TrueTears question thread

[TrueTears]

From a balloon ascending with a velocity of 10 m/s. A stone was dropped and reached the ground in 12 seconds. Given the gravitational acceleration is 9.8m/s. Find the height of the balloon when the stone was dropped.

[Mao]

btw, acceleration is in ms^-2

[TrueTears]

thanks mao

[TrueTears]

A particle travels in a straight line(going right) with constant velocity of 4 m/s for 12 seconds. It is then subjected to a constant acceleration in the opposite direction for 20 seconds which returns the particle to its original position. Find the acceleration of the particle.

This is my working:

Left - negative  and Right - positive

so the distance travelled right is just (4)(12) since it has constant velocity, a = 0, so it travelled 48 m right.

so next we have the going back journey which is going left back to original position.

a = -a, x = -48 and t = 20 u = 0, since the particle is instantaneously at rest when its turning around

using the formula



However the answer is -0.64 m/s^2. Can anyone see what i did wrong?

[/0]

You still have to take into account the particle is moving 4m/s to the right when it receives the negative acceleration, so u = 4.

[TrueTears]

o i see. thanks again ST

[TrueTears]

1. The maximum rate at which a bus can accelerate or decelerate is 2 m/s^2. It has a max speed of 60 km/h. Find the shortest time the bus can take to travel between two bus stops 1km apart on a straight stretch of road.

2. Two cars A and B, each moving with constant acceleration, are travelling in the same direction along the parallel lanes of a divided road. When A passes B, the speeds are 64km/h and 48km/h respectively. Three minutes later, B passes A, travelling at 96km/h. Find the distance travelled by A and B this instant(since they first passed) and the speed of A.

[Mao]

1,
it takes to accelerate from 0 to 60 (and from 60 to 0)

this amounts to ~138.9 m in total, and takes seconds

The rest of the way, m, takes seconds

In total, shortest time would be seconds.

[Mao]

2.

acceleration of B: m/s²

distance travelled by B:

acceleration of A: m/s²

velocity of A:

[TrueTears]

thanks again mao XD

[TrueTears]

just another Q,

The acceleration of an object whose displacement is x metres from the origin is given by . If the object starts from the origin with a velocity of m/s, then its velocity when displacement is metres from the origin is given by:

This is a multiple choice question, and my working is:
a = -x and the given information is when






subbing in the formation gets

so



But when x = 0

so the negative answer is discarded and the answer should be

But my books answer says its . why is that?

[Flaming_Arrow]

velocity can be negative

[TrueTears]

velocity can be negative

yeah i know but for that question if u sub in x = 0 v should = (positive) since its given in the question. But if u have + or - out on the front, u cant have as an answer since when x= 0 v doesnt equal . So i dono why the answer has + or - as the answer

[/0]

It only says the object 'starts' from the origin at . It could return to the origin later with a speed of

In response to your PM, when a rock is thrown down from a cliff, it will begin with a positive velocity vector downwards. Since gravity is also a positive vector of magnitude 10 downwards, it will speed up the rock.
If a rock is thrown up from a cliff, it will have a positive velocity upwards, and gravity vector can be turned so it has a negative vector upwards, so it will slow the rock down until it has a positive velocity vector downwards.

[TrueTears]

thanks ST