BlogNCERTImportant Topic of Physics: Relative velocity

Important Topic of Physics: Relative velocity


When travelling by vehicle, bus, or train, you would notice that the trees, homes, and other objects outside are moving in the opposite direction. Is it true, however, that they have been going backward? No, you’re well aware that your vehicle is moving while the trees remain motionless on the ground. But, if that’s the case, why are all the trees travelling backward? Also, even though they are moving, your fellow passengers seem to be stationary to you.

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    It’s since you and your fellow travellers are moving together in your frame. That is to say, you and also the passengers have no relative velocity. While you are moving, the trees remain immobile. As a result, trees move at a certain speed in relation to yourself as well as the other rider. The difference in velocities for both you and the tree is the relative velocity. The velocity of an object or observer B in the hold mode of some other object or observer A is called relative velocity. Below is the formula for velocity:

    B’s velocity in relation to A is = Vb – Va

    The idea of relative velocity can only be described using this formula. When two items are going in the same direction, they are said to be going the same way.

    Vab= Va + Vb

    When two items are travelling in opposite directions, they are said to be in opposition.

    Vab= Va – Vb

    A Brief outline

    • Distinguish two trains travelling in the very same direction and at the same speed. Even if both trains are moving in relation to the houses and trees on both sides of the track, the other train appears to be stationary to the train observer. The other train looks to be moving at a standstill.
    • Assume you’re driving at 50 miles per hour. Your relative velocity to the earth’s surface is 50 miles per hour. At the same time, your relative velocity to me is zero if I’m seated next to you.
    • If we’re on a bus and you walked at 1 mph, then relative velocity on the planet would be 51 mph, and your relative velocity to me would be 1 mph. Any object’s relative velocity is its speed in comparison to any other object, irrespective of its speed.

    Important concepts

    1. There is no way to tell the difference between an item at rest and one travelling at a constant velocity in an inertial (non-accelerating) reference frame, according to physics. As a result, there is no “right” answer to the question “how fast is the train glass of water moving?” We are absolutely correct in saying that the glass is travelling 50 meters per second to the right, as well as in stating that the glass is motionless.
    2. Assume you’re on a smooth plane with all of the window drapes lowered. It’s literally impossible to tell if you’re flying at 300 m/s or if you’re sitting motionless on the runway.
    3. The Earth is a fantastic frame of reference for physics difficulties in most cases. However, there are situations when computing an object’s velocity relative to various reference frames is beneficial.
    4. Assume you’re competing in a canoe race on a river. It may be necessary to know not just your speed in relation to the river’s flow, but also your speed in relation to the riverbank and even your speed in relation to your racing opponent’s canoe.

    This appears to be more difficult than it is. Let’s have a peek at the few examples of how it’s used.

    Question1: In relation to the earth, a train moves at 60 m/s to the east. A gentleman on the train is moving at a speed of 5 m/s west of the train. Estimate the man’s velocity relative to the ground.

    Answer: First, figure out what kind of information you’ve been given. You know the train’s velocity with respect to the ground (vTG=60 m/s) if you call east the positive direction. You also know the man’s velocity in relation to the train (vMT=-5 m/s). By combining these, you can calculate the man’s velocity in relation to the ground.

    VMG = VMT + VTG = -5 m/s + 60 m/s = 55 m/s

    Question2: On the same straight track, two trains, each running at 20 kmph, approach each other. When two trains are 40 kilometres apart, a bird capable of flying at 40 kilometres per hour takes off from one and makes straight for the other. When it reaches the other train, it returns to the first train and so on. The total distance travelled by the bird before the trains collide is

    1. A) 20km
    2. B) 40km
    3. C) 60km
    4. D) 80km

    Answer: B) 40 km. The speed of the bird is 40km/hr. Henceforth the distance travelled=Speed× time= 40km

    Question3: A plane’s captain, Ravi, wishes to fly due west. The plane’s cruising speed in relation to the air is 251 m/s. A 65 m/s wind is going from south to the north, according to the weather forecast. What direction should Ravi point the plane in relation to the air, measured from due west?

    1. 10°
    2. 15°
    3. 20°

    Answer: C) 15°

    Significance of relative velocity for NEET exam

    1. The concept of relative motion is at the heart of the relative motion section, and it will help us comprehend why things appear to move differently in different frames.
    2. If we use an example, we may say that when we say an item has a given velocity, we mean that this velocity is in relation to some frame called the reference frame. Whenever we assess the velocity of the object in everyday life, we use the ground or the earth as our reference frame.
    3. NEET Physics needs round-the-clock practice as well as a thorough understanding of ideas and formulae. To get it on the NEET Physics course, first understand and grasp all the major topics, particularly formulas in the relative velocity section, as stated in the curriculum. It is the first step toward easily solving difficult numerical problems.
    4. Students consider physics numericals to be a nightmare, thus they tend to disregard them and therefore do not practice them enough. As an outcome, they focus on fixing other aspects of NEET. This is believed to be the primary reason why some people do not perform ok in NEET Physics, adversely affecting their entire NEET score.

    Also read: Multiplication of Vectors by a Real Number

    Frequently asked questions (FAQs)

    Question 1: Find whether the subsequent statement is true or false: Negative relative velocity is possible.

    Ans: The above statement is accurate. Negative relative velocity is possible. It is possible for relative velocity to be negative because it is the difference between two velocities regardless of their magnitude.

    Question 2: What’s the difference between relative velocity and velocity?

    Ans: The contrast between velocity and relative velocity would be that relative velocity is restrained in relation to a reference point that is placed at a different location. Though absolute velocity is measured in a frame where an object is either at rest or moving with regard to the absolute frame, the relative velocity is found in a frame in which an object is either at rest or flowing with respect to the absolute frame.

    Question 3: Why is it necessary to use relative velocity?

    Ans: The usage of relative velocity is necessary since it is used to determine whether or not an object is moving.

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