Submitted by Anonymous (not verified) on
Forums:
A pebble is throw with a horizontal velocity of 4m/s and has a mass of 3kg (assume not vertical velocity).
The cliff is 51m high.
I went about solving this in the way the question directed you to: Calculate KE and GPE at beginning and then calculate KE at end which gave me 1520 J.
I then used this to calculate the final speed of 31.9 m/s.
My confusion comes from the fact that if I use SUVAT I get a different answer:
S = 51
U = 0
V = ?
A = 3g
T = x
$$ v^2=u^2+2as$$
$$ v=\sqrt {306g}$$
$${v}_{vert}=54.8 {ms}^{-1}$$
$${v}_{res}=\sqrt {54.9^2+4^2}$$
$${v}_{res}=54.9{ms}^{-1}
$$
I thought that suvat is valid because there are no resistive forces - therefore vertical acceleration is constant - probs just being stupid but help would be appeciated :) (by the way 31.9 m/s is right so the second part must be wrong)
acceleration is g not 3g. you
Submitted by Shakira (not verified) on
acceleration is g not 3g. you've used a force as the acceleration.
Oh dear :(
Submitted by O________O (not verified) on
Thank you that was very silly :/
easy mistake to make. had a
Submitted by Shakira (not verified) on
easy mistake to make. had a friend who thought acceleration was a force in an exam and used F=ma as F=mF throughout the whole exam, oddly all this masses came out as 1 kg ahaha!
M2 question confusion
Submitted by stcs on
I'm coming to this a bit late.
I have every sympathy with the original post: putting numbers into these questions just obscures what is going on and makes it easy to slip up. That's why there are rather few STEP mechanics questions that involve numbers (and when they do, it is usually only at the final stage).
If the question was really as stated, then it was malicious; the mass of the particle was not required for this stage so the only reason for including it was to encourage you to go wrong.
Stephen