Physics

Circular Motion



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An object moves around a circle is called in circular motion. Sometimes the speed of the object is constant. This motion is called uniform circular motion. There are several important terms in circular motion. I will discuss them subsequently.

The first is the frequency. Suppose an object moves in circular motion. It will pass the same point again and again. The object makes one complete cycle if it reaches the same point at which it departs. The number of cycles made in a second is called the frequency. Its reciprocal is called period. What is the period? It is the time needed for an object to make one complete cycle.

In a uniform circular motion, the frequency is constant. The periode is constant too. Constant circular motion is important in our life. The rotation of the machine should be kept constant. The unstable rotation of a machine can damage the working parts.

There are two kinds of velocity in circular motion. They are tangential velocity and angular velocity. Tangential velocity equals to the distance travelled per unit time. This velocity is always perpendicular to the radius of the rotation. The angular velocity is the angle travelled divided by time.

The angular velocity of rotational motion is directly proportional to the frequency. Increasing the frequency will increase the angular velocity. On the contrary, increasing the period will decrease the angular velocity. This is because the period is the reciprocal to the frequency. The angular velocity is inversely proportional to the period.

The tangential velocity is directly proportional to two quantities. They are the angular velocity and the radius of the rotation. A faster rotation means greater tangential velocity. Increasing the radius of motion will increase the tangential velocity too. Actually, the tangential velocity is the product of the angular velocity and the radius of the rotation.

In uniform circular motion, although the magnitude of the velocity is constant due to the constant frequency, but the velocity vector itself is change. The direction of the velocity vector changes during the motion. To make a circular path, the object must bend it motion. This will change the direction of the motion and the direction of the velocity too.

What does make the motion of the object bend? In circular motion, there must be a force directed toward the centre of motion. This is the centripetal force. Without this force, there will be no circular motion. This centripetal force gives rise to the centripetal acceleration. This acceleration changes the direction of the tangential velocity. Centripetal acceleration is directly proportional to the square of tangential velocity. It is inversely proportional to the radius of rotation. The centripetal force equal to the product of the mass of the object and the centripetal acceleration.

Where do the centripetal forces come? This force comes from several sources. It is depending on the kind of the motion. Imagine a car is turning around a circular park. The friction between the wheels and the road provides the centripetal force. In a slippery wet road, the friction is small. It is not large enough to bend the car. When it is happen, the car will out from the track. The motor racer must incline its motor to turn in the curve track. They do this in order to provide more centripetal force to bend their motion.

Unfortunately, a car cannot incline its body. A train cannot either. How can we provide additional centripetal force? For the slippery surface, inclining the surface will help. The inclined road gives a horizontal force toward the centre. This is come from the horizontal component of the normal force. The normal is force always perpendicular to the contact surface. The incline surface rotates the normal force from the vertical line. There will be an angle between the normal force and the vertical line. The normal force has two components now. They are the horizontal and the vertical component.

Tie a stone at the end of the rope. Hold the other end of the rope. Rotate it in horizontal to make a circle. The stone move in a circular motion. The forces from our hand that hold the rope give the centripetal force indirectly. Why I say it is indirectly? The force from our hand does not in contact with the stone. The rope bends the stone motion. The force in the rope originates from our hand. Rotating the stone faster will increase the centripetal force. If the hand fails to provide this large force, the rope will escape.

Have you ever thought, what keeps the moon orbiting the Earth? The centripetal force comes from the attractive gravitational force. According to Newton, two masses have mutual attractive forces. These forces are equal in magnitude. They are in opposite direction one to another. The same situation occurs in a satellite orbiting the earth. The planets in solar systems also have something in common about this force.

There is a pitfall in understanding circular motion. I will back to our stone example. People think a force pull the stone away from our hand. They call it centrifugal force, directed away from the centre of motion. This is wrong. The stone escape because we fail to provide enough centripetal force. The force provided is not large enough to bend the faster motion. As a result, the motion will not bend any more. There is no such a mysterious centrifugal force.

Circular motion is an important concept in our life. Many phenomenons involve circular motion. Knowing a little about it will help to give more value to life. I hope this writing can help you all.

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