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Simply put : Speed is the magnitude of motion. Velocity, on other hand, is magnitude plus direction of motion.

Motion is the change of position with respect to time. Speed quantifies this change in position, but notably without direction. It tells us exactly : how rapidly this change is taking place with respect to time.

Motion
Speed is the rate of change of distance with respect to time and is expressed as distance covered in unit time.

v = Δ s Δ t

Δs = v Δt

Evaluation of ratio of distance and time for finite time interval is called “average” speed, where as evaluation of the ratio for infinitesimally small time interval, when Δt-->0, is called instantaneous speed. In order to distinguish between average and instantaneous speed, we denote them with symbols v a and v respectively.

Determination of speed allows us to compare motions of different objects. An aircraft, for example, travels much faster than a motor car. This is an established fact. But, we simply do not know how fast the aircraft is in comparison to the motor car. We need to measure speeds of each of them to state the difference in quantitative terms.

Speed is defined in terms of distance and time, both of which are scalar quantities. It follows that speed is a scalar quantity, having only magnitude and no sense of direction. When we say that a person is pacing at a speed of 3 km/hr, then we simply mean that the person covers 3 km in 1 hour. It is not known, however, where the person is actually heading and in which direction.

Dimension of speed is L T - 1 and its SI unit is meter/second (m/s).

    Some values of speed

  • Light : 3 x 10 8 m/s
  • Sound : 330 m/s
  • Continental drift : 10 - 9 m/s

Distance .vs. time plots

Motion of an object over a period of time may vary. These variations are conveniently represented on a distance - time plot as shown in the figure.

Distance time plot

Distance is given by the vertical segment parallel to the axis representing distance.

The figure above displays distance covered in two equal time intervals. The vertical segment DB and FC parallel to the axis represents distances covered in the two equal time intervals. The distance covered in two equal time Δ t intervals may not be equal as average speeds of the object in the two equal time intervals may be different.

s 1 = v 1 Δ t = DB 

s 2 = v 2 Δ t = FC 

and DB FC

The distance - time plot characterizes the nature of distance. We see that the plot is always drawn in the first quadrant as distance can not be negative. Further, distance – time plot is ever increasing during the motion. It means that the plot can not decrease from any level at a given instant. When the object is at rest, the distance becomes constant and plot is a horizontal line parallel to time axis. Note that the portion of plot with constant speed does not add to the distance and the vertical segment representing distance remains constant during the motion.

Distance time plot

Static condition is represented by a horizontal section on distance - time plot.

Average speed

Average speed, as the name suggests, gives the overall view of the motion. It does not, however, give the details of motion. Let us take the example of the school bus. Ignoring the actual, let us consider that the average speed of the journey is 50 km/ hour. This piece of information about speed is very useful in planning the schedule, but the information is not complete as far as the motion is concerned. The school bus could have stopped at predetermined stoppages and crossings, besides traveling at different speeds for variety of reasons. Mathematically,

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
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John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
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emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
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Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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Source:  OpenStax, Physics for k-12. OpenStax CNX. Sep 07, 2009 Download for free at http://cnx.org/content/col10322/1.175
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