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# Someone answer this kinematics question for me? watch

1. I have found answers myself but I don't want to post them in case they're embarrassingly wrong lmao.

The question:

A stone is thrown vertically upwards with speed 16 ms^-1 from a point h metres above the ground. The stone hits the ground 4s later. Find:

a) the value of h
b) the speed of the stone as it hits the ground
2. (Original post by emilyrose2001)
I have found answers myself but I don't want to post them in case they're embarrassingly wrong lmao.

The question:

A stone is thrown vertically upwards with speed 16 ms^-1 from a point h metres above the ground. The stone hits the ground 4s later. Find:

a) the value of h
b) the speed of the stone as it hits the ground
You will learn more by posting embarassingly wrong answers so that other people can see what you don't understand.

You could split the problem into two phases of motion, upward travel where g slows the stone to 0 m/s and then free falling under the acceleration of g BUT this is very fiddly. You should remember that s is displacement so much more simply the displacement of the stone from it's start point is h in a downward direction.
Choosing the downward direction to be positive and s=ut+0.5at^2 you can easily determine h (note that since the initial velocity is upwards u=-16)

For b) what kinematics equation connects v, u, a and s?
3. So I've ended up with :
a)
Spoiler:
Show

Displacement downwards s = 0.5(v+u)t
so s= 0.5(23.25)2.37
Displacement downwards = 27.55
27.55-13.04 = 14.51
So h = 14.51m

b)
Spoiler:
Show

t= (v-u)/a, so t= -16/-9.81 = 1.63s travelling upwards
Which means for the stone travelling upwards, s= 1/2 * 16 * 1.63 = 13.04m
T for the stone falling back down is 4-1.63 = 2.37
so v= 0+(9.81*2.37) = 23.25ms^-1

And I've kind of done the question backwards and am now confused
4. (Original post by emilyrose2001)
So I've ended up with :
a)
Spoiler:
Show

Displacement downwards s = 0.5(v+u)t
so s= 0.5(23.25)2.37
Displacement downwards = 27.55
27.55-13.04 = 14.51
So h = 14.51m

b)
Spoiler:
Show

t= (v-u)/a, so t= -16/-9.81 = 1.63s travelling upwards
Which means for the stone travelling upwards, s= 1/2 * 16 * 1.63 = 13.04m
T for the stone falling back down is 4-1.63 = 2.37
so v= 0+(9.81*2.37) = 23.25ms^-1

And I've kind of done the question backwards and am now confused
Other than rounding issues introduced when you rounded t early in your working that method is fine. I would use the exact result for t for the upward period of motion. My suggestion regarding displacement is less fiddly.
5. (Original post by gdunne42)
Other than rounding issues introduced when you rounded t early in your working that method is fine. I would use the exact result for t for the upward period of motion. My suggestion regarding displacement is less fiddly.
Thank you

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