# Projectile motions help

Watch
Announcements
#1
How do I answer this question?

a) A rugby player is aiming for a conversion. He kicks the ball at 15ms−1 at an angle of 50∘ to the horizontal. At the time, he is 20m from the posts. How much time will the ball take to reach the posts?

b) How high will the ball be when it reaches the posts?

I've been struggling on this question for ages. So any answers and help would be really helpful...
0
3 years ago
#2
(assuming no air resistance)

use trig to get the horizontal component of the initial velocity... this will get you the time to reach the posts v=s/t

use trig to get the vertical component of the initial velocity
then use suvat with acceleration = g to get the vertical displacement of the ball after the time you found in the earlier section
0
#3
(Original post by Joinedup)
(assuming no air resistance)

use trig to get the horizontal component of the initial velocity... this will get you the time to reach the posts v=s/t

use trig to get the vertical component of the initial velocity
then use suvat with acceleration = g to get the vertical displacement of the ball after the time you found in the earlier section
Okay, so calculating the horizontal component of V gives '9.6418.. m/s' which then gives a a time of 2.074 s from '20/9.6418'

Then moving onto part b, I calculated the the vertical component of V which is '11.49 m/s'.

Subbing all this into the suvat equation of 's = ut + at^2 and using the value of time from the earlier part along with '11.49 m/s' gives an answer of 2.8 to 1dp.
0
#4
(Original post by Joinedup)
(assuming no air resistance)

use trig to get the horizontal component of the initial velocity... this will get you the time to reach the posts v=s/t

use trig to get the vertical component of the initial velocity
then use suvat with acceleration = g to get the vertical displacement of the ball after the time you found in the earlier section
One other Q I have is how would you do this too?

A cricket batsman hits a ball at a speed of 27m/s at an angle of 60 degrees to the horizontal. How far would you have to stand in order to catch it (assuming you have to catch it before it hits the ground)?

How would I attempt this one since I don't know time or dist???
0
3 years ago
#5
(Original post by Yatayyat)
One other Q I have is how would you do this too?

A cricket batsman hits a ball at a speed of 27m/s at an angle of 60 degrees to the horizontal. How far would you have to stand in order to catch it (assuming you have to catch it before it hits the ground)?

How would I attempt this one since I don't know time or dist???
well you still need to find t (common theme to this type of question)

this time around you'd look at the vertical component of velocity first & find the the time(s) taken for the ball to reach 'catching height'. if you assume the batsman hits the ball at knee height and the fielder catches the ball at chest height then there might be a couple of solutions - one with the ball still on it's way up to the maximum height and one with the ball on it's way back down again.

the first case would correspond to the wicket-keeper making a quick reaction catch close to the batsman and the second case to a more conventional catch in wide field.

you'd use the time (s) and the horizontal component of velocity to find the distance horizontally.
0
1 year ago
#6
(Original post by Joinedup)
well you still need to find t (common theme to this type of question)

this time around you'd look at the vertical component of velocity first & find the the time(s) taken for the ball to reach 'catching height'. if you assume the batsman hits the ball at knee height and the fielder catches the ball at chest height then there might be a couple of solutions - one with the ball still on it's way up to the maximum height and one with the ball on it's way back down again.

the first case would correspond to the wicket-keeper making a quick reaction catch close to the batsman and the second case to a more conventional catch in wide field.

you'd use the time (s) and the horizontal component of velocity to find the distance horizontally.
i used 1.5m as my vertical height but i got answer 63.45m. isaac physics accepts 64m. bit ambiguous.
0
1 year ago
#7
(Original post by lyds01)
i used 1.5m as my vertical height but i got answer 63.45m. isaac physics accepts 64m. bit ambiguous.
I guess the fielder would be able to stick an arm out by the 55cm
0
1 year ago
#8
(Original post by Yatayyat)
Okay, so calculating the horizontal component of V gives '9.6418.. m/s' which then gives a a time of 2.074 s from '20/9.6418'

Then moving onto part b, I calculated the the vertical component of V which is '11.49 m/s'.

Subbing all this into the suvat equation of 's = ut + at^2 and using the value of time from the earlier part along with '11.49 m/s' gives an answer of 2.8 to 1dp.
why does s = ut + at^2 work but the actual suvat equation is 𝑠 = 𝑢t + (at^2)/2... what happened to the /2 part of the equation?
0
11 months ago
#9
Quick question i know im super late but i just got set this question for homework - why do you use 1.5m as the vertical height, where did you get that from, if you dont mind me asking?
0
11 months ago
#10
(Original post by Lina_Amiour)
Quick question i know im super late but i just got set this question for homework - why do you use 1.5m as the vertical height, where did you get that from, if you dont mind me asking?
Where does the bat hit the ball?
0
11 months ago
#11
how did u get 2.8? I am not getting it. can you show your working, please?
1
X

new posts Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

Yes, and I've sent off my application! (156)
55.12%
I've made my choices but havent sent my application yet (41)
14.49%
I've got a good idea about the choices I want to make (31)
10.95%
I'm researching but still not sure which universities I want to apply to (26)
9.19%
I haven't started researching yet (16)
5.65%
Something else (let us know in the thread!) (13)
4.59%