# How to solve this question

#### ljanes

A football player kicks for a field goal 30.91m from the posts. The ball is kicked with a speed of 18.6 m/s at an angle of 45 degrees above horizontal

a) how long will it take the ball reaches the posts
b) how high will it be when it goes through the posts
c) What is the magnitude of the velocity of the ball as it travels through the posts
d) Calculate the ball's horizontal range
e) When will it land?

Thank you for anybody who takes the time to help me with this! I am new to physics

#### Woody

This is actually more a mathematics question than a physics question.
Thus I will assume that such real world issues as aerodynamic drag can be ignored.

So it starts off as a geometry problem,
how much of the speed is upwards and how much is horizontal?
Then how long will it take to reach the posts given the horizontal speed you have just calculated.

The next parts require you to include gravity
You should know the standard equation:
V=U+At
where:
U is the initial vertical component of the speed,
V is the current velocity (at time t)
A is the acceleration due to gravity.

Note that A is in the opposite direction to U
(i.e. if we say U is positive A must be negative).

You worked out earlier how long the ball takes to reach the posts
this is the time you need in the above equation.

For the last part you need the standard equation:
S=Ut+½At²
Where S is the distance traveled
since we have split the speed into vertical and horizontal components,
just consider the vertical component.
So if U is the initial vertical component of the speed and A is the acceleration (or if you prefer deceleration, because it is negative in this case) due to gravity
Then S is the height above the ground.

The question is now when is S zero?

There are two times when S is zero, one at the start (when it is first kicked) and one at the end (when it hits the ground).

Note that S=Ut+½At² is a quadratic equation.
There is a standard way of finding the roots of a quadratic equation (the points at which it is zero).

• 1 person

#### GeorgeGR

Hello. I attached an image with solutions. Please tell me if you can see them.

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#### ljanes

thank you, i can see it thank you for your help!

#### Woody

Hi George,
As a matter of policy this forum frowns on just supplying the answers to what look like homework questions.
We will try to provide pointers and hints, but try to stop short of just doing their homework for them.

One of the major purposes of homework is to highlight to the tutor is a person is struggling to answer questions which the tutor thinks they should be able to answer given the information delivered in the classes so far.

If they can't, it possibly indicates that the tutor is pacing the lessons incorrectly
or needs to spend a bit of extra time with this individual.
or it might mean that the individual has not been paying attention in lessons.

Whatever the reason, us helping to cover over the problem by simply supplying the answers is not ultimately being helpful.

#### GeorgeGR

Hi Woody,
You are totally right! Thank you for the advice.

#### Curiousphysicist

I think that sometimes the worked out solution is so much more helpful than blurting out the right answer. - Someone who doesn't really know what steps to take will really benefit from seeing the steps that give you the final answer, though. 