I am 80ft from a wall that is 60ft tall...(Physics question)?

Question

I am 80ft from a wall that is 60ft tall. How fast should I throw a rock if I throw it at 60degrees, and I want it to just clear the height of the wall? Can you show your work too please... thanks alot.

its irrelevant, or at least it doesn't say in my question.

Answer 1

The weight of the rock doesn't matter. Remeber the story of Galileo dropping objects from the Leaning Tower - gravitational acceleration is independent of mass.

The relevant formulas are:
x= vtcos(60) where v is speed of the throw, t is time since the throw and x is the horizontal distance travelled.

y = vsin(60)t - .5gt^2 where v and t are the same as above, g is the gravitational constant and y is the vertical position.

Since we want to clear a 60' wall 80' away we set x = 80 y = 60. The gravitational constant is 32 ft/sq^2.

Use the first formula to solve for t: t= 80/[vcos(60)]

plug this value for t into the second formula and solve for v:

60 = vsin60[80/[vcos(60]] -.5(32)[80/[vcos(60)]^2
v = 19.65 ft/sec

You should check my arithmetic, and at least try to do your own homework :)

Answer 2

How much does the rock weigh?

It's not irrelevant, you can't calculate that unless you know the mass of the rock.