# How Do You Calculate Initial Velocity in a Vertical Ball Throw?

• Erenjaeger
In summary, the conversation discusses a problem where a ball is thrown vertically upward and returns to its point of release after 8.21 seconds. The goal is to find the initial velocity of the ball, neglecting air resistance. The correct equation to use is xf=xo+vot+1/2at2, and the mistake in the conversation was not including vo*t in the equation. After correcting this, the final solution should be divided by t to get the correct initial velocity, being careful with the signs.
Erenjaeger

## Homework Statement

Concept Simulation 2.3 provides some background for this problem. A ball is thrown vertically upward, which is the positive direction. A little later it returns to its point of release. The ball is in the air for a total time of 8.21 s. What is its initial velocity? Neglect air resistance.

[/B]

## Homework Equations

xf=xo+vo+1/2at2[/B]

## The Attempt at a Solution

known values
xf = 0 because it comes back to where it was released from
xo = 0 because initial displacement is going to be 0
a = -9.8m/s2
t = 8.21 s
so couldn't you solve by using the kinematic formula xf=xo+vo+1/2at2
and then solving for vo
leaving you with -vo=xo+1/2at2-xf
and because both the initial displacement and final displacement are 0
-vo = 1/2 at2 ?
so -vo=-9.8⋅8.212
which gives me -330.62 which would be just 330.62m/s but it is incorrect. pls helpp
[/B]

Erenjaeger said:
xf=xo+vo+1/2at2
Can you spot the mistake in this equation?

a
TSny said:
Can you spot the mistake in this equation?
ahhhh i haven't put in vo t
because the equation is really xf=xo+vot+1/2at2 right?

Erenjaeger said:
ahhhh i haven't put in vo t
because the equation is really xf=xo+vot+1/2at2 right?
Yes, good.

TSny said:
Yes, good.
so by just dividing my answer by t (8.21) ill get the vo right?

so by just dividing my answer by t (8.21) ill get the vo right?
Yes. But be careful with the signs. In your first post you wrote
so -vo=-9.8⋅8.212which gives me -330.62
But you have a negative sign on both sides, so this will give a positive value for v0.

TSny said:
Yes. But be careful with the signs. In your first post you wrote

But you have a negative sign on both sides, so this will give a positive value for v0.
yeah okay, thank you!

## 1. What is the kinematics ball throw problem?

The kinematics ball throw problem is a physics problem that involves calculating the motion of an object (the ball) as it is thrown into the air. It takes into account factors such as initial velocity, acceleration due to gravity, and time.

## 2. How do you solve the kinematics ball throw problem?

To solve the kinematics ball throw problem, you can use the kinematic equations, which describe the relationship between displacement, velocity, acceleration, and time. By plugging in the given values and solving for the unknown variable, you can determine the motion of the ball.

## 3. What are the key variables in the kinematics ball throw problem?

The key variables in the kinematics ball throw problem are initial velocity (v0), acceleration due to gravity (g), time (t), and displacement (d). These variables are used to calculate the final velocity (vf), and can be rearranged to find other unknown values.

## 4. Is air resistance a factor in the kinematics ball throw problem?

In most cases, air resistance is not considered a factor in the kinematics ball throw problem. This is because the problem assumes that the ball is thrown in a vacuum, where there is no air resistance. However, in real-life situations, air resistance can affect the motion of the ball and must be taken into account.

## 5. How is the kinematics ball throw problem used in real life?

The kinematics ball throw problem is used in real-life situations to understand and predict the motion of objects. It is commonly used in sports, such as basketball and baseball, to analyze the trajectory of the ball and determine factors such as the optimal angle and speed for a successful throw. It is also used in engineering and physics to design and test structures and machines that involve throwing or launching objects.

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