by the nail. Newton described such paired forces with his third law: If object 1 and object 2 interact, the force F12 exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force F21 exerted by object 2 on object 1.

Newton's third law constantly affects our activities in everyday life. Without it, no locomotion of any kind would be possible, whether on foot, on a bicycle, or in a motorized vehicle.

Applications of Newton’s laws

When we apply Newton's law to an object, we are interested only in those forces which act on the object. The most important step in solving a problem by means of Newton's second law is to drawthe correct free-body diagram. Include only those forces that act directly on the object of interest.

A zero net force on a particle does not mean that the particle isn't moving. It means that the particle isn't accelerating. If the particle has a nonzero initial velocity and is acted upon by a zero net force, it continues to move with the same velocity.

Forces of friction

An object moving on a surface or through a viscous medium, such as air or water, encounters resistance as it interacts with its surroundings. This resistance is called friction. Forces of friction are essential in our everyday lives. Even standing in one spot would be impossible without friction, as the slightest shift would instantly cause you to slip and fall. The force that counteracts F and keeps is called the force of static friction. We call the friction force for an object in motion the force of kinetic friction. Friction makes it possible to grip and hold things, drive a car, walk, and run.

1.Can we control the direction of applied force? What does it

mean?

2.What concept introduced Michael Faraday?

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3.Formulate Newton's first law.

4.What is inertia?

5.Formulate Newton's second law.

6.Give the definition of the gravitational force.

7.Formulate Newton's third law.

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

3. Fill in the gaps with the missing words from the list:

inertia, reaction force, gravitational force, action force, forces of friction, motion

1._____ can be used to explain the operation of one type of seat belt mechanism.

2.Force causes changes in _____.

3.The _____ on Earth due to Moon is much weaker than the gravitational force on Earth due to the Sun.

4.In applying Newton’s third law, remember that an _____ and

_____ always act on different objects.

5._____ are important in the analysis of the motion of cars and other wheeled vehicles.

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UNIT 5

ENERGY

Read the following texts. Study the thesaurus on fig. 5, answer the questions after the texts

Energy is one of the most important concepts in the world of science. In everyday use, energy is associated with the fuel needed for transportation and heating, with electricity for lights and appliances, and with the foods we consume. These associations, however, don't tell us what energy is, only what it does, and that producing it requires fuel.

Energy is present in the Universe in a variety of forms, including mechanical, chemical, electromagnetic, and nuclear energy. Even the inert mass of everyday matter contains a very large amount of energy. Although energy can be transformed from one kind to another, all observations and experiments to date suggest that the total amount of energy in the Universe never changes. This is also true for an isolated system - a collection of objects that can exchange energy with each other, but not with the rest of the Universe. If one form of energy in an isolated system decreases, then another form of energy in the system must increase. For example, if the system consists of a motor connected to a battery, the battery converts chemical energy to electrical energy, and the motor converts electrical energy to mechanical energy. Understanding how energy changes from one form to another is essential in all the sciences.

Using an energy approach to solve certain problems is often much easier than using forces and Newton's three laws. These two very different approaches are linked through the concept of work.

Work

Work has a different meaning in physics than it does in everyday usage. In the physics definition, a programmer does very little work typing away at a computer. In physics, work is done only if an object is moved through some displacement while a force is applied to it. It's easy to see the difference between the physics definition and the everyday definition of work. The programmer exerts very little force on the keys of a keyboard, creating only small displacements, so relatively little physics work is done. The mason must exert much larger forces on the concrete blocks and move them significant distances, and so performs a much greater amount of work. Even very tiring tasks, however, may not constitute work according

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to the physics definition. A truck driver, for example, may drive for several hours, but if he doesn't exert a force, and he doesn't do any work.

Kinetic energy

Solving problems using Newton's second law can be difficult if the forces involved are complicated. An alternative is to relate the speed of an object to the net work done on it by external forces. If the net work can be calculated for a given displacement, the change in the object's speed is easy to evaluate. We can think of kinetic energy as the work a moving object can do in coming to rest. For example, suppose a hammer is on the verge of striking a nail. The moving hammer has kinetic energy and can therefore do work on the nail.

Gravitational Potential energy

An object with kinetic energy (energy of motion) can do work on another object, just like a moving hammer can drive a nail into a wall. A brick on a high shelf can also do work. It can fall off the shelf, accelerate downwards, and hit a nail squarely, driving it into the floorboards. The brick is said to have potential energy associated with it, because from its location on the shelf it can potentially do work. Potential energy is a property of a system, rather than of a single object, because it's due to a physical position in space relative a center of force.

Gravitational Work and Potential Energy

For most trajectories - say, for a ball traversing a parabolic arc - finding the gravitational work done on the ball requires sophisticated techniques from calculus. Fortunately, for conservative fields there's a simple alternative: potential energy. Gravity is a conservative force, and for every conservative force a special expression called a potential energy function can be found.

Spring Potential energy

Springs are important elements in modern technology. They are found in machines of all kinds, in watches, toys, cars, and trains. Work done by an applied force in stretching or compressing a spring can be recovered by removing the applied force, so like gravity, the spring force is conservative.

Pushing a block against the spring compresses it a distance x. While x appears to be merely a coordinate, for springs it also represents a displacement from the equilibrium position. F s = - k x , where k is a constant of proportionality, the spring constant, carrying units of newtons per meter and called Hooke's law.

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