[thorn0123]

A ball is projected upwards from a point which is 4m above the ground with speed 18ms^-1 find:

a)The speed of the ball when it is 15m above its point of projection
b)The speed with which the balls hits the ground

So I drew a diagram and I tried to attempt this question a different way than what the book did:



I ignored the red particle and started from the green:

from the green I said u = 0, a = 9.8 and s = 19 and found the velocity to be 19.29

I then done it another way with u = 18, a = -9.8 and s = -4 and I get the velocity to be about 20ms^-1

these two are very similar so im not sure whats going wrong

EDIT: I realised that for some reason i'm assuming that the 15m is its maximum distance, is this why they are different?

[ghostwalker]

(Original post by thorn0123)
EDIT: I realised that for some reason i'm assuming that the 15m is its maximum distance, is this why they are different?

Yes. At 15m its speed is 5.48 m/s (3 sig.fig.)

[raheem94]

(Original post by thorn0123)
A ball is projected upwards from a point which is 4m above the ground with speed 18ms^-1 find:

a)The speed of the ball when it is 15m above its point of projection
b)The speed with which the balls hits the ground

So I drew a diagram and I tried to attempt this question a different way than what the book did:



I ignored the red particle and started from the green:

from the green I said u = 0, a = 9.8 and s = 19 and found the velocity to be 19.29

I then done it another way with u = 18, a = -9.8 and s = -4 and I get the velocity to be about 20ms^-1

these two are very similar so im not sure whats going wrong

EDIT: I realised that for some reason i'm assuming that the 15m is its maximum distance, is this why they are different?

Your thinking is correct, 15m isn't the max distance.

You have assumed the speed to be zero when the particle is 19m above the ground, while this is not the case, its speed at that instant is

EDIT: Beaten to it!

[SpiggyTopes]

(Original post by thorn0123)
A ball is projected upwards from a point which is 4m above the ground with speed 18ms^-1 find:

a)The speed of the ball when it is 15m above its point of projection
b)The speed with which the balls hits the ground

So I drew a diagram and I tried to attempt this question a different way than what the book did:



I ignored the red particle and started from the green:

from the green I said u = 0, a = 9.8 and s = 19 and found the velocity to be 19.29

I then done it another way with u = 18, a = -9.8 and s = -4 and I get the velocity to be about 20ms^-1

these two are very similar so im not sure whats going wrong

EDIT: I realised that for some reason i'm assuming that the 15m is its maximum distance, is this why they are different?

a) The mistake you made here was to assume that S is the distance from the ground, it isn't, it is the displacement.

S= 15
U=18
V=To be found <--Up is positive
A=-9.8
T=Unknown

V^2=U^2+2as
V^2=294
V=17.1ms-1

b)

S=-4
U=18
V=To be found <---upwards is positive
A=-9.8
T=Unknown

V^2=U^2+2as
V^2=402.4
V=20.1ms-1


I hope my calculations are right, I used a pathetic calculator on my computer!

[aznkid66]

(Original post by SpiggyTopes)
a) The mistake you made here was to assume that S is the distance from the ground, it isn't, it is the distance travelled.

Actually, both answers were for part (b)... And if something goes up, comes down, and hits the ground, S is the distance from the ground, not distance traveled.

[SpiggyTopes]

(Original post by aznkid66)
Actually, both answers were for part (b)... And if something goes up, comes down, and hits the ground, S is the distance from the ground, not distance traveled.

I'm sorry, I meant displacement! Schoolboy error!