# Having trouble with some problems in my homework -- Balls being rolled and tossed

1. Oct 10, 2014

### lynaa

1. The problem statement, all variables and given/known data
While rolling balls down an inclined plane, Galileo observes that the ball rolls 1 cubit (the distance from elbow to fingertip) as he counts to 10. How far will the ball have rolled from its starting point when he has counted to 20?

The answer I get is supposed to be 4 cubits

2. Relevant equations

d = 1/2 a t^2

3. The attempt at a solution
Ball is rolling at 1 cubit / 10 count.

t = 20 count = two 10counts = 2t

d = 1/2 a t^2
d = 1/2 a (2t)^2
d = 1/2 * 4t^2 * a
2d = 4t^2 * a
I don't know where to go from here

1. The problem statement, all variables and given/known data

Someone standing at the edge of a cliff (as in the following figure) throws a ball nearly straight up at a certain speed and another ball nearly straight down with the same initial speed. If air resistance is negligible, which ball will have the greater speed when it strikes the ground below?

This time I do not know what the correct answer will be

2. Relevant equations

None, theory problem

3. The attempt at a solution

I think that the ball thrown up will have a greater acceleration because it will fall from a higher point (once it reaches that highest point). So it will have a longer time to accelerate.

1. The problem statement, all variables and given/known data
A freely falling object falls 5.0s after it is released from a position of rest. How far will it fall in this time?

Express your answer to two significant figures and include the appropriate units.

2. Relevant equations

d = 1/2 a t^2

3. The attempt at a solution

a = g = -10m/s^2
t = 5s

d = 1/2 (-10m/s^2) (5s)^2
d = -5m/s^2 (25s^2)
d = -125m
want distance fallen so take absolute value of d = -125
d = 125m

But it tells me that I am wrong. :(

2. Oct 10, 2014

### RUber

for the first problem $1/2 a$ is constant in both instances. You have all the information needed to make a comparison.

3. Oct 10, 2014

### RUber

For the second problem, ask yourself how fast will the ball that was thrown upward be travelling when it gets back to the man who threw it. What is that velocity relative to the velocity he threw the downward ball?

4. Oct 10, 2014

### RUber

For the third problem, I suspect there are some rounding errors since it is asking for two significant digits.
Gravity is 9.8m/sec/sec, so there is one possible source of error.

5. Oct 10, 2014

### lynaa

Because they are constant that means that I can I just set it to 1 and ignore it right? I can see how to solve it based on that, but why is it constant? Why am I able to just say it's equal to whatever when solving the problem?

Oh, it would be the same velocity as when he threw it up right? So that means that it will be the same thing right?

We were told to use 10m/s^2 for everything in gravity. It is only an introductory physics course, so my professor said on the first day to not worry about using 9.8 as it is easier to learn to the concepts with 10m/s^2. Is there another spot that I made an error in?

Also thank you for helping me!

6. Oct 10, 2014

### Staff: Mentor

In 1. you could determine your value for a where a is the acceleration directed along the slope due to a component of gravity. Then use that same value for your 2t calculations.

What is 125 expressed to 2 significant figures?

7. Feb 23, 2015

3rd question