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Power

What is Power?

crane-powerIn physics, power is the rate of doing work. Because work does not deal with time, it only tells us that how much energy is consumed in performing the work. Since the same work can be done at different rates. For example, you and your friend are pushing the boxes having the same weight. Now, your friend reaches the target in 60 seconds and you reach the same target in 100 seconds. Both of you have expanded the same amount of energy to push the box. But, your friend did the work faster than you, so its power was more than you.

How is Power measured?

Power is measured in watts in the international system of units. One watt of power is equal to one joule energy expanded in one second. The mathematical formula for calculating the power is:

Power = Work/time

Or

P = W/t

Example: Two persons are carrying boxes towards their home. The first person is reaching the home in 100 seconds, whereas the second person in 40 seconds. Calculate the power of both persons, if the work performed by both is 400 joules.

Solution:

Power of the first person is:

P1 = work / time = 400 joules / 100 seconds = 4 watts

Power of the second person:

P2 = work / time = 400 joules / 40 seconds = 10 watts

Clearly, the second person is doing the work with more power than the first person.

The power is also measured in another unit, which is called horsepower. Horsepower is bigger unit than a watt and is mostly used for heavy duty machines. One horsepower is equal to 745.7 watts.

Electrical Power

Our rate of electricity consumption is also measured in watts. To find out how much power we are using, just multiply the current flowing in the circuit with the voltage of the circuit.

Light-Bulb

P = Voltage*Current

Facts

  • Chemicals that release energy very fast result into the explosions.
  • Your electricity bill is calculated in kilowatt-hours. This is the unit of energy, not the power.
  • In real practice, machines with high power consume more energy than the machines with low power to perform the same Because increasing power can decrease the efficiency.
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Work

You might think about work as, reading books and doing homework. You may have also heard the sentence “hard work leads to success”. But, in physics work has its own meaning and it is different from the previously described example.

What is Work?

In physics, work is done when a force is applied to an object and it travels some distance. For example, you kicked a ball with a certain amount of force and ball traveled few meters, then you had done a certain amount of work on the ball.
boy-kicking-a-ball

How is Work measured?

Work is the scalar quantity, means it doesn’t need direction to define it. Work is measured in joule or newton-meter. This is the same unit in which energy is measured. The work done on an object is found by multiplying the force exerted on an object with the distance that it traveled.

Work = Force*distance

But, there are also some complications in calculating the work which may sometimes confuse you. Here are the few things to remember when calculating the work.

  1. If you are applying a force on an object but it is not moving, then work will be zero. Because object didn’t travel any distance. For example, you are pushing a wall with all of your force, but the wall is not moving. So, your work done will be zero.
  2. If you are applying a force on an object and it is moving in a path that brings it back from where it started. Then, work done on that object is also zero. Because the net distance traveled by the object is zero. For example, a race car is starting from a starting point and traveling in a circular path and it comes back again at the same starting point. Then, work done by the car is zero because it didn’t travel any net distance.

Example: A man is pushing a box with a force 50 newtons. The box has traveled the distance of 100 meters. Calculate the work done by the man.
worker-draging-load

Solution:

Applied Force = 50 N

Distance covered = 100 meters

Work = Force*distance = 50*100 = 5000 joules or N-m

Facts

  • If a force is applied in opposite direction to the covered distance, then the work is said to be negative work.
  • An object that falls from a height also performs work due to gravitational force and height.
  • In space when an object is accelerated to a certain speed then the force is removed from it. But, the object doesn’t lose its speed because there is no any air friction in space. So, work done by objects flying in space is also zero.
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Gravity

Gravity is force that attracts us and any other object towards earth’s center. When we jump gravity pull us back and we fall back on earth. There is a difference between gravity and gravitation. Gravitation is the force of attraction between objects and gravity is the gravitational force between earth and other objects that means gravity is special case of gravitation.

earth-gravity

On the earth gravity keeps air around us from escaping into space. Tides into the ocean are caused by the gravitational pull of moon and sun. By knowing gravity you can calculate when a ball will touch the ground after released into air. You can measure and graph the trajectory of a projectile. You can calculate how much propellant is required to push a rocket into the space and many more.

Gravity at the surface of earth is 9.8 that means an object falling towards earth gains 9.8 m/s speed in every second if air resistance is neglected. If a ball is dropped from a building it will have 9.8 m/s speed at the end of 1st second, 19.6 m/s at end of 2nd second and 29.4 at the end of 3rd second and so on. Value of gravity decreases for an object which is going away from earth or going closer to the center of earth. Gravity at the center of the earth is zero. Every object is accelerated at the rate to earth no matter it is a paper clip or paper. If two masses are dropped from a height both will hit the ground at same time. You are thinking why a paper falls slowly than a paper clip when both are dropped from same height? It is because of air resistance, paper clip has lower surface area than paper so resistance for paper is high. If you drop both paper and paper clip into a vacuum chamber both will hit the bottom at same time.

Gravitation

Gravitational force exists since the formation of universe. It is the fundamental force of nature. It is the force that attracts atoms together to form stars and planets. It keeps the moon in orbit around earth and earth around sun. How high is the gravitational pull between two objects? It depends on mass and distance between two objects. Larger the mass and less the distance greater would be the gravitational pull.

Sir Isaac Newton was first who mathematically defined gravity and gave law of universal gravitation. But later, Albert Einstein made improvements to it in theory of relativity because in some cases newton law of universal gravitation was inaccurate.

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What is Friction

You may be listening to the word friction from your childhood that it slows down the vehicles and wear out the machine part. That is true! Friction causes resistance in motion. It is also a force that acts in opposite direction of motion.
Friction-force

When you put a brake on your bicycle, it stops suddenly because you increase the friction between the wheels and road. It is hard to push a box on a cemented surface than a smooth surface because cemented surface causes more friction due to its irregular surface. On the other hand, if you push the same box on a glass table, it will slide very easily because it has a very smooth surface that causes lower friction.

Types of friction

Friction has been divided into 3 types; static friction, kinetic friction, and fluid friction. All the three types of friction act on any object differently. Let’s see!

  1. Static Friction

    Static friction, as the name suggests occurs between the objects that are in contact with each other but not moving. So, that means if you place a heavy box on a table, will it experience the static friction? No!

    pushing-box-sliding-friction

    OK so, when static friction actually occurs? Static friction will only occur between two static objects that are not moving even when the force is applied on them. For example, When you put a box on the table it won’t experience static friction until you apply a force to slide it but it still doesn’t move, then it is experiencing the static friction. But when you push the box even harder, the box will move. Now the static friction will be converted to kinetic friction.

  2. Kinetic Friction

    Now it is time learn about the kinetic friction. Let’s start with an example. Suppose you are riding a bicycle very fast. Tell me what will happen when you suddenly stop paddling? You will notice that the bicycle will start to slow down until it comes to stop completely. This is what you might guess, the kinetic friction!
    bicycle-friction

    If one object is moving on another object than both objects will experience kinetic friction. Kinetic friction always occurs between moving objects (In our case, the wheels of bicycles and the road). Normally kinetic friction is less than the static friction due to the motion of the objects.

    Kinetic friction is further divided into two types.

    1. Sliding friction: When two objects slide on each other, both will feel the sliding friction. Pushing a box and rubbing your hands both are the examples of sliding friction.
    2. Rolling friction: When one object rolls on another object or both are rolling on each other, they will experience rolling friction. It is less than the Sliding friction that is why pushing a weight on wheels is easier than sliding it. The bicycle that you were riding was experiencing rolling friction.
      wheel-rolling-friction

  3. Fluid Friction

    Solids offer two kinds of friction to an object, but there is also another type of friction that is provided by the fluids, e.g. liquids and gases. Let’s see an example to understand the concept more easily.
    coin-droping-in-liquid-fluid-friction

    What will happen when you drop a coin in oily liquid (your cooking oil)? It will sink slowly to the bottom. Can you tell me why? Yes, you guessed correctly. This is because of the fluid friction. The fluid (oil) is resisting the motion of the coin. Fluid friction increases with two things; one is the velocity of an object and other is viscosity (thickness of liquid) of the fluid. The honey is more viscous than the cooking, so it will provide more friction to the coin. Try it!

The importance of Friction in Everyday Life.

  • It helps us to walk on the road. Otherwise, we would skid away.
  • You warm your hand by rubbing them together.
  • We can stop of vehicles suddenly by putting brakes on them.
  • It helps you to hold a pencil or anything tightly. Without friction, the object you hold would easily slip away from your hand.

Why is Friction Avoided?

  • Friction cause wears in the machinery parts. So, we lubricate the machines to reduce friction
  • Our body joints are well lubricated to reduce friction.
  • Reduced fluid friction can increase the mileage of your car. After the speed of 80 km/h, we spend the majority of our fuel to fight friction.
    moving-car-friction
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