# Determine initial velocity of a vertical throw

• duchuy
In summary: Sorry, that's not right at all.1) That's not what the equations tell you.2) A ball is moving downwards when it hits the ground after being thrown up.There are lots of videos on line about throwing an object up. I think you need to develop some sort of physical understanding of what is happening.
duchuy
Homework Statement
t = 4s , g = 10 m/s^2
Relevant Equations
I don't really know which equation to use...
Hi,
I was given this problem saying that a ball is thrown vertically up in the air and returns to its initial position after 4 seconds. The acceleration due to gravity is given to be equal to 10m/s^2.
I tried to attempt this problem by using the equation :
v^2 - v0^2 = 2ah by considering terminal velocity = 0. Despite this, i still can't determine the total distance travelled.
Sorry in advance if I have misused any vocabulary sincei translated this from french.
Thank you!

Do you have any ideas?

PeroK said:
Do you have any ideas?
Well, I'm also trying with v = v0 + at but I didnt really work out..
This is why I chose medecine physics is so so hard..

duchuy said:
Well, I'm also trying with v = v0 + at but I didnt really work out..
What about $$s = v_0t + \frac 1 2 at^2$$

PeroK said:
What about $$s = v_0t + \frac 1 2 at^2$$
But in this case wouldn't I have to values of gravitational acceleration?
When the object is going up, acceleration is negative and when it goes down, it's positive.
Therefore I couldn't use the 4 seconds value because ( i think ), the time the object takes to go up is different from the time it takes to go down right?

duchuy said:
Well, I'm also trying with v = v0 + at but I didnt really work out..
This is why I chose medecine physics is so so hard..
That equation is a good start. You know that the ball is in the air for 4 s, and you know the acceleration. If the velocity is +v0 when the ball is thrown up, what would it be at the moment it comes back down after 4 s?

kuruman said:
That equation is a good start. You know that the ball is in the air for 4 s, and you know the acceleration. If the velocity is +v0 when the ball is thrown up, what would it be at the moment it comes back down after 4 s?
I think the thing that what's stopping me is that I don't know whether the time an object takes to go up is different from the time it takes to go down..
Because I've been considering that it's different, so i didnt really manage to write the equation..
At 4s, wouldn't the velocity be 0?
If I consider that the time is equal, I'm getting v0 = 20 m/s ...

duchuy said:
But in this case wouldn't I have to values of gravitational acceleration?
When the object is going up, acceleration is negative and when it goes down, it's positive.
Therefore I couldn't use the 4 seconds value because ( i think ), the time the object takes to go up is different from the time it takes to go down right?
If you're not sure, you could split the motion into two parts (up and down). You can then show using the kinematic equations whether the time to go up is the same as the time to come down.

duchuy said:
At 4s, wouldn't the velocity be 0?
If I consider that the time is equal, I'm getting v0 = 20 m/s ...
If you throw a ball up at ##20 m/s##:

1) What is the displacement after ##4s##?

2) What is the velocity after ##4s##?

duchuy said:
I think the thing that what's stopping me is that I don't know whether the time an object takes to go up is different from the time it takes to go down..
Because I've been considering that it's different, so i didnt really manage to write the equation..
At 4s, wouldn't the velocity be 0?
If I consider that the time is equal, I'm getting v0 = 20 m/s ...
At 4 s the ball returns to the height from which it was launched. Just before it is stopped, it is still moving, That's the velocity I was asking about. Anyway, I don't want to detract from the course that @PeroK has set so I will cease and desist.

duchuy and PeroK
PeroK said:
If you throw a ball up at ##20 m/s##:

1) What is the displacement after ##4s##?

2) What is the velocity after ##4s##?
Oh ok ok I got that thank you two so much!
For the displacement after 40swouldn't it be 40m?
For the velocity I'm not quite sure.. I really thought that it would be 0 since it's the moment the ball landed..

PeroK
duchuy said:
Oh ok ok I got that thank you two so much!
For the displacement after 40swouldn't it be 40m?
For the velocity I'm not quite sure.. I really thought that it would be 0 since it's the moment the ball landed..
Sorry, that's not right at all.

1) That's not what the equations tell you.

2) A ball is moving downwards when it hits the ground after being thrown up.

There are lots of videos on line about throwing an object up. I think you need to develop some sort of physical understanding of what is happening.

But, also, you have to develop the ability to use equations.

## 1. How do you determine the initial velocity of a vertical throw?

The initial velocity of a vertical throw can be determined by using the formula v = u + at, where v is the final velocity, u is the initial velocity, a is the acceleration due to gravity (9.8 m/s^2), and t is the time in seconds. Rearranging the formula to solve for u, we get u = v - at. This will give us the initial velocity of the vertical throw.

## 2. What is the importance of determining the initial velocity of a vertical throw?

Determining the initial velocity of a vertical throw is important because it allows us to understand the motion and trajectory of an object. This information is crucial in various fields such as sports, engineering, and physics. It also helps us make predictions about the future position and speed of the object.

## 3. What factors can affect the initial velocity of a vertical throw?

The initial velocity of a vertical throw can be affected by factors such as the angle of release, air resistance, and the mass of the object. The angle of release determines the direction of the initial velocity, while air resistance can slow down the object's speed. The mass of the object also plays a role in determining the initial velocity, as a heavier object will require more force to achieve the same initial velocity as a lighter object.

## 4. Can the initial velocity of a vertical throw be negative?

Yes, the initial velocity of a vertical throw can be negative. This means that the object is thrown downwards, towards the ground. In this case, the acceleration due to gravity will also be negative, as it is acting in the opposite direction of the initial velocity.

## 5. How does the initial velocity of a vertical throw relate to the final velocity?

The initial velocity and the final velocity of a vertical throw are related by the formula v = u + at. As the object moves upwards, the final velocity decreases until it reaches its maximum height, where the final velocity is 0. On its way back down, the final velocity increases until it reaches the initial velocity at the point of release. This relationship between the initial and final velocity is important in understanding the motion of the object.

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