# Homework Help: HELPPPP! working with mass and velocity

1. Feb 13, 2012

### desire0701

This is my first physics class, I am having a lot of trouble doing this homework. Please help.

BELOW ARE 6 BALLS OF DIFFERENT MASS THROWN HORIZONTALLY AT DIFFERENT SPEEDS FROM THE SAME HEIGHT ABOVE THE GROUND. ASSUME THE EFFECTS OF AIR ARE NEGLIGIBLE.

BALL A- 1.0 kg , 30 m/s
BALL B- 0.5 kg, 30 m/s
BALL C- 1.0 kg, 40 m/s
BALL D- 0.3 kg, 60 m/s
BALL E- 1.0 kg, 10 m/s
BALL F- 0.5 kg, 40 m/s

a) rank these baseballs on the basis of the elapsed time before they hit the ground.
Largest 1._____ 2._____3._____4._____5._____6_____ Smallest

b) rank these baseballs on the basis of the horizontal distance traveled before they hit the ground.
Largest 1._____ 2._____3._____4._____5._____6_____ Smallest

2. Feb 13, 2012

### PeterO

To get an idea of how A and B [or C and F] compare you could try putting two objects of different mass beside each other on a table, then using a ruler, push them both off withe the same speed and see what happens, both from the point of view of time and distance.

Don't use marshmellows or ping-pong balls as air resistance might have an effect.

3. Feb 13, 2012

### desire0701

thank you. Although im still very confused on what formula to use in order to answer these 2 questions :/

4. Feb 13, 2012

### fluidistic

What does your intuition tells you here?

5. Feb 13, 2012

### PeterO

I am suggesting pushing things off a table so that you might use Physics rather than a formula to get the answer.

6. Feb 13, 2012

### desire0701

for part a I believe that since they all have the same heights they will all fall at the same time. Im not quite sure

for part b Im unsure about what formula to use or how to use it im completely confused :/

7. Feb 13, 2012

### PeterO

Note: you could try flicking two masses of the edge if the table at different speed - but at the same time. Quite difficult to manage, but you could try.

8. Feb 13, 2012

### desire0701

thanks you :)

9. Feb 13, 2012

### desire0701

both objects having different masses fell at the same time. Therefore part A would b unable to rank considering they all have the same heightand mass does not affect, only gravity. Thank you once again.

Do you know how I can solve part B?

10. Feb 13, 2012

### PeterO

Judging the time is easy, as you will have heard the impacts with the floor at the same time.

To get an idea for different speeds, you could try flicking a mass off at different speeds, but concentrate on the sound.
You will hear two clicks - one when the ruler hits the mass and a second when the mass hits the floor.

If you lined up a series of coins on the edge of a table and worked you way through them, you would hear each pair of clicks while the time interval from the previous one is fresh in your mind.

11. Feb 13, 2012

### fluidistic

If you believe that for part a) they should all hit the ground at the same time, you believe the time they take to fall is independent of the mass and on the horizontal velocity of the balls.
Let's suppose for the sake of it, that they all take 1 s to fall and hit the ground. What will be the horizontal distance that each traveled?

Edit: To check out if you are right that they all take the same time to fall, you can do PeterO's experiment.

12. Feb 13, 2012

### desire0701

for part b they require me to show my work in formula... I began working with d=Votf + at^2 but it doesnt seem to be giving me the right answer

13. Feb 13, 2012

### fluidistic

Before to run blindly into any formula, check out our help.

14. Feb 13, 2012

### desire0701

I did PeterO's experiment by dropping a pencil and a heavy pencil case from the same height and at the same time and realized they both hit the ground at the same time despite the difference in mass. Now i am unsure about part b.

15. Feb 13, 2012

### fluidistic

Ok perfect!
For part b, my tip was "Let's suppose for the sake of it, that they all take 1 s to fall and hit the ground. What will be the horizontal distance that each traveled?"
If you go at 30 m/s and you're stopped after 1 s, what is the distance you traveled?

16. Feb 13, 2012

### desire0701

d=Votf + 1/2at^2
d=1/2(10)(1)^2
d=5m

would this be correct?

17. Feb 13, 2012

### fluidistic

I'm afraid, no.
I'm asking a question that does not require the use of this kind of formula.
An equivalent question could be "you're in a car travelling at a constant speed of 100 km/h. After one hour, what is the distance you've travelled"?

18. Feb 13, 2012

### desire0701

if my car is travelling 100km/h in 1 hour i would have travelled 100km

19. Feb 13, 2012

### desire0701

therefore both balls A & B would have travelled 30 m in 1 sec. Balls C & F travelled 40 m in 1 sec. Ball D 60 m in 1 sec and Ball E 10m in 1 sec

20. Feb 13, 2012

### fluidistic

You got it.
I'll help you a bit more and tell you that if you change the 1 s by any number of second, call it "j", balls A & B would travel 30 jm within the j seconds, etc.
So now you know the actual ranking for part b).
If you need to work with formula's, I'd start to calculate the time it takes for all the balls to touch the ground if they're released by an arbitrary height "h".
P.S.:Any other helper is welcome, I must go to sleep soon...