Calculating Rock Speed on Impact from 10m

[Ugnius]

Homework Statement

Rock is falling from 10meters 18km/h speed , what is speed of rock just before it touches the ground

Homework Equations

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 I am stuck

 

[mfb]

so t=2s

Why?
The speed won't stay constant.

v=t*d=2s*10m=20m/s

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.

 

[Ugnius]

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.

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

 

[mfb]

The mass will drop out of the calculations.

You can also start with "energy per mass" everywhere, doesn't matter.

 

[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).

 

[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

 

[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.

 

[mfb]

could you atleast tell me what formulas should i use

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.

 

[Ugnius]

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

 

[Planet_Earth]

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

Your answer makes sense but always check it again.

 

[Ugnius]

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

 

[Planet_Earth]

That is probably the most likely case.

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

 

[gneill]

We won't confirm possible guesses. We don't know w

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

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).