# Motion problem

1. Sep 8, 2008

### homeworkboy

A toy rocket moving vertically upward passes by a 2.2 m high window whose sill is 9.0 m above the ground. The rocket takes 0.14 m/s} to travel the 2.2 m height of the window.

What was the launch speed of the rocket? Assume the propellant is burned very quickly at blastoff.

How high will the rocket go?

2. Sep 8, 2008

### homeworkboy

bump please help

3. Sep 8, 2008

### LowlyPion

What do you think you should do to solve the problem?

4. Sep 8, 2008

### homeworkboy

well since it says it tokk .14 sec to travel 2.2 m then velocity (v) = 2.2/.14 then we get the final velocity...we know a = -9.8 m/s2........and X = 2.2+9 =11.2m...so we find initial velocity but it says that its the wrong answer

so if u teach me the method it would be great...

5. Sep 9, 2008

### LowlyPion

You need to use the relationship that Vf2 - Vi2 = 2 a x where a is 9.8 m/s2

You are given 2 distances here. Choose one formula from launch.

Vo2 - Vb2 = 2 a x ... where Vb is V bottom and x is height to bottom of window which is 9

You also know that since gravity is slowing things down that
Vb = Vt + .14 (9.8) where Vt is velocity at the top of the window.

Then you can make use of the other equation to the top of the window:
Vo2 - Vt2 = 2 a x ... where Vt is V top and x is height to top of the window which is 11.2

3 equations. 3 unknowns. Solve.

Last edited: Sep 9, 2008
6. Sep 9, 2008

### homeworkboy

I dint understand your explaination....is there anyone else who can help me..

7. Sep 9, 2008

### homeworkboy

bump...anyone

8. Sep 9, 2008

### LowlyPion

What have you tried? You have some of the values. And you have 3 equations that relate the variables you don't have to each other.

You need to use algebra to solve the equations and eliminate your unknowns.

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