Chapter No. 3 Motion and force Physics Notes QUE.1: Define rest and motion ? Ans: (i) Rest: A body is said to be at rest if it do not change its position with respect to the surrounding. (ii) Motion : A body is said to be in motion if it changes its position with respect to the surrounding. QUE.2: Define distance and displacement ? Ans: (1) Distance: The length of actual path traversed by a body in motion is known as distance. The actual path may be straight or curved. The distance is a scalar quantity. Its S.I unit is meter. (ii) Displacement : The shortest distance b/w two points is known as displacement. In given figure, a body reaches from point ‘O’ to ‘C’ through ‘A’ and ‘B’. Thus the path ‘OABC’ represents the actual path traversed by the body during its motion. So it is called distance. While ‘OC’ represents the shortest distance b/w the initial point ‘O’ and final point ‘C’. Thus ‘OC’ is known as displacement. It is a vector quantity. Its S.I unit is meter. QUE.3: Define the following terms ? Position, speed, uniform speed, variable speed, average speed, Instantaneous speed. Ans: (1) Position: The location of an object relative to same reference point [origin] is known as position of that object. In the given figure, an object is placed at point ‘p’. If we join the point with origin ‘o’, then op=r represents the distance of object from the origin ‘o’ from this line, we can also find the angle made by point ‘p’ with x-axis or y-axis. The position of point ‘P’ is represented by p(x,y), where ‘x’ represents the perpendicular distance of point ‘P’ from y-axis and ‘y’ represents the perpendicular distance of point ‘P’ from x-axis respectively. In vector terminology, op= r is known as position vector. (2) Speed : The distance covered by a moving body in one second is known as speed. i,e Speed = Distance covered / time è V= S/t The speed is a scalar quantity. Its S.I unit is “m/sec”. (3) Uniform speed [Constant speed]: A body is said to be moving with uniform speed, if it covers equal distance in equal intervals of time. (4) Variable Speed: A body is said to be moving with variable speed if it covers unequal distance in equal intervals of time. (5) Average Speed: The average speed of a moving body can be obtained by dividing total distance covered by total time taken. i,e Average speed = Total Distance covered / Total time taken ---> < V > = s/t (6) Instantaneous Speed : The speed of a body at any particular instant of time is known as instantaneous speed. For such speed, we take time interval ‘∆t’ to be vary small such that ‘∆t’ approaches to zero. i,e limit ∆t--->0 Vins = Lt ∆t----> o ∆s/∆t QUE.4: Define the following terms ? velocity, uniform velocity, variable velocity, Average Velocity, Instantaneous velocity ? Ans: (1) Velocity: The distance covered by a body in one second along a particular direction is known as velocity. OR The rate of change of displacement is known as velocity. Mathematically, we have, Velocity = Displacement / time ----> V = S/t Velocity is a vector quantity. Its standard unit is “m/sec”. (2) Uniform velocity : A body is said to be moving with uniform velocity, if it covers equal displacement in equal intervals of time. (3) Variable Velocity: A body is said to be moving with variable velocity, if it covers unequal displacement in equal intervals of time. (4) Average velocity: The average velocity can be obtained by dividing total displacement by total time taken, i,e Average velocity = Total Displacement / Total time taken ----> < V > = S/t (5) Instantaneous velocity : The velocity of a body at any particular instant of time is known as instantaneous velocity. i,e Vins = Lt ∆t----> o ∆S/∆t QUE.5: Define the following terms ? Acceleration, Uniform acceleration, variable acceleration, radial acceleration, linear acceleration, positive acceleration, negative acceleration, Instantaneous acceleration ? Ans: (1) Acceleration : “The rate of change of velocity is known as acceleration.” Mathematically, we have Acceleration = velocity / time ----> A = ∆v / ∆t It is a vector quantity. It SI unit is m/sec² Dimensions of Velocity : We known that , V = Displacement / time = m/sec = length / Time ---> V= [LT-²] (2) Uniform acceleration: A body is said to be moving with uniform acceleration if equal change occurs in velocity in equal intervals of time. (3) Variable Acceleration: A body is said to be moving with variable acceleration if unequal change occurs in velocity in equal intervals of time. (4) Radial Acceleration: The acceleration which is produced due to change in direction of velocity is known as radial acceleration. (5) Linear Acceleration: The acceleration which is produced due to change in magnitude of velocity is known as linear acceleration. (6) Positive acceleration : The acceleration is said to be positive, if the magnitude of velocity increases with respect to time. (7) Negative acceleration : The acceleration will be negative, if the magnitude of velocity decreases with respect to time. (8) Instantaneous Acceleration: The acceleration of a body at any particular instant of time is known as instantaneous acceleration. I,e ains = lt ∆t----> o ∆v/∆t QUE 6 : Discuss the displacement – Time Graph ? Ans: The displacement- time graph gives us a certain slope or gradient from which we can calculate the velocity of a body. For example, fig (a) shows the displacement – time graph of a moving car. From the slope of the graph, the velocity of the car can be calculated in the following manner. i,e Velocity = slope of the graph ----> Velocity = ∆y/∆x ----> Velocity = 100/10 m/sec ----> V = 10 m/sec (1) If the slope of the displacement – time graph increases, it means that the object is speed up or accelerating. as shown in fig (b) (2) If the slope of displacement – time graph decreases, if means that the object is slowing down (or) decelerating as shown in fig (c). (3) If the slope of the displacement – time graph remains constant, then it means that the body is moving with constant velocity as shown in fig (d). In fig (e) , (i) the line ab shows that the object is moving with constant velocity. (ii) The line be shows that there is no change in displacement of the body, so the body is at rest. (iii) The line cd shows that the body begins to reverse its journey with constant velocity.
Posted on: Fri, 03 Oct 2014 10:31:04 +0000
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