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Revision Notes on Motion
A Reference Point is used to describe the location of an object. An object can be referred through many reference points.
Origin – The reference point that is used to describe the location of an object is called Origin.
For Example, a new restaurant is opening shortly at a distance of 5 km north from my house. Here, the house is the reference point that is used for describing where the restaurant is located.
If the location of an object changes with time the object is said to be in motion.
Distance – The distance covered by an object is described as the total path length covered by an object between two endpoints.
Distance is a numerical quantity. We do not mention the direction in which an object is travelling while mentioning about the distance covered by that object.
Figure 1 – Distance and Displacement
According to the figure 1 given above, if an object moves from point O to point A then total distance travelled by the object is given as 60 km.
Displacement – The shortest possible distance between the initial and final position of an object is called Displacement.
Consider the figure 1 given above, here the shortest distance between O and A is 60 km only. Hence, displacement is 60 km.
Displacement depends upon the direction in which the object is travelling.
Displacement is denoted by Δx.
Δx = x_{f }− x_{0}
Where,
x_{f} = Final position on the object
x_{0} = Initial position of the object
Zero Displacement – When the first and last positions of an object are same, the displacement is zero.
For Example, consider the diagrams given below.
Figure 2 – Example for zero displacement
Displacement at point A = 0 because the shortest distance from A to A is zero.
Figure 3 – Example for negative and positive displacement
Here, displacement of object B is negative
ΔB = B_{f }− B_{0} = 7–12 = – 5
A negative sign indicates opposite direction here.
Also, displacement of object A is positive
ΔA = A_{f }− A_{0} = 7– 0 = 7
A scalar quantity describes a magnitude or a numerical value.
A vector quantity describes the magnitude as well as the direction.
Hence, distance is a scalar quantity while displacement is a vector quantity.
Distance provides the complete details of the path taken by the object
Displacement does not provide the complete details of the path taken by the object
Distance is always positive
Displacement can be positive, negative or zero
It is a scalar quantity
It is a vector quantity
The distance between two points may not be unique
Displacement between two points is always unique
When an object travels equal distances in equal intervals of time the object is said to have a uniform motion.
When an object travels unequal distances in equal intervals of time the object is said to have a non-uniform motion.
Speed of an object is defined as the distance traveled by the object per unit time.
SI Unit: Meter (m)
Symbol of Representation: m/s or ms^{-1}
Speed = Distance/Time
Average Speed – If the motion of the object is non-uniform then we calculate the average speed to signify the rate of motion of that object.
For Example, If an object travels 10m in 3 seconds and 12m in 7 seconds. Then its average speed would be:
Total distance travelled = 10 m + 12 m = 22m
Total Time taken = 3s + 7s = 10s
Average speed = 22/10 = 2.2 m/s
To describe the rate of motion in a direction the term velocity is used. It is defined as the speed of an object in a particular direction.
Velocity = Displacement/Time
Symbol of Representation: M/s or ms^{-1}
Average Velocity (in case of uniform motion)-
Average Velocity = (Initial Velocity + Final Velocity)/2
Average Velocity (in case of non-uniform motion)-
Average Velocity = Total Displacement / Total Time taken
The magnitude of speed or velocity at a particular instance of time is called Instantaneous Speed or Velocity.
Figure 4 - Instantaneous Speed / Velocity
Uniform Motion – In case of uniform motion the velocity of an object remains constant with change in time. Hence, the rate of change of velocity is said to be zero.
Non-uniform Motion – In case of non-uniform motion the velocity of an object changes with time. This rate of change of velocity per unit time is called Acceleration.
Acceleration = Change in velocity/ Time taken
SI Unit: m/s^{2}
Uniform Acceleration – An object is said to have a uniform acceleration if:
It travels along a straight path
Its velocity changes (increases or decreases) by equal amounts in equal time intervals
Non - Uniform Acceleration – An object is said to have a non-uniform acceleration if:
Acceleration is also a vector quantity. The direction of acceleration is the same if the velocity is increasing in the same direction. Such acceleration is called Positive Acceleration.
The direction of acceleration becomes opposite as that of velocity if velocity is decreasing in a direction. Such acceleration is called Negative Acceleration.
De-acceleration or Retardation – Negative acceleration is also called De-acceleration or Retardation
1. Distance – Time Graph
It represents a change in position of the object with respect to time.
The graph in case the object is stationary (means the distance is constant at all time intervals) – Straight line graph parallel to x = axis
Figure 5 - Distance-time Graph in case of Stationary object
The graph in case of uniform motion – Straight line graph
Figure 6 - Distance-time Graph in Uniform Motion
The graph in case of non-uniform motion – Graph has different shapes
Figure 7- Distance-time Graph in Non-Uniform Motion
Constant velocity – Straight line graph, velocity is always parallel to the x-axis
Uniform Velocity / Uniform Acceleration – Straight line graph
Non-Uniform Velocity / Non-Uniform Acceleration – Graph can have different shapes
Consider the graph given below. The area under the graph gives the distance traveled between a certain interval of time. Hence, if we want to find out the distance traveled between time interval t_{1} and t_{2}, we need to calculate the area enclosed by the rectangle ABCD where area (ABCD) = AB * AC.
Similarly, to calculate distance traveled in a time interval in case of uniform acceleration, we need to find out the area under the graph, as shown in the figure below.
To calculate the distance between time intervals t_{1} and t_{2} we need to find out area represented by ABED.
Area of ABED = Area of the rectangle ABCD + Area of the triangle ADE = AB × BC + 1/ 2 * (AD × DE)
The equations of motion represent the relationship between an object's acceleration, velocity and distance covered if and only if,
The object is moving on a straight path
The object has a uniform acceleration
1. The Equation for Velocity – Time Relation
v = u + at
2. The Equation for Position – Time Relation
s = ut + 1/2 at^{2}
3. The Equation for the Position – Velocity Relation
2a s = v^{2 }– u^{2}
u: initial velocity
a: uniform acceleration
t: time
v: final velocity
s: distance traveled in time t
Figure 12
Study the graph above. The line segment PN shows the relation between velocity and time.
Initial velocity, u can be derived from velocity at point P or by the line segment OP
Final velocity, v can be derived from velocity at point N or by the line segment NR
Also, NQ = NR – PO = v – u
Time interval, t is represented by OR, where OR = PQ = MN
Acceleration = Change in velocity / time taken
Acceleration = (final velocity – initial velocity) / time
a = (v – u)/t
so, at = v – u
We know that, distance travelled by an object = Area under the graph
So, Distance travelled = Area of OPNR = Area of rectangle OPQR + Area of triangle PQN
s = (OP * OR) + (PQ * QN) / 2
s = (u * t) + (t * (v – u) / 2)
s = ut + 1/2 at^{2 }[because at = v – u]
We know that, distance travelled by an object = area under the graph
So, s = Area of OPNR = (Sum of parallel sides * height) / 2
s = ((PO + NR)* PQ)/ 2 = ( (v+u) * t)/ 2
2s / (v+u) = t [equation 1]
Also, we know that, (v – u)/ a = t [equation 2]
On equating equations 1 and 2, we get,
2s / (v + u) = (v – u)/ a
2as = (v + u) (v – u)
2 a s = v^{2 }– u^{2}
If an object moves in a constant velocity along a circular path, the change in velocity occurs due to the change in direction. Therefore, this is an accelerated motion. Consider the figure given below and observe how directions of an object vary at different locations on a circular path.
Uniform Circular Motion – When an object travels in a circular path at a uniform speed the object is said to have a uniform circular motion.
Non-Uniform Circular Motion – When an object travels in a circular path at a non-uniform speed the object is said to have a non-uniform circular motion
Examples of uniform circular motion:
The motion of a satellite in its orbit
The motion of planets around the sun
Velocity = Distance/ Time = Circumference of circle / Time
v = 2πr/ t
where,
v: velocity of the object
r: radius of the circular path
t: time taken by the object
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