# Help! 3 part physics assignment?

Doing rough in AP physics. I really can’t seem to get this.

A ball of mass m,initially at rest , is kicked directly toward a fence from a point D meters away, as shown above. The velocity of the ball as it leaves the kicker's foot is v, at an angle above the horizontal The top of the fence is H meters high. The ball hits nothing while in flight and air resistance is negligible .

a. write an equation for the time (t) it takes for the ball to reach the plane of the fence

b.will the ball hit the fence? justify your answer

c. sketch the horizontal and vertical components of the velocity of the ball as functions of time until the ball reaches the plane of the fence

### 2 Answers

- NCSLv 71 month agoFavorite Answer
a) the horizontal velocity is constant, so

x = v*cosΘ*t = 20m/s*cos37º * t = 16m/s * t

b) time to hit the fence t = x / v*cosΘ = 32m / 16m/s = 2.0 s

height at that time:

h = vsinΘ*t + ½at² = 20m/s*sin37º*2s - ½*9.8m/s²*(2.0s)²

h = 4.4 m

It looks like your fence is 2.5 m high (very blurry). If so, then the ball clears the fence by 1.9 meters.

c) To help with the sketch, find the time for the ball to reach its maximum height (and zero vertical velocity):

t = v*sinΘ / g = 20m/s*0.6 / 9.8m/s² = 1.2 s

Sketch below.

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- oubaasLv 71 month ago
Vo = 20 m/sec

Voy = Vo*sin 37° = 20*0.6 = 12.0 m/sec

Vox = Vo*cos 37° = 20*0.8 = 16.0 m/sec

time t to reach the fence = d/Vox = 32/16 = 2.0 sec

height h at the fence = Voy*t-g/2*t^2 = 12*2-4.9*2^2 = 24-19.6 = 4.4 m

the ball will clear the fence by 4.4-2.5 = 1.9 m