Kinematics

(a) state what is meant by speed and velocity.

Definition is given in units' Table

(b) calculate average speed using distance travelled/time taken.

average speed =

Question:

Calculate the speed of an object which has covered a distance of 5 km in 2 minutes.

Important note: standard unit of distance is not kilometer but meter and standard unit of time is not minutes but seconds. In exams, we must keep our units in standard form unless told otherwise.

Solution:

Distance = 5 km = 5000 m and Time = 5 minutes = 5 x 60 = 120 seconds

Using above formula and pluging values

Avg. speed = 5000 / (2 x 60) = 41.67 m/s


(c) state what is meant by uniform acceleration and calculate the value of an acceleration using change in
velocity/time taken.


Definition of acceleration is given in units' section above.

Uniform Acceleration: Uniform acceleration means constant acceleration.

Since acceleration means change in speed, if the object is speeding up with the same value each second then we can say that object is moving with uniform acceleration. Suppose, if the speed is zero at t= 0 sec then after t=1 sec speed has increased to 10 m/s then at t=2 sec speed has increased to 20 m/s then at t= 3 sec speed has increased to 30 m/s and so on. So after each second speed is increasing by 10 m/s or at constant rate. Therefore, we can say that object is accelerating at 10 m/s2 (m/s2 is the unit of acceleration.)

acceleration =

a =

acceleration's definition is already given in table of section 1.1.

Acceleration happens when speed increases but what if speed decreases? This is called deceleration or retardation. It's the opposite of acceleration.

So if in calculation you are getting a negative acceleration it means the speed is decreasing, which means object is deceleration.

 

Caution:

Suppose you are solving a question to find acceleration and its value is coming as a negative number, which means the object is decelerating. Then how should you write your answer as


a)    Acceleration = - 5 m/s2        or         b) Deceleration = -5 m/s2


c)    Deceleration = 5 m/s2


The correct answer is either (a) or (c ) and if you are thinking for (b) then you will lose marks for your entire answer in exam paper.
This is because –ve acceleration automatically means deceleration. And if placing –ve sign after deceleration would change its meaning to acceleration as


-ve acceleration = deceleration   AND –ve deceleration = acceleration.

Note: Two types of questions will be asked
Type 1: problems in which initial speed, final speed and time will be given. You just have to use formula to find acceleration. These are dead easy, just apply formula and then plug and play.
Type 2: find acceleration by using graphs. Only speed-time or velocity-time graphs will be given. Because--- gradient is the most ________ thing in graphs.