Measuring the Rate of Change of Motion

Average Speed

The magnitude of displacement may be different from the actual path length. To describe the rate of motion over the actual path, we introduce another quantity called average speed. Average speed is defined as the total path length travelled divided by the total time interval during which the motion has taken place :

• Average speed = Total path length/Total time interval
• SI unit = m s^–1
• It does not tell us in what direction an object is moving.
• It is always positive (in contrast to the average velocity which can be positive or negative).
• If the motion of an object is along a straight line and in the same direction, the magnitude of displacement is equal to the total path length. In that case, the magnitude of average velocity is equal to the average speed.
• If the motion of an object is not along a straight line and it’s motion may be same or different, the magnitude of displacement is not equal to the total path length. In that case, the magnitude of average velocity is not equal to the average speed.

Velocity (Rate of change of position)

The average velocity tells us how fast an object has been moving over a given time interval but does not tell us how fast it moves at different instants of time during that interval. For this, we define instantaneous velocity or simply velocity “v” at an instant “t”. The velocity at an instant is defined as the limit of the average velocity as the time interval ∆t becomes infinitesimally small. In other words,

• v_inst = dx/dt
• Where (dx/dt) is the rate of change of position with respect to time, at that instant.
• The SI unit for velocity is m/s or m s^–1.

Instantaneous Speed or Speed

• Instantaneous speed or simply speed is the magnitude of velocity.
• Example: A velocity of + 24.0 m s^–1 and a velocity of – 24.0 m s^–1; Both have an associated speed of 24.0 m s^–1.
• It should be noted that though average speed over a finite interval of time is greater or equal to the magnitude of the average velocity.
• Instantaneous speed at an instant is equal to the magnitude of the instantaneous velocity at that instant.
• SI unit = m/sec
• It does not tell us in what direction an object is moving.

Watch the video and learn about the measurement of motion

Frequently Asked Questions – FAQs

Q1

What is speed?

Speed can be considered as the rate at which a body covers distance.

Q2

What is the most common formula for calculating the speed of a moving body?

Speed is normally calculated by dividing the distance travelled by the time spent travelling the particular distance.

\(\begin{array}{l}Speed = \frac{Distance}{Time}\end{array} \)

Q3

What is velocity?

Velocity can be defined as the directional speed of a body in movement as an implication if the body’s rate of change in position is perceived from a particular point of reference and as calculated by a specific standard of time. Example: 125km/h to the south.

Q4

What is average velocity?

The average speed of a body is calculated by dividing the total displacement by the total time taken.

Average speed = Total distance travelled/Total time taken

Q5

What is acceleration?

Acceleration is defined as the rate of change of motion of a body. In other words, the measure of the rate of change in its speed along with direction with respect to time is called acceleration.

Q6

What is momentum?

In Newtonian mechanics, momentum is the product of the mass of the moving body and its velocity. It is a vector quantity, having a direction and a magnitude.

The below video provides the Top 10 NTSE Important Questions on Motion Class 9

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