Question about freely falling objects

[RuthlessTB]

Homework Statement

Q: A stone is thrown vertically upwards with initial speed of 5 m/s from edge of a cliff 30 m high. Find the time needed for the stone to reach the ground (in seconds).

Homework Equations

1. (h= Vo t - 0.5 g t^2)
2. Quadratic equation:

The Attempt at a Solution

First I will use this equation: (h= Vo t - 0.5 g t^2)
-30 = (5) t - 5 t^2
Now I divide by 5 and it will be like this
t^2 - t - 6
Then, I use the quadratic equation
[itex]\frac{-(-1)+\sqrt{(-1)^2-4(1)(-6)}}{2(1)}[/itex]

The answer is 3, is that right?

Another question, I am not sure..
maybe I need to calculate the upward vertical distance it will go first.. then I sum it with the actual height which is 30 m?

 

[lep11]

The answer is 3, is that right?

Looks right to me.

 

[raghav singh]

simple calculation..
considering down as positive...
h=ut+1/2at.t
h=30 m
a= 10 m/s.s
u=-5m/s

30=-5t+5t.t
t=3 s

 

[raghav singh]

simple calculation..
considering down as positive...
h=ut+1/2at.t
h=30 m
a= 10 m/s.s
u=-5m/s

30=-5t+5t.t
t=3 s

 

[Nickie]

Your attempt at solving the problem is correct. However, there are a few things to consider:

1. When using the equation h=Vo t - 0.5 g t^2, it is important to note that h is the displacement from the initial position. In this case, the initial position is the edge of the cliff, so the displacement should be -30m (negative because it is in the downward direction).

2. When dividing by 5, you should divide all terms by 5, including the -30. This will give you t^2 - t - 6 = 0.

3. Your use of the quadratic formula is correct, but you should include the units when writing your final answer. In this case, the units for time are seconds, so the final answer should be t=3 seconds.

4. It is not necessary to calculate the upward vertical distance first and then add it to the actual height. The equation takes into account the initial velocity and acceleration due to gravity, so you can simply solve for time using the given information.

Overall, your approach and answer are correct. However, it is always a good idea to double check your units and make sure they are consistent throughout your calculations. Keep up the good work!