What is Power?
In 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
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.
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.
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.
P = Voltage*Current
- 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.
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.
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.
- 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.
- 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.
Applied Force = 50 N
Distance covered = 100 meters
Work = Force*distance = 50*100 = 5000 joules or N-m
- 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.
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.
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.
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.
Friction causes resistance in motion. It is also a force that acts in opposite direction of motion. When you put brake on your bicycle, it stops suddenly because you increase the friction between wheels and road. It is hard to push a box on cemented surface because cemented surface causes more friction due to its irregular surface and on other hand if push same box on glass table it will slide very easily because it has smooth surface that cause lower friction than cemented surface.
Types of friction
Friction has three types:
Static Friction occurs between objects that are in contact with each other are forced to move but both of them are not moving. That means if you put a box on a table or any surface and trying to push it but it is not moving than it feels static friction.
if one object is moving on another than both them are experiencing kinetic friction. Kinetic friction always occurs to moving objects. Normally kinetic friction is less than the static friction.
There are two types of kinetic friction
1. Sliding friction: When two objects slides on one another both feels sliding friction. Pushing a box and rubbing your hands both are examples of sliding friction.
2. Rolling friction: When one objects rolls on another or both are rolling on one another both feels Rolling friction. It is less than the Sliding friction that is why pushing a weight on wheels is easier than sliding it.
Like solids, liquids and gases also offer resistance in motion of an object. This kind of resistance is called Fluid friction. Drop a coin in oily liquid, it will sink slowly because of fluid friction. While riding a bicycle you feel backward push by air, it is also fluid friction.
In our everyday life friction is very important.
- It helps walking on the road.
- You warm your hand by rubbing them together.
- You driving a car and brake it for sudden stop.
- It helps you to hold a pencil or anything tightly.
Sometimes friction is avoided
- To reduce the loss of energy into machines.
- Our body joints are well lubricated to reduce friction.