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March 28, 2015

March 28, 2015

Posted by **eddy** on Tuesday, February 23, 2010 at 4:41pm.

(a) friction is negligible

(book answer is 10m/s)

(b) a friction force of 3000n opposes the motion?

(book answer is 5.1 m/s)

dont know where to begin to solve these 2questions. I tried differant formulas, but nothing I tried came out to the answers in the book.

help

- College Physics -
**bobpursley**, Tuesday, February 23, 2010 at 4:43pmsee this very related problem:

http://www.jiskha.com/display.cgi?id=1266960070

- College Physics -
**drwls**, Tuesday, February 23, 2010 at 4:53pmMany if not most problems in physics are not solved by plugging directly into a single equation. It is better to prceed in logical steps using formulas you are familiar with.

In this case, the place to start is with conservation of energy. The vertical drop of the runway is

H = 15 sin 20 = 5.13

The Potential energy loss MgH equals the Kinetic Energy gain, so

MgH = (1/2) M V^2

V = sqrt(2 g H) = 100.3 m/s

(I used 9.81 m/s^2 for g)

Note that you did not need to use the mass for this one.

To do the second case, subtract friction work done

3000N*15 m = 45,000 J from the potential energy decrease, and solve for V again.

M g H - 45,000 = (1/2) M V^2

You will need to use the mass this time.

- College Physics -
**Aweis Mahamud**, Wednesday, February 24, 2010 at 2:44pmTwo long, parallel wires separated by 30 cm each carry currents of 4.0 A in a horizontal direction.

B)Find if they are in opposite directions.

**Answer this Question**

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