Home » Ask & Discuss » Physics. » Mechanics « Back to Discussion

Mechanics

Cool goIITian

Joined:	7 Jun 2007
Post:	77

18 Sep 2007 22:00:34 IST

0 People liked this

988

Conceptual Ques

None

A heavy stone is thrown from a cliff of height h with a speed v. The stone will hit the ground with max speed if it it thrown

a) vertically upward

b) vertically downward

c) horizontally

d) the speed does not depend on the initial direction

plz explai

ans is (d)

Share this article on:

Comments (13)

anish phophaliya

New kid on the Block

Joined: 10 Aug 2007

Posts: 28

18 Sep 2007 22:19:26 IST

Like 0 people liked this

pls recheck the answer coz i think its (b)+(a)
since when thrown horizontally its vertical initial velocity is 0
but in the other cases it is v downwards.

pls check again

Gaurav |spideyunlimited| Ragtah

Moderator

Joined: 16 Dec 2006

Posts: 3373

18 Sep 2007 22:40:21 IST

Like 0 people liked this

both (a) and (b) are correct.
as velocity in vertical plane will be v. but if it's thrown horizontally , then velocity in vertical plane will be zero and only provided by g.

it cant be d. *ever*

it would be (d) if there was no initial velocity v.

*PLZ RATE*

jayesh burnwal

New kid on the Block

Joined: 8 Sep 2007

Posts: 11

18 Sep 2007 23:07:54 IST

Like 0 people liked this

if v consider this ques by work-energy principle rather than kinematics:
change in k.e is made up by dec in pot energy(which depends only on vertical height)
so there is always the same final velocity (irrespective) of manner it is thrown.

ramyani chakrabarty

Blazing goIITian

Joined: 22 Apr 2007

Posts: 2537

18 Sep 2007 23:14:50 IST

Like 0 people liked this

Yes, the speed does not depend on the initial direction.

paris mulye

Hot goIITian

Joined: 19 Jun 2007

Posts: 143

19 Sep 2007 09:56:17 IST

Like 0 people liked this

d is not the correct option
by simple calculations we can derive that the object thrown vertically upwards and the other vertically downwards hit the ground at the same speed provided their initial velocity is the same
they have their y component ut-1/2gt^2 but projectile has its x component also

aNdRoMeDa

Blazing goIITian

Joined: 10 Sep 2007

Posts: 1319

19 Sep 2007 10:19:40 IST

Like 1 people liked this

ya, i 2 feel dat speed, vel. do not depend on d initial direction as

v=u+at

thus, v depends on u ,a& t

v^2=u^2+ 2as

here v depds on u, a,& s(displacement)

displacement is same 4 any case

thus, ans is (d)

sankydreams

Blazing goIITian

Joined: 14 May 2007

Posts: 822

19 Sep 2007 15:23:39 IST

Like 0 people liked this

The answer cant be 'D' . It wud have been d had there been no initial velo.
Let me prove it
For the case when its thrown horizontallly
Considering vertical motion
u = 0
a = g
s = h
final velo = (0 + 2ag) ^ 1/2

For the case when its thrown vertically downwards
u = v
a = g
s = h
final velo = (v^2 + 2ag) ^ 1/2
which is ofcourse grtr than the final velo in previous case
Answer wud be A and B

Krishna Gopal Singh

Forum Expert

Joined: 29 Dec 2006

Posts: 5153

19 Sep 2007 22:24:33 IST

Like 0 people liked this

Answer is (d) only and deedee has given a perfect answer for it. sanky in horizontal throw you are missing the initial horizontal speed of v which will remain till end. When the body reaches the ground its final velocity will be sqrt(2gh) vertically and v horizontaly so its speed still is sqrt(v^2+2hg)

Gaurav |spideyunlimited| Ragtah

Moderator

Joined: 16 Dec 2006

Posts: 3373

19 Sep 2007 22:33:22 IST

Like 0 people liked this

oh yes we forgot that the speed will be the horizontal plus the vertical component.. gud gud
tht was the only confusion. allright den :)

thevyzz

Blazing goIITian

Joined: 5 May 2007

Posts: 686

19 Sep 2007 22:44:39 IST

Like 0 people liked this

it can be simply put on conservation of energy principle

Initial energy = mv^2/2 + mgh

final energy = mV^2/2 +0

so all the potential energy will be converted into kinetic energy

so the velocity will be the same

siddharth sarangdhar

New kid on the Block

Joined: 13 Sep 2007

Posts: 9

20 Sep 2007 10:31:36 IST

Like 0 people liked this

in case of vertically upward n downward throws, vel. when it touches ground will be same...........max vel will be in the case when it is thrownhorizontally as it has a component velocity in hori direction also....
option D is completely invalid...............velocity does depend upon direction..............consider a case of oblique downward projection where velocity will be max

Prantik Chatterjee

Cool goIITian

Joined: 9 Sep 2007

Posts: 36

20 Sep 2007 11:44:49 IST

Like 0 people liked this

If the body is projected horizontally then obviously the vertical component is 0 and so it will behave as free fall.So the answer will be D when the initial vertical component is 0

yash shah

Cool goIITian

Joined: 10 Apr 2007

Posts: 59

20 Sep 2007 12:06:38 IST

Like 0 people liked this

ans d as it acts under a constant gravitation field

Quick Reply

Reply

	Some HTML allowed.
	Keep your comments above the belt or risk having them deleted.
	Signup for a avatar to have your pictures show up by your comment
	If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team