How to calcuate the change in velocity

SgStudentStressed SG

we all know that change in velocity= final velocity - intial velocity. ( taking a specific direction) so like throw a ball at 3m/s2 > then bounce off <1m/s2. then we take > as +ve so change in velocity= -1-3=-4m/s2

however, if the question is like this: tom throws a ball at 8m/s2 towards the right horizontally. the ball then deflects downwards directly at 5m/s2. what is the change in velocity. we cant find it by minusing it since there r 4 different directions so we cant use a specific direction as a reference point. so how do we solve the question? i heard of using a vector diagram but my teacher said something i didnt really understand. if possible can u explain it to me. n if there r other methods pls use use secondary school methods cos im in sec 3 this year. thanks pls help me

wee_ws SG

Hi,

In the first scenario, we are simply looking at the horizontal directions of left and right. Simple addition and subtraction are needed only, to obtain the change of velocity.

In the example of the ball, we have to look at the vertical direction, in addition to the horizontal one. A vector diagram will be required in this case:

A                        O
     --------<---------
    |
    v
    |
B

1. The vector OA represents -V_i.

2. The vector AB represents V_f.

3. Now V_f - V_i refers to the vector OB, whose value can be found by Pythagoras's Theorem.

4. The direction can be found by trigonometry. Let alpha be the angle that OB makes with respect to the horizontal. Then

tangent alpha = opposite side / adjacent side

= AB / OA

= 5/8.

Thanks.

Cheers,
Wen Shih

eagle SG

Hi, simple way of looking at this....

The change in velocity on the horizontal direction is 8m/s2 towards the left.
The change in velocity on the vertical direction is 5m/s2 down.

Hence, we have a net change in velocity of magnitude sqrt(8^2 + 5^2) = 9.43 m/s2
--->---- (8m/s2)
|\
| \
v   \
|     \
|       \
(5m/s2)

The direction is tan inverse (5/8) degrees below the horizontal.

SgStudentStressed SG

oh ok thanks u guy :)

my teacher said that since we have to use a vector digram we have to use vector addition, so we have to change iv to +(-iv)

so eventually the formula would be

change in velocity= fv+(-iv)

so when i draw the vector diagram, the iv direction would be change its direction in 180 degrees but the final would remain the same. Connect the lines from the final to initial to get the change in velocity and itll be the change in volcity.

Thanks for the help Wen Shih and Eagle