# Homework Help: Acceleration and Displacement

1. Aug 3, 2011

### andorei

Hey im kinda new here. :D

a coin is thrown down into a well with an initial speed of 3m/s. The coin hits the bottom 2 seconds later.

1. How deep is the well?
2. What is the coin's speed as it hits the ground?

Please show solution that I can study it.

2. Aug 3, 2011

### NewtonianAlch

What are the equations you have learnt so far dealing with velocity, acceleration, distance, and time? List them out, if you can't remember them, check it up.

Once you do that, it may become clear as to what you need to do.

3. Aug 3, 2011

### andorei

The formulas in my book isn't very informative so I'm really having troubles in this particular question and I need this for an upcoming College Admission Test.

4. Aug 3, 2011

### NewtonianAlch

What are the formulas given in your book regarding it?

5. Aug 3, 2011

### PeterO

The formulas involve five variables. One set of letters is
Initial velocity - u
Final Velocity - v
Displacement - s
Acceleration - a
Time - t

[Some texts/people use different letters - the most common difference being a d instead of the s]

There are 5 formulae [though often only 4 are referred to], each with one variable missing.

For example v = u + a.t the formula with no reference to displacement.

To solve a problem, write out the three variables you do know, and identify the variable you want to find.
Then identify the formula using those 4 variable; substitute and solve carefully.

6. Aug 3, 2011

### andorei

There are too many formulas but I'm pretty sure it is in freefall unless it was thrown downward.
I just need tips to find the coin's speed.

Here are the choices in the question

1.How deep is the well?
a. 11.56m
b. 27.32m
c. 25.6m
d. 30.25m

2. What is the coin's speed as it hits the ground
a. 30.5m/s
b. 22.6m/s
c. 11.82m/s
d. 56.72m/s

7. Aug 3, 2011

### PeterO

Which of the 5 variable I mentioned do you know? What value do they have?

8. Aug 3, 2011

### andorei

Initial Velocity is - 3m/s
Final Velocity is unknown
Displacement is also unknown
time - 2 seconds

9. Aug 3, 2011

### NewtonianAlch

http://www.physicsforidiots.com/dynamics.html

Take a look at those 6 equations in the second paragraph, which one would suit your needs to solve for displacement?

Remember, like you said, you have initial velocity (u), acceleration (a), and time (t).

10. Aug 3, 2011

### PeterO

Great!

Lets do velocity. You know u , a and t

which formula involves v, u, a and t ?

While we are at it
Let's do Displacement (s)
You still know u, a and t
Which formula involves u, a, s and t ?

11. Aug 3, 2011

### andorei

v = u + a.t
is it this one? Tho the displacement is not supplied in the formula?

12. Aug 3, 2011

### PeterO

That is the formula for finding the final velocity YES.

You use a different formula to find the displacement.

Edit:
That great link supplied by "NewtonianAlch" lists all the formulas in case your notes/text doesn't have them

13. Aug 3, 2011

### andorei

Hahaha. I got number 2 even before the formula was given. I experimented. HAHAHA

For the displacement?

s=ut+1/2at^2

Is this the one that I should use?

14. Aug 3, 2011

### PeterO

Exactly.

Now just substitute in for the variables you know and the answers should come out.

15. Aug 3, 2011

### andorei

Fantastic! Everything was correct.
Thanks for the great help guys.

4 more days to review tho. It's really frustrating.

I'll ask more questions to be achieve higher grades though. I wish you guys could help me again.