# Homework Help: Projectile Motion - Horizontal Component

1. Apr 25, 2010

### Procrastinate

A rock is thrown off a 100.0m cliff upward at an angle of 20o to the horizontal. It has an initial velocity of 15ms-1 and strikes the rocks below.

Find how far away the rock is from the base of the cliff.

I calculated total time of flight (projectile parabola + 100m vertical descent) which equalled around 4.3-5.0 (verified by textbook.)

The total time of flight for the parabolic section was about 1.046s

Horizontal component = 15cos20 = 14.095ms-1

SH = ut = 14.095 x 1.046 = 14.74m.

However, the answer is 70m and you can only acquire that by 14.095 x 5 which doesn't make sense because I thought after being projected at an angle it would make a completely vertical descent thus having no effect whatsoever on the horizontal component.

2. Apr 25, 2010

### kuruman

Exactly what do you mean by "parabolic section"? Is the entire trajectory of the rock a parabola or only a piece of it?

3. Apr 25, 2010

### pat666

I got t equals 4.022 taking g as 9.81. from that s=ut+1/2at^2.... where u = 15cos(20) and a = 9.81... comes out to 76.43m. ignoring drag.

4. Apr 25, 2010

### pat666

sorry didnt see the answer you provided the book is right but i assume that you take gravity as 10m/s^2