A playground is surrounded by a fence 2.0 m high. A boy who plays in the
playground kicks a ball vertically upward from a height of 0.2 m with a speed of
10 m/s. How much time does the ball spend above the height of the fence?

Question

A playground is surrounded by a fence 2.0 m high.  A boy who plays in the 
playground kicks a ball vertically upward from a height of 0.2 m with a speed of 
10 m/s. How much time does the ball spend above the height of the fence?

anonymous · Answer

do you know the equation for height in meters? i forget. somethng like 
$$h(t)=-4.9t^2+v_0t+h_0$$ right?

mertsj · Answer

Yes.  That is right, smartie.

anonymous · Answer

ok then if that is it solve 
$$-4.9t^2+10t+.2>2$$

anonymous · Answer

ick this is a real pain

anonymous · Answer

$$-4.9t^2+10t-1.8=0$$
$$49t^2-100t+18=0$$ 
difference of the roots is 
$$\frac{\sqrt{b^2-4ac}}{a}$$

mertsj · Answer

I think it might be easier to set it equal to 0 and solve the equation.  You will get 1.84 and .2  The ball will be higher when the x value is between those two values.  So 1.84-.2=1.64 sec it will be higher than the fence.

anonymous · Answer

i get 
$$\frac{2 \sqrt{1618}}{49}$$

anonymous · Answer

1.64 seconds rounded

anonymous · Answer

my answer 0.82s. is it correct?

anonymous · Answer

no , i think it should be 1.64

anonymous · Answer

ok2... my fault..sory..thanx all my friends

anonymous · Answer

yw