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if acceleration wasnt constant, it would be impossible to answer. so you must assume that it is. (for a homework sheet i mean)

There is no acceleration, only a collision where the speed changes.

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Fenstermacher hits a golf shot that travels at 114.8km/hr. The shot strikes a tree and bounces back to the tee at 25% of its original speed. The ball required 3.82 seconds for the round trip. How far away is the tree?

 

The answer is 24.4m. Can someone (ehem, Haku? :ph34r) explain how that's the answer.

 

114.8km/hr is ~31.88m/s. 7.97m/s is 25% of that.

 

I think the avg speed would be 19.875m/s. And the total distance the ball travelled is 75.9m. I just don't know how to figure out how far the tree was!

Suppose the starting point is A, and Tree is at point B. The distance between A and B is d. (we need to find this distance)

Speed at A is, V_A. Speed after bouncing is V_B. (you already calculated the two speeds)

Suppose time taken for the ball to go from A to B is t1.

 

Motion from A to B:

V_A . t1 = d --> t1 = d/V_A (eqn 1)

 

Motion from B to A:

V_B . t2 = d --> t2 = d/V_B (eqn 2)

 

round trip time = t1 + t2 = T (is given).

 

add eqn1 and eqn2: d(1/V_A + 1/V_ B ) = T

Now simply plug in the numbers for V_A, V_B and T to solve for d.

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Suppose the starting point is A, and Tree is at point B. The distance between A and B is d. (we need to find this distance)Speed at A is, V_A. Speed after bouncing is V_B. (you already calculated the two speeds)Suppose time taken for the ball to go from A to B is t1.Motion from A to B:V_A . t1 = d --> t1 = d/V_A (eqn 1)Motion from B to A:V_B . t2 = d --> t2 = d/V_B (eqn 2)round trip time = t1 + t2 = T (is given).add eqn1 and eqn2: d(1/V_A + 1/V_ B ) = TNow simply plug in the numbers for V_A, V_B and T to solve for d.

Inspiring. Thankyou Haku, I really do appreciate it :flower:

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this is how I did it!

 

3.82 = x/31.889 + x/7.972

3.82 = x/31.889 + 4x/31.889

3.82 = 5x/31.889

5x = 3.82x31.889

x = 121.816/5

x = 24.36 m

 

EDIT: I guess it's the same as what Hakpoo did.

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hmm a small subtlety that i think we both ignored is that you should average speed with respect to time and not distance.

 

why am i doing a physics degree..

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After reading through this thread I believe that I have proven Spider correct when in a recent thread he started that I have been a confident moron. The more I read and watch about the actual math involved in deciphering the universe, the more I respect the ability of some to explain what the hell they are talking about. Like multilinguistic translators of dead and forgotten tongues. When I am in charge of the world and all its resources, as parallel universes must provide for now that I've declared it, I will fully fund all of their research forever if they all swear to go away and never speak to me of its myriad processes until they have the answer to everything. I feel like a dog trying to understand algebra.

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nice. I designate you to be the MMs official physics tutor.

 

I don't really like physics that much, just really enjoy newtonian motion + I did a maths module on this + projectiles etc and it was fun. But nooo way will I tutor in physics lol.

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I don't really like physics that much, just really enjoy newtonian motion + I did a maths module on this + projectiles etc and it was fun. But nooo way will I tutor in physics lol.

too late to back down. sorry

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