# Homework Help: Projectile Problem

1. Feb 24, 2012

### vivekrai

1. The problem statement, all variables and given/known data

http://u.spig.in/forum/105599049.jpg [Broken]

A projectile is fired at t=0 with velocity v = 20√2 m/s at an angle of 45° degrees as shown. A wall inclined at the same angle and at a distance of 40 m starts moving towards the projectile at the same time (ie., t=0) with a speed of 20m/s. Find the time at which the ball collides with the the wall.

2. Relevant equations

General Kinematical Equations

3. The attempt at a solution

I'm getting answer as 2(√3-1) s. Strikingly this is very similar to my friend's answer i.e, 2(√2-1) s. I don't know whose one is correct. Help me.

My Solution,

Let the base of the wall has moved a distance 40-x m at the time the ball collides with the wall. Let the height at which the collision occurs is y.
Then we have,
20t = 40-x ..... (1) (for wall)
y = 20t-5t2 ...... (2) (for ball)
and x+y=20t ..... (3) (General diagram and since angle is 45 deg)

Solving for ' t ' , a quadratic I got, t = 2(√3-1) s (Other value being negative)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: May 5, 2017
2. Feb 24, 2012