Unit of Velocity - Definition, Difference Between Unit of Speed and Velocity

Q: Differmtiate between the unit of speed and velocity

Speed Velocity Speed is the measure of how fastly an object is moving. the direction of the movement of an object. It is a scalar quantity It is a vector quantity rate of change of distance Rate of change of displacement Speed of an object moving is never negative velocity of a moving object can be zero, positive or negative. indicator of the rapidity of the object. indicator of the position as well as the rapidity of the object. It can be defined as the distance covered by an object in unit time. Velocity can be defined as the displacement of the object in unit time.

Unit of Velocity - Definition, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:39 PM IST

What is the SI unit of velocity or Define velocity and its SI unit.

The concept of velocity and speed can help us identify the moving objects in a given area. For instance, if two objects are moving in the same direction, it can be easily done to tell the fastest moving object. However, if the two are in the opposite direction, it can be hard to determine the fastest moving object.

Unit of Velocity - Definition, FAQs

Definition of velocity units physics and SI unit of velocity and acceleration:

Velocity is the rate at which the position changes. The average velocity is the displacement or position change (a vector quantity) per time ratio. The SI unit of velocity is metre per second (ms-1). When the amount or direction of a body's velocity changes, the body is said to be accelerating. The velocity is measured in metre per second whereas knot is the unit of one nautical mile per hour.

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Constant velocity

The simplest form of motion is that which has a constant velocity. When an object slides over a horizontal, low friction surface, we see continual motion (when a puck slides over a hockey rink.)

Constant velocity

(Source self made using paint)

A graph of displacement versus time for a body travelling at a constant velocity is shown above. The graph's straight line can be expressed algebraically as follows:

x = x0+vt In the equation, x0 represents the displacement at time t0 , and v represents the body's constant velocity, v=dx/dt.

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SI unit of velocity in physics and velocity unit of measurement:

m/s (ms-1) is the SI unit of velocity.

Other velocity units and dimensions are shown in the table below.

Common symbols	v, v, v⃗
SI unit	m/s
Other units	mph, ft/s
Dimension	LT−1

(Source self made using tables):

SI system:

Quantity	Quantity symbol	CGS unit of velocity
velocity	v	metre per second
acceleration	a	m/s2
force	F	newton

CGS system:

Quantity	Quantity symbol	CGS unit name
velocity	v	centimetre per second
acceleration	a	gal
force	F	dyne

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Unit of speed and velocity

For most of us, the concepts of unit of speed and velocity are a little perplexing. The distinction between unit of speed and velocity is that speed informs us how rapidly an object is going, but velocity tells us not only how fast an object is moving but also in which direction it is travelling. Velocity is a function of displacement, whereas speed is a function of distance travelled. The velocity of a body at any particular time is known as instantaneous velocity.

The average velocity is calculated by multiplying the total displacement by the entire time, v = xt, where x is the total displacement of the body and t is the time. Average speed is always less than or equal to average velocity; This is due to the fact that displacement can never exceed the distance travelled, yet the distance travelled can exceed displacement.

Difference between unit of speed and velocity

Speed	Velocity
Speed is the measure of how fastly an object is moving.	the direction of the movement of an object.
It is a scalar quantity	It is a vector quantity
rate of change of distance	Rate of change of displacement
Speed of an object moving is never negative	velocity of a moving object can be zero, positive or negative.
indicator of the rapidity of the object.	indicator of the position as well as the rapidity of the object.
It can be defined as the distance covered by an object in unit time.	Velocity can be defined as the displacement of the object in unit time.

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is the SI unit of velocity?

m/s (ms-1) is the SI unit of velocity.

2. What is the unit of velocity in cgs system?

cm/s (cms-1) is the cgs unit of velocity.

3. What are the unit of speed and velocity?

The units for measuring speed and velocity are the same. The metre is the SI unit for measuring distance and displacement. The second is the SI unit of time. The metre per second – the SI unit of speed and velocity

4. Differmtiate between the unit of speed and velocity

Speed	Velocity
Speed is the measure of how fastly an object is moving.	the direction of the movement of an object.
It is a scalar quantity	It is a vector quantity
rate of change of distance	Rate of change of displacement
Speed of an object moving is never negative	velocity of a moving object can be zero, positive or negative.
indicator of the rapidity of the object.	indicator of the position as well as the rapidity of the object.
It can be defined as the distance covered by an object in unit time.	Velocity can be defined as the displacement of the object in unit time.

5. How does velocity differ from speed?

Velocity differs from speed in that it includes direction. Speed is a scalar quantity that only describes how fast an object is moving, while velocity specifies both the speed and the direction of motion. For example, a car traveling at 60 km/h north has a different velocity than a car traveling at 60 km/h south, even though their speeds are the same.

6. What is velocity?

Velocity is a vector quantity that describes both the speed and direction of an object's motion. It represents how fast an object is moving and in which direction. Unlike speed, which is a scalar quantity, velocity provides information about both magnitude and direction of motion.

7. Why is velocity considered a vector quantity?

Velocity is considered a vector quantity because it has both magnitude (speed) and direction. This means that to fully describe an object's velocity, you need to specify not only how fast it's moving but also in which direction it's traveling.

8. What does a velocity of zero mean?

A velocity of zero means that an object is not changing its position over time. This could indicate that the object is at rest, or it could mean that the object has momentarily stopped, such as at the highest point of a ball's trajectory before it starts falling back down.

9. Can velocity be negative?

Yes, velocity can be negative. A negative velocity doesn't mean the object is moving slower; it indicates that the object is moving in the opposite direction of what is defined as the positive direction. For example, if moving right is considered positive, then an object moving left would have a negative velocity.

10. What is the standard unit of velocity?

The standard unit of velocity in the International System of Units (SI) is meters per second (m/s). This unit combines distance (meters) and time (seconds) to describe how quickly an object changes its position in a specific direction.

11. How is average velocity calculated?

Average velocity is calculated by dividing the total displacement (change in position) by the total time taken. The formula is: average velocity = (final position - initial position) / (final time - initial time). It's important to note that this uses displacement, not distance traveled.

12. Why is it important to specify the frame of reference when discussing velocity?

Specifying the frame of reference is crucial when discussing velocity because velocity is relative. The same object can have different velocities when observed from different perspectives. For example, a passenger walking in a moving train has one velocity relative to the train and a different velocity relative to the ground outside.

13. How does acceleration relate to velocity?

Acceleration is the rate of change of velocity over time. It describes how quickly an object's velocity is changing, either in magnitude (speed) or direction, or both. When an object accelerates, its velocity is changing; this could mean it's speeding up, slowing down, or changing direction.

14. Can an object have constant speed but changing velocity?

Yes, an object can have constant speed but changing velocity. This occurs when an object moves in a circular path at a constant speed. Although the speed remains the same, the direction of motion is constantly changing, which means the velocity is changing. This change in velocity without a change in speed is called centripetal acceleration.

15. How can velocity be represented graphically?

Velocity can be represented graphically in several ways:

16. How does the concept of relative velocity apply in everyday life?

Relative velocity is important in many everyday situations. For example:

17. What is the significance of the velocity-time graph?

A velocity-time graph provides important information about an object's motion:

18. How does air resistance affect an object's velocity?

Air resistance, or drag, is a force that opposes an object's motion through air. It affects velocity in several ways:

19. How does velocity change during projectile motion?

In projectile motion, velocity changes in a specific way:

20. What is the relationship between velocity and kinetic energy?

Kinetic energy is directly related to velocity through the equation: KE = (1/2)mv², where m is mass and v is velocity. This quadratic relationship means that doubling the velocity quadruples the kinetic energy. Understanding this relationship is crucial in many applications, from vehicle safety to understanding the destructive power of fast-moving objects.

21. What is the difference between average velocity and instantaneous velocity?

Average velocity is the total displacement divided by the total time taken, giving an overall picture of motion. Instantaneous velocity is the velocity at a specific instant or point in time. While average velocity provides a broad view of motion over a time interval, instantaneous velocity gives precise information about the object's motion at a particular moment.

22. What is the relationship between velocity and wavelength in wave motion?

In wave motion, velocity (v), wavelength (λ), and frequency (f) are related by the equation v = fλ. This means that for a given frequency, the velocity of a wave determines its wavelength, or vice versa. This relationship is fundamental in understanding various wave phenomena, from sound waves to electromagnetic radiation.

23. How does the concept of relative velocity apply in special relativity?

In special relativity, the concept of relative velocity becomes more complex. Unlike in classical mechanics, velocities don't simply add linearly. Instead, relativistic velocity addition takes into account the speed of light as a universal speed limit. This leads to effects like time dilation and length contraction, which become significant as velocities approach the speed of light.

24. What is the significance of escape velocity?

Escape velocity is the minimum velocity an object needs to break free from a celestial body's gravitational field without further propulsion. It's crucial in space exploration and satellite deployment. The escape velocity depends on the mass and radius of the celestial body. For Earth, it's about 11.2 km/s at the surface. Understanding escape velocity is essential for planning space missions and understanding planetary formation.

25. What is the relationship between velocity and time dilation in special relativity?

In special relativity, time dilation is directly related to velocity. As an object's velocity increases relative to an observer, time appears to slow down for that object from the observer's perspective. This effect becomes significant as velocities approach the speed of light. The time dilation factor is given by the Lorentz factor: γ = 1 / √(1 - v²/c²), where v is the relative velocity and c is the speed of light.

26. How does velocity affect the Doppler effect?

The Doppler effect is the change in frequency of a wave for an observer moving relative to its source. The relative velocity between the source and observer determines the magnitude of the frequency shift. For approaching objects, the observed frequency increases (blueshift), and for receding objects, it decreases (redshift). This principle is used in various applications, from radar speed guns to studying the motion of distant galaxies.

27. What is the significance of group velocity in wave mechanics?

Group velocity is the velocity at which the overall shape of a wave's amplitudes propagates through space. It's particularly important when dealing with wave packets or modulated waves. In dispersive media, where wave speed depends on frequency, the group velocity can differ from the phase velocity. Understanding group velocity is crucial in fields like fiber optics, where it determines the speed of information transmission.

28. What is the relationship between velocity and angular momentum?

Angular momentum (L) is related to velocity through the equation L = r × p, where r is the position vector and p is linear momentum (mv). In circular motion, angular momentum can be expressed as L = Iω, where I is the moment of inertia and ω is the angular velocity. The conservation of angular momentum is a fundamental principle in physics, playing a crucial role in understanding rotational motion in systems ranging from spinning tops to galaxies.

29. How does velocity relate to the concept of relativistic mass?

In special relativity, the concept of relativistic mass shows how an object's mass appears to increase as its velocity approaches the speed of light. The relativistic mass is given by m = γm_0, where m_0 is the rest mass and γ is the Lorentz factor (1 / √(1 - v²/c²)). This relationship demonstrates that as velocity increases, the object's mass appears to increase, approaching infinity as v approaches c. This concept is crucial in understanding high-energy particle physics and the limitations on accelerating particles to very high speeds.

30. What's the difference between instantaneous velocity and average velocity?

Instantaneous velocity is the velocity of an object at a specific instant or point in time, while average velocity is the overall velocity over a period of time. Instantaneous velocity can be thought of as the velocity at a single point on a velocity-time graph, while average velocity represents the slope of the line connecting two points on a position-time graph.

31. What does the slope of a position-time graph represent?

The slope of a position-time graph represents the velocity of the object. A steeper slope indicates a higher velocity, while a flatter slope indicates a lower velocity. A horizontal line on a position-time graph represents zero velocity (the object is not moving), and a negatively sloped line represents negative velocity (the object is moving in the opposite direction).

32. What's the relationship between displacement and velocity?

Displacement is the change in position of an object, while velocity is the rate of change of displacement over time. In other words, velocity describes how quickly displacement is occurring. Mathematically, velocity is the derivative of displacement with respect to time, and displacement is the integral of velocity over time.

33. How does the unit of velocity (m/s) relate to other common units of speed?

The unit of velocity, meters per second (m/s), can be related to other common units of speed through conversion factors:

34. What is terminal velocity and how is it related to air resistance?

Terminal velocity is the maximum velocity attainable by an object falling through a fluid (like air or water) under the force of gravity. It occurs when the downward force of gravity equals the upward force of air resistance. At this point, the net force on the object is zero, and it no longer accelerates. The value of terminal velocity depends on the object's mass, shape, and the properties of the fluid it's falling through.

35. How does velocity relate to momentum?

Velocity is a key component of momentum. The formula for momentum is p = mv, where m is mass and v is velocity. This linear relationship means that doubling the velocity doubles the momentum. The vector nature of velocity also makes momentum a vector quantity, which is important in understanding collisions and conservation of momentum.

36. How does the concept of velocity apply in circular motion?

In circular motion, velocity is constantly changing in direction, even if the speed remains constant. This is because velocity is a vector quantity, and the direction of motion is continuously changing as the object moves in a circle. The change in velocity (without a change in speed) results in centripetal acceleration, which is directed towards the center of the circle.

37. What is the significance of the tangential velocity in circular motion?

Tangential velocity in circular motion is the instantaneous linear velocity of an object moving in a circular path. It is tangent to the circle at the object's position and perpendicular to the radius. The magnitude of tangential velocity is equal to the angular velocity multiplied by the radius of the circle. Understanding tangential velocity is crucial in many applications, from planetary motion to the design of rotating machinery.

38. How does velocity factor into the concept of work in physics?

Velocity plays a crucial role in the concept of work. Work is defined as the dot product of force and displacement (W = F · d). However, when considering work done over time, velocity becomes important. The power (rate of doing work) is given by P = F · v, where v is the velocity. This relationship shows how velocity directly affects the rate at which work is done or energy is transferred.

39. How does velocity affect fluid dynamics?

In fluid dynamics, velocity plays a crucial role:

40. What is the concept of phase velocity in waves?

Phase velocity is the rate at which the phase of a wave propagates in space. It's given by v_p = ω/k, where ω is the angular frequency and k is the wave number. In some cases, like in dispersive media, the phase velocity can differ from the group velocity (the velocity at which the overall shape of the wave's amplitudes propagates). Understanding phase velocity is crucial in optics, telecommunications, and quantum mechanics.

41. How does velocity relate to the concept of impulse in physics?

Impulse is defined as the change in momentum, and since momentum is the product of mass and velocity (p = mv), velocity is directly related to impulse. The impulse-momentum theorem states that the impulse applied to an object equals its change in momentum: F∆t = m∆v. This relationship is crucial in understanding collisions, explosions, and the design of safety features like airbags.

42. How does velocity factor into the concept of centripetal force?

Centripetal force is the force required to keep an object moving in a circular path. It's directly related to velocity through the equation F_c = mv²/r, where m is mass, v is velocity, and r is the radius of the circular path. This shows that the centripetal force increases with the square of the velocity. Understanding this relationship is crucial in analyzing circular motion, from planets orbiting the sun to electrons in an atom.

43. How does velocity affect the concept of work-energy theorem?

The work-energy theorem states that the net work done on an object equals its change in kinetic energy. Since kinetic energy is given by KE = (1/2)mv², velocity plays a crucial role. The theorem can be expressed as W_net = ∆KE = (1/2)m(v_f² - v_i²), where v_f and v_i are the final and initial velocities. This relationship is fundamental in understanding energy transfer and conservation in mechanical systems.

44. What is the significance of drift velocity in electric current?

Drift velocity is the average velocity of charge carriers (usually electrons) in a conductor when an electric field is applied. It's typically much slower than one might expect, often on the order of millimeters per second. The drift velocity is related to the current (I) through the equation I = nAvq, where n is the number density of charge carriers, A is the cross-sectional area of the conductor, v is the drift velocity, and q is the charge of each carrier. Understanding drift velocity is crucial in analyzing electric circuits and semiconductor physics.