# How far above the top of a fence?

#### noaster

A ball of mass 0.4 kg, initially at rest, is
kicked directly toward a fence from a point
20 m away, as shown below.
The velocity of the ball as it leaves the
kicker’s foot is 19 m/s at angle of 61 ◦ above
the horizontal. The top of the fence is 12 m
high. The kicker’s foot is in contact with the
ball for 0.04 s. The ball hits nothing while in
flight and air resistance is negligible.
The acceleration due to gravity is 9.8 m/s2

1. Determine the magnitude of the average
net force exerted on the ball during the kick.
Answer in units of N.

2. Determine the time it takes for the ball to reach the plane of the fence. Answer in units of s.

3. Hint: Consider the diameter of the ball to be
negligible. How far above the top of fence will the ball
pass? Answer in units of m.

4. What is the vertical component of the velocity
when the ball reaches the plane of the fence?
Answer in units of m/s.

I have gotten all of the parts except 3. I have tried submitting .969, 1.148, and .8377 nut they are all wrong

1 is 190
2 is 2.1739
4 is -4.68649

THESE THREE ARE RIGHT FOR SURE. I SUBMITTED THEM

Last edited:
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#### Doc Al

Mentor
What's the height of the ball as a function of time?

#### nrqed

Science Advisor
Homework Helper
Gold Member
A ball of mass 0.4 kg, initially at rest, is
kicked directly toward a fence from a point
20 m away, as shown below.
The velocity of the ball as it leaves the
kicker’s foot is 19 m/s at angle of 61 ◦ above
the horizontal. The top of the fence is 12 m
high. The kicker’s foot is in contact with the
ball for 0.04 s. The ball hits nothing while in
flight and air resistance is negligible.
The acceleration due to gravity is 9.8 m/s2

1. Determine the magnitude of the average
net force exerted on the ball during the kick.
Answer in units of N.

2. Determine the time it takes for the ball to reach the plane of the fence. Answer in units of s.

3. Hint: Consider the diameter of the ball to be
negligible. How far above the top of fence will the ball
pass? Answer in units of m.

4. What is the vertical component of the velocity
when the ball reaches the plane of the fence?
Answer in units of m/s.

I have gotten all of the parts except 3. I have tried submitting .969, 1.148, and .8377 nut they are all wrong

I am not sure how you got those values but I get something different

Find the time to reach the fence (using the motion along X). Then plug that time in the Y equation.

#### Delphi51

Homework Helper
I agree with your time of 2.17 s. I got 12.9 - 12 for the height, which agrees with your first answer to my 1 digit accuracy. I suspect the difficulty is in accuracy. That 12.9 - 12 kills two digits of accuracy so if you need 3 digit accuracy in the final answer you need to do everything to 5 or 6 digits. Even g = 9.81 doesn't cut it.

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