# Homework Help: Falling rock

1. Jan 30, 2017

### Ugnius

1. The problem statement, all variables and given/known data
Rock is falling from 10meters 18km/h speed , what is speed of rock just before it touches the ground

2. Relevant equations

3. The attempt at a solution
So in meters its like 5m/s and h=10m so t=2s if h=d , v=t*d=2s*10m=20m/s but i think i somehow should include gravity too so im stuck

2. Jan 30, 2017

### Staff: Mentor

Why?
The speed won't stay constant.
This formula assumes the rock starts at rest. It does not.

Don't blindly put values into formulas. Think about what is happening and then which formula could be useful.

Conservation of energy is the easiest approach here, using the kinematic formulas (correctly) is possible as well.

3. Jan 30, 2017

### Ugnius

How could i use kinetic energy when i have no mass and no energy only height and velocity

4. Jan 30, 2017

### Staff: Mentor

The mass will drop out of the calculations.

5. Jan 30, 2017

### Planet_Earth

Mfb is correct.

If my assumptions are correct, you are still in the early stages of learning Physics where mass can often be ignored, mainly just for practice.

Jump into the earlier chapters of your book (if you aren't there already) and skim through any sections that focus on "Particle Under Constant Acceleration."

Just keep in mind that this early stage of physics requires tons of assumptions and ignorance. Don't let it fluster you (read a head and practice with other variables if you are inclined).

6. Jan 30, 2017

### Ugnius

I was just watching the Particle Under Constant Acceleration , but there's given time , and acceleration , could you atleast tell me what formulas should i use

7. Jan 30, 2017

### Planet_Earth

As much as I hate doing this, because this is how I was treated, you should find it yourself.

The location of the formula was given, along with the general name of it, and you already know you don't have to worry about mass. I opened my book right up to chapter 2, skimmed, found the name and equation, and posted on this thread. Go for it. It's easy.

8. Jan 30, 2017

### Staff: Mentor

That is the point of your (!) homework problem.

Multiplying or adding numbers is something your calculator can do. Figuring out which numbers to multiply or add is the actual physics.

9. Jan 30, 2017

### Ugnius

If my calculations were correct should i get 15,15m/s?

10. Jan 30, 2017

### Planet_Earth

My calculations were 14.66, but I did them off the top of my head.

11. Jan 30, 2017

### Ugnius

Maybe that's because i took 10m/s^2 and you 9.8m/s^2 this is possible too

12. Jan 30, 2017

### Planet_Earth

That is probably the most likely case.

And just as a heads up, physics is wholly about logical problem solving a **** ton of research. If the research isn't your thing, get better at it or trying a different field (not attempting to deture you).

13. Jan 30, 2017

### Staff: Mentor

We won't confirm possible guesses. We don't know w
You should become familiar with what are known as the SUVAT equations (look it up!). For kinematic problems such as this you will almost always find that one of the SUVAT equations suits the situation by involving the right mix of variables (the givens and the unknown).