desperately need physics help!!

[1cat2ferrets]

This is what my son has so far:

This has to do with free fall problems.

 

1. what is the instantaneous speed at the end of 5 secs of a flower pot which has been knocked from a window ledge 200meters from[/the ground? My son figured it out to be 50m/s.B]

 

He was told that in all cases g=10m.s^2, BTW.

 

2. How far will the flower pot have fallen in 5 seconds?

using this equation: d=1/2at^2 he got the answer 125 meters

 

3. Someone reaching out an upper story window catches the flowerpot and then throws it straight back up in the sir at the rate of 10 meteres per second. What would be the instantaneous speed of the flowerpot at the end of 1 second? My son chose 20m/s.

 

Now we're both stuck on these last two:

 

4. How many seconds elapse between the time the person throws the flowerpot up and catches it again? The choices are: 1 sec, 2 secs, 10 secs, or none of these.

 

5. What is the speed of the flowerpot when the person catches it for the second time? The choices are: 0m/s, 5m/s, 50m/s 10m/s, or 20m/s.

 

Please help!!

 

 

TIA

[AngieW in Texas]

1. final velocity = initial velocity + (acceleration)(time)

The initial velocity is 0 because the flower pot is in free fall. It wasn't thrown. The acceleration is given as 10m/s^2. The time is given as 5 seconds.

final velocity = 0 + (10)(5) = 50 m/s

 

2. distance = 0.5(acceleration)(time squared)

distance = 0.5(10)(5^2)

distance = 125 m

 

So those two are correct.

 

3. final velocity = initial velocity + (acceleration)(time)

This problem gets tricky because your initial velocity and your acceleration are in opposite directions. The initial velocity is upwards, but gravity is downwards. This means that your initial velocity and your acceleration have opposite signs.

final velocity = 10 - 10(1) = 0 m/s

After one second, the flowerpot is as high as it will go. After this, it will fall back down.

 

4. It took one second to travel up. It will take one second to travel down. The answer is 2 seconds.

 

5. final velocity = initial velocity + (acceleration)(time)

You can use your work from problems 3 and 4 for this one. You know that at the end of problem 3, the speed was 0. You also know that it takes just one second to go up or down.

final velocity = 0 + (10)(1) = 10 m/s

 

 

The only time it's going to make a difference whether you use a positive or negative sign for acceleration is when you have them working in opposite directions. Outside of that circumstance, you just need to bear in mind which direction the object is moving in.

Edited February 1, 2010 by AngieW in Texas

[1cat2ferrets]

Thanks for taking the time in answering my questions and in explaining them, Angie! You've been a Godsend.

 

I may have one or two more for you, if you're up to it. LOL!!!

 

Thanks again!

[1cat2ferrets]

He's using the same textbook as one of your kids, Conceptual Physics by Hewitt. He likes it, does yours?

[AngieW in Texas]

My 16yo doesn't like math or science, but she is very good at math. She likes physics better than any of the other science courses we've done because the math can carry her through it. She's taking science at the cc next year and physics is what she's chosen to do. For her, any science that has math is going to be easier than a science without math.

 

I prefer math-based stuff too.