Equation of motion??

Question

i have been solving several questions on motion in physics. there are several questions in which the initial and final velocity along with acceleration and time is given and we have to find the distance. if i use 2as= v2-u2 my answers are right. but if i use s=ut 1/2 at2 i get wrong answers. is there a rule that for a certain question only a particular equation can be used?

Answer 1

You must be doing something wrong. If a question gives enough information for you to use either of those equations and get the result in one step, maybe the question contains inconsistent information.

Example:

u = 10 m/s.

a = 3 m/s^2

Find distance travelled in 30 sec.

Using s = ut + at^2 / 2:

s = 10 * 30 + 3 * 30^2 / 2

= 300 + 3 * 900 / 2

= 300 + 2700 / 2

= 300 + 1350

= 1650 m.

To use v^2 = u^2 + 2as, you first need to find v. If v were given, and was not 100 m/s, then something is wrong.

Using v = u + at to find what v should be:

v = 10 + 3 * 30

= 10 + 90

= 100 m/s.

Using v^2 = u^2 + 2as:

100^2 = 10^2 + 2 * 3s

10000 = 100 + 6s

6s = 9900

s = 1650 m.