Материал: Андрианова English Thesaurus of Student-Physicist 2013

The energy either leaves the system and goes into the surrounding environment, or it stays in the system and is converted into a. nonmechanical form such as thermal energy. A simple example is a block sliding along a rough surface. Friction creates thermal energy, absorbed partly by the block and partly by the surrounding environment. When the block warms up, something called internal energy increases. The internal energy of a system is related to its temperature, which in turn is a consequence of the activity of its parts, such as the moving atoms of a gas or the vibration of atoms in a solid. Energy can be transferred between a nonisolated system and its environment.

Power

The rate at which energy is transferred is important in the design and use of practical devices, such as electrical appliances and engines of all kinds. The issue is particularly interesting for living creatures, since the maximum work per second, or power output, of an animal varies greatly with output duration. Power is denned as the rate of energy transfer with time. If an external force is applied to an object, and if the work done by this force in the time interval, then the average power delivered to the object during this interval is the work done divided by the time interval.

The SI unit of power is the joule/sec, also called the watt, named after James Watt. The horsepower was first defined by Watt, who needed a large power unit to rate the power output of his new invention, the steam engine. In electric power generation, it's customary to use the kilowatthour as a measure of energy.

1.What forms of energy do you know?

2.Give the physics definition of work.

3.What is kinetic energy?

4.Potential energy is a property of a system, rather than of a single object. Why?

5.Gravity is a conservative force, is not it?

6.What do you know about spring constant?

7.Give the definition of power.

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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:

system, work, foot-pound, potential energy, center of mass, internal

1._____ is a scalar quantity - a number rather than a vector - and consequently is easier to handle.

2.The U.S. customary unit of work is the _____, because distances are measured in feet and forces in pounds in that system.

3._____ 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.

4.We define a _____ as a collection of objects interacting via forces or other processes that are _____ to the system.

5._____ is the point in the body at which all the mass may be considered to be concentrated.

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UNIT 6

MOMENTUM AND COLLISIONS

Read the following texts. Study the thesaurus on fig. 6, answer the questions after the texts

What happens when two automobiles collide? How does the impact affect the motion of each vehicle, and what basic physical principles determine the likelihood of serious injury? How do rockets work, and what mechanisms can be used to overcome the limitations imposed by exhaust speed? Why do we have to brace ourselves when firing small projectiles at high velocity? Finally, how can we use physics to improve our golf game?

To begin answering such questions, we introduce momentum. Intuitively, anyone or anything that has a lot of momentum is going to be hard to stop. In politics, the term is metaphorical. Physically, the more momentum an object has, the more force has to be applied to stop it in a given time. This concept leads to one of the most powerful principles in physics: conservation of momentum. Using this law, complex collision problems can be solved without knowing much about the forces involved during contact. We'll also be able to derive information about the average force delivered in an impact. With conservation of momentum, we'll have a better understanding of what choices to make when designing an automobile or a moon rocket, or when addressing a golf ball on a tee.

Momentum and impulse

In physics, momentum has a precise definition. A slowly moving brontosaurus has a lot of momentum, but so does a little hot lead shot from the muzzle of a gun. We therefore expect that momentum will depend on an object's mass and velocity. The linear momentum p of an object of mass m moving with velocity v is the product of its mass and velocity.

Changing the momentum of an object requires the application of a force. This is, in fact, how Newton originally stated his second law of motion. Changing an object's momentum requires the continuous application of a force over a period of time, leading to the definition of impulse. Impulse is a vector quantity with the same direction as the constant force acting on the object. The impulse of the force acting on an object equals the change in momentum of that object.

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Conservation of momentum

When a collision occurs in an isolated system, the total momentum of the system doesn't change with the passage of time. Instead, it remains constant both in magnitude and in direction. The momenta of the individual objects in the system may change, but the vector sum of all the momenta will not change. Action and reaction, together with the accompanying exchange of momentum between two objects, is responsible for the phenomenon known as recoil.

Fig. 6. Thesaurus for Unit 6

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We can summarize the types of collisions as follows:

In an elastic collision, both momentum and kinetic energy are conserved.

In an inelastic collision, momentum is conserved but kinetic energy is not.

In a perfectly inelastic collision, momentum is conserved, kinetic energy is not, and the two objects stick together after the collision, so their final velocities are the same.

Collision

A collision may be the result of physical contact between two objects. For any type of collision, the total momentum of the system just before the collision equals the total momentum just after the collision as long as the system may be considered isolated. An elastic collision is defined as one in which both momentum and kinetic energy are conserved. The collision of a rubber ball with a hard surface is inelastic, because some of the kinetic energy is lost when the ball is deformed during contact with the surface. Elastic and perfectly inelastic collisions are limiting cases; most actual collisions fall into a range in between them.

In application, medical professionals use a device called a tonometer to measure the pressure inside the eye. This device releases a puff of air against the outer surface of the eye and measures the speed of the air after reflection from the eye. At normal pressure, the eye is slightly spongy, and the pulse is reflected at low speed. As the pressure inside the eye increases, the outer surface becomes more rigid, and the speed of the reflected pulse increases. In this way, the speed of the reflected puff of air can measure the internal pressure of the eye.

An important subset of collisions takes place in a plane. The game of billiards is a familiar example involving multiple glancing collisions of objects moving on a two-dimensional surface.

Rocket propulsion

When ordinary vehicles such as cars and locomotives move, the driving force of the motion is friction. In the case of the car, this driving force is exerted by the road on the car, a reaction to the force exerted by the wheels against the road. Similarly, a locomotive "pushes" against the tracks; hence, the driving force is the reaction force exerted by the tracks on the locomotive.

However, a rocket moving in space has no road or tracks to push against. In fact, reaction forces also propel a rocket. When the explosion

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