I'm missing something - help

[speg]

I'm missing something - fixed :P

Ok, doing practice stuff over the summer ( expect more of these :P ) and anyway this should be easy but I can't get it to work.

A diver ( mass = 70 kg ) jumps off a board 10m above the water. If his downward motion is stopped 2.0s after he enters the water, what average upward force did the water exert on him?

Ok, I get his speed when he hits the water to be:
(2*9.8*10)^(1/2) = 14 m/s

Then it takes him 2s to stop... so we can find that acceleration:
a = 14/2 = 7

F=ma = 70*7 = 490 N

But the back of the book tells me it should be 1180 N.

Sorry about the presentation, I'll have to learn how to enter this stuff soon. :)

Haha, silly me forgot about gravity :O How do I keep all these things straight - there's so much stuff to worry about!

 

[Cummings]

yes, the force required is the force to act against gravity (70*9.8) plud the force required to act against the 14m/s motion (70*7)

so (70*9.8)+(70*7) = 1176 N

 

[M98Ranger]

No problem, we all make mistakes! Glad you were able to figure it out though. It's important to remember all the factors and equations when solving physics problems. Keep practicing and it'll become second nature to you. And don't worry, I'm sure you'll get the hang of entering equations and formatting soon enough. Keep up the good work! :)

 

[M98Ranger]

Don't worry, it's completely normal to feel overwhelmed with physics concepts and equations. It takes time and practice to fully understand and remember everything. Let's break down the problem together and see where the discrepancy in our calculations is.

First, we need to calculate the velocity of the diver as he hits the water. You were correct in using the equation (2*9.8*10)^(1/2) = 14 m/s.

Next, we need to find the acceleration of the diver as he stops in the water. This is where the missing factor comes in - gravity. We need to take into account the force of gravity acting on the diver as he falls. This means that the acceleration is not just 14/2 = 7, but 14/2 + 9.8 = 16.9 m/s^2.

Now, we can calculate the average upward force exerted by the water on the diver using the formula F=ma. Plugging in the mass of the diver (70 kg) and the acceleration (16.9 m/s^2), we get F = 1180 N, which matches the answer in the back of the book.

So, the missing factor was the force of gravity, which is an important concept to remember in physics problems. Don't worry, with more practice, you'll become more familiar with these equations and concepts. Keep up the good work!