## Friday, October 16, 2009

### Acceleration and deceleration

Dear our form 4 students answer following question before star your lesson.

Pretest:

1. state the meaning and unit for the symbol
i) a
ii) v
iii) u
iv) t

2. Define acceleration and deceleration

Lesson start:

Acceleration is the rate of change of velocity. ( 1 mark in paper 2 )

An object accelerates when it moves with increasing velocity.

An object decelerates when is getting slower, its velocity is decreasing.

Equation :

where

a: acceleration ( ms-2)

u: initial velocity ( ms-1)

v: final velocity ( ms-1)

t: time ( second)

Calculation:

Tips:

First of all find out all useful info as highlighted. Then substitute the info given in equation, while you are calculating check whether need to change units or not. (e.g., from km to m, hour to seconds)

1.An athletic runs from the starting line and achieves a velocity of 12 m/s in 3 s. Find his/her acceleration. (ans: 4 ms-2)

2. A track travels along a road with a velocity of 20 m/s the driver slam on the brake when a chicken suddenly runs across the road. Assuming the truck take 8s to complete stop from the time the driver hit the brake, determine the acceleration of the lorry. (ans: -2.5 ms-2)

3. A motorcycle move with a velocity of 25 m/s. the motorcyclist brakes and its velocity decreases uniformly to 10 m/s in 3s. The motorcycle continues to slow down with the same acceleration until it comes to a stop. Find the time taken for the motorcycle to stop. (ans: 5s)

* Step 1 : Determine its acceleration in the first 3s,

Step 2 : Determine the time taken .

For more detail study the graph below.

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