# Homework Help: Explain Displacement problem

1. Mar 8, 2008

### mirza21

Hi friends am new to Physics. Am facing a problem while caculating displacment.

Acceleration of free falling object is 9.8m/s/s. Am using formula 1/2*b*h to get the displacement.According to me if object acceleration is 9.8 m/s/s then after 1 second it's displacment should be 9.8 meters and after 2 second it should be positioned at 29.4 m.
If i use the formula to calculate displacment then it is positioned at 4.9 m at end of 1st second and a the end of 2nd second it 19.6 m. Please explain why object is positon at 4.9 m by using this formula.

At t = 1 s
d = (0.5) * (9.8 m/s2) * (1 s)2 = 4.9 m

At t = 2 s

d = (0.5) * (9.8 m/s2) * (2 s)2 = 19.6 m

At t = 5 s

d = (0.5) * (10 m/s2) * (3 s)2 = 44.1 m

For refrence see http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L5d.html [Broken]

Last edited by a moderator: May 3, 2017
2. Mar 9, 2008

### physixguru

The statement in red is totally wrong.It also depends on the initial velocity of the object from which it was thrown downwards.
NOW for the hint part:

It seems you are confused with the formula part.
The formula s=1/2 at^2 [u=0] does not give the displacement in that particular time interval,in fact It gives ya the total displacement from the initial time interval.
To get the displacement in a particular time interval you must calculate the displacements using the formula and substituting the correct values and then calculate the difference.

Last edited by a moderator: May 3, 2017