What is motion?

    •  An object is called to be in motion when its position changes with time and with a frame of reference.
    • The location of any object by specifying a reference point. 
    • All motions are considered relative to some frame of reference.
    • For example, a body on the surface of the Earth may appear to be at rest, only because the observer (frame of reference) is also on the surface of the Earth. The Earth in motion itself, together with both the body and the observer, is moving in its orbit around the Sun.
    • Rest is opposite of motion means there is no change of position of the object.

     

    Distance 



    • The total path length covered by an object is said to be the distance traveled by it.

     

    Displacement



    •  The shortest path length measured from the initial to the final position of an object is known as displacement.
    • Displacement is simply the distance between where the object started and where it ended up measured with a straight line.

     

    Distance and displacement are two quantities that may seem to mean the same thing but they have different meanings in physics.


    • Distance is a scalar physical quantity that indicates "how much ground an object has covered" during its motion.

    • Displacement is a vector physical quantity that indicates "how far out of place an object is"; it is the object's overall change in position.

     

    Consider the motion shown in the diagram below. A boy walks 4 meters East, 2 meters south, 4 meters west, and finally 2 meters north

     


     

    During his motion, He has "covered 12 meters of the ground" (distance = 12 m).

     

    Displacement is a vector quantity, must give attention to direction. The 4 meters east cancel out the 4 meters west, and the 2 meters south cancel out the 2 meters north. Vector physical quantities such as displacement are direction aware.

     


    Uniform motion: 

    • If any object covers equal distances in equal intervals of time, it is said to be in uniform motion.
    • Acceleration of objects in uniform motion is zero.
    • Distance time graph for uniform motion in a straight line.
    • Slope of the distance-time graph represents the speed of the object.
    • Speed of any object under uniform motion object is constant.
    • Distance traveled by an object is determined by multiplying speed and time.
    • The area under the speed-time graph gives the distance traveled.

     

    Non-uniform motion: 

    • Motions in which objects cover unequal distances in equal intervals of time.
    • Acceleration of objects in non-uniform motion is non-zero.
    • Distance time graph for any object under non-uniform motion is the curve line.
    • For non-uniform motion, the average speed of an object is obtained by dividing the total distance traveled by an object by the total time taken.

     

    Speed: 

    • The distance traveled by an object in unit time is referred to as speed. Its unit is m/s.
    • Speed is a scalar physical quantity that can be expressed by magnitude only.

    Speed is taken as directly proportional to distance when time is constant: v ∝ s (t constant)

    Speed is taken as inversely proportional to time when distance is constant: v ∝ 1/t (s constant).

    Adding these two rules together gives the definition of speed in symbolic form.

    v = s/t

    Thus speed = distance/ time

     

    Instantaneous speed:

    The speed is determined over a very small interval of time as an instant. Ideally, the interval should be as close to zero as possible.

    A car's speedometer shows its instantaneous speed.

     

    Average speed:

    Dividing the total distance traveled by the total time spent traveling. It will give you your average speed. 

    So if A car traveled 150 km in 3 hours, 120 km in 2 hours, and 90 km in 1 hour, his average speed was about 60 kmph.


    Velocity: 

    • Speed of an object moving in a definite direction is called the velocity of the object. S.I. unit is m/s.    
    • Velocity is a vector physical quantity. 

     

    Acceleration:

    • Simple meaning of acceleration refers to speeding up.
    • Acceleration is taken positively when the object is speeding up and negative when speeding down.
    • Change in the velocity of an object per unit time is called acceleration.
    • According to Newton’s Second Law, the net unbalanced force which acts on the object causes acceleration in the object.
    • Acceleration is a vector quantity. 

     

    Velocity-time graph moving with uniform velocity

    The slope of a Velocity–time graph of any object moving in rectilinear motion with uniform velocity in a straight line and parallel to x-axis when velocity is taken along y-axis and time is taken along the x-axis.


     

    Velocity – Time Graph of any object moving with uniform acceleration

    When the velocity-time graph is plotted for any object moving with uniform acceleration, the slope of the graph is a straight line.

     

    Velocity time graph of any object moving with uniform decreasing velocity:


    Velocity time graph of an object moving with non-uniform velocity:

     


    • The slope of a velocity graph expresses the acceleration of the object.
    • The area under the line on a velocity-time graph is the same as the displacement of the object.

     

    Equations of motion 

    The three equations are,

    1. v = u + at
    2. v² = u² + 2as
    3. s = ut + ½at²

     

    Where, s = displacement

                 u = initial velocity

                 v = final velocity

                  a = acceleration

                  t = time of motion

     

    Uniform circular motion

    • Motion of a particle moving at a constant speed on a circle.

    Example - if the athlete moves with a velocity of constant magnitude along the circular path the only change in his velocity is due to the change in the direction of the motion.

     

    Chapter Test