# Uniformly accelerated linear motion downwards

Homework Statement:
A small body is freely released from a platform located 605 meters above the ground. If g=10m/s2
and
we ignore air resistance find:
a) the time required to reach the ground;
b) the velocity at the ground;
c) the
velocity at 425 m from the ground;
d) the distance traveled in the first 4 seconds of flight;
e) the distance
traveled in the last 4 seconds of flight;
f) the distance traveled in the third second of flight and in the
seventh.
Relevant Equations:
motion law, velocity law
Could you check if what I did is right and help me at e) and f)? Thank you!

## Answers and Replies

Homework Statement:: A small body is freely released from a platform located 605 meters above the ground. If g=10m/s2
and
we ignore air resistance find:
a) the time required to reach the ground;
b) the velocity at the ground;
c) the
velocity at 425 m from the ground;
d) the distance traveled in the first 4 seconds of flight;
e) the distance
traveled in the last 4 seconds of flight;
f) the distance traveled in the third second of flight and in the
seventh.
Relevant Equations:: motion law, velocity law

Could you check if what I did is right and help me at e) and f)? Thank you!
EDIT: I tried e) again and got this:
tAD = tAO - 4
tAD = 7s
yDO = yA - 1/2 * g * (tAD)^2
yDO = 605 -245
yDO = 360m
is this right?

•It seems to me that d. part is not correct.

•distance travelled in nth second means distance travelled between time interval of ##n-1## to ##n## seconds.

•e part—>
If total time interval is t seconds and you want to find the distance traveled in last n seconds you can calculate the the distance traveled in time interval of ##t-n## seconds and then subtract it from distance traveled in total time interval.

•It seems to me that d. part is not correct.

•distance travelled in nth second means distance travelled between time interval of ##n-1## to ##n## seconds.

•e part—>
If total time interval is t seconds and you want to find the distance traveled in last n seconds you can calculate the the distance traveled in time interval of ##t-n## seconds and then subtract it from distance traveled in total time interval.
Thank you!

• Hemant
Lnewqban
Gold Member
EDIT: I tried e) again and got this:
tAD = tAO - 4
tAD = 7s
yDO = yA - 1/2 * g * (tAD)^2
yDO = 605 -245
yDO = 360m
is this right?
That result seems to be correct.

After following your work for a), b), c) and d), I believe that you take many steps that are not necessary and could introduce errors.

For resolving f), you could calculate the velocities at times 2 seconds and 7 seconds first.
Then, include each of those values into the equation to calculate the distance traveled during the following second.

That result seems to be correct.

After following your work for a), b), c) and d), I believe that you take many steps that are not necessary and could introduce errors.

For resolving f), you could calculate the velocities at times 2 seconds and 7 seconds first.
Then, include each of those values into the equation to calculate the distance traveled during the following second.
Thank you, I'll try to be more careful