Turn on thread page Beta
    • Thread Starter
    Offline

    13
    ReputationRep:
    17) A child throws a ball horizontally from a window 6m above horizontal ground. The ball just clears a vertical wall 2.5m high and 12m from the house. By modelling the ball as a particle calculate the speed of projection. The ball hits the ground at Q. Calculate the distance of Q from the wall.

    Vertically:
    s=12
    a=0

    Horizontally:
    s=2.5
    a=9.8
    u=0

    Resolving vertically: s=ut+1/2at^2
    t=5/7s

    Resolving horizontally: s=ut+1/2at^2
    12=5/7u
    u=16.8m/s

    b) Horizontally:
    v=16.8
    u=16.8
    a=0

    Vertically:u=0
    a=9.8
    s=5

    Resolving vertically: s=ut+1/2at^2
    5=4.9t^2
    t=1.01s

    Resolving horizontally: s=ut+1/2at^2
    =16.8*1.01
    =16.97

    Ergo, Horizontal distance from wall=16.97m-12=4.97

    The textbook has the answer of 4.97m. However, what I am asking is that it asks for the distance of Q from wall, this is not the horizontal disance. So shouldn't it be:

    Root of 4.97^2+2.5^2

    18) A tennis ball is served horizontally at a speed of 24m/s from a height of 2.7m. The net is 1m high and 12m horizontally from the server. Model the ball as a particle and hence determine whether the ball cclears the net and if so by what distance.

    How do you do 18? I have little idea.
    Offline

    1
    ReputationRep:
    First - Didn't read all the working, but it just means the horizontal distance. That's the shortest distance to the wall. You worked out the distance to the top of it I think?)

    Second - So you can look at horizontal and vertical motion. You know horizontal distance and speed - can you see how to use this? And then how you can use what you find in the vertical motion?
    Spoiler:
    Show
    find the time at which the ball has gone 12m horizontally
    Spoiler:
    Show
    use this time and the acceleration of gravity to find how far it's fallen
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by gcseeeman)
    First - Didn't read all the working, but it just means the horizontal distance. That's the shortest distance to the wall. You worked out the distance to the top of it I think?)

    Second - So you can look at horizontal and vertical motion. You know horizontal distance and speed - can you see how to use this? And then how you can use what you find in the vertical motion?
    Spoiler:
    Show
    find the time at which the ball has gone 12m horizontally
    Spoiler:
    Show
    use this time and the acceleration of gravity to find how far it's fallen
    I did it as such:

    Horizontal:
    u=24
    v=24
    a=0
    s=12
    t=?

    s=ut+1/2at^2
    12=24t
    t=2

    Now what?
    Offline

    1
    ReputationRep:
    (Original post by Deep456)
    I did it as such:

    Horizontal:
    u=24
    v=24
    a=0
    s=12
    t=?

    s=ut+1/2at^2
    12=24t
    t=2

    Now what?
    If 12=24t, t isn't 2 (I guess you see the silly mistake). But use that for vertical motion and see how far it falls in that time.
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by gcseeeman)
    If 12=24t, t isn't 2 (I guess you see the silly mistake). But use that for vertical motion and see how far it falls in that time.
    Ah yes, that was a school boy error.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 16, 2010
The home of Results and Clearing

1,287

people online now

1,567,000

students helped last year

University open days

  1. Keele University
    General Open Day Undergraduate
    Sun, 19 Aug '18
  2. University of Melbourne
    Open Day Undergraduate
    Sun, 19 Aug '18
  3. Sheffield Hallam University
    City Campus Undergraduate
    Tue, 21 Aug '18
Poll
A-level students - how do you feel about your results?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.