Calculating initial velocity for object

In summary, the conversation is about calculating the initial velocity, final velocity, and maximum height of a ball thrown directly upwards. The question is whether to use -9.8m/s^2 or +9.8m/s^2 for acceleration in the equations. The expert suggests always using -9.8m/s^2 for gravity acting downwards. The calculations for initial velocity and maximum height are confirmed to be 61.7m/s and 193 meters, respectively.
  • #1
babbagee
74
0
Calculating initial velocity for object...

I will give the problem first and then explain what i am having trouble with.

a.) With what speed must a ball be thrown directly upward so that it remains in the air for 12.6 seconds?
b.) What will be its speed when it hits the ground?
c.) How high does the ball rise?

a.) For the first part i just used the equation, v=v0 + at, and I assumed that v = -v0, so my question is do i use -9.8m/s2 or +9.8m/s2. But after thinking about it I think I should use -9.8m/s2 because when the ball reaches its maximum hight it falls back with a negative velocity, right. If someone else has another way of think about this please tell me. Oh, and I got 61.7 m/s, can some one confirm that.

b.) The velocity will be the same as the intial velocity but negative.
c.) I did this one and used -9.8m/s2, and i got 193 meters, can someone confirm that also.

I am having a hard time chosing weather to use -9.8m/s2 or +9.8m/s2. I guess what I am trying to say is what is a simple of thinking about this.

Thank you
 
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  • #2
In kinematics questions ALWAYS take g=-9.8m/s2.Reason being that gravity always acts downwards.So if you take downwards vel. as -ve you must also take g to be -ve.
 
  • #3
ok, i understand

can someone confirm my answers.

thanks
 

1. What is initial velocity?

Initial velocity is the speed and direction at which an object is moving at the beginning of its motion.

2. How do you calculate initial velocity?

The formula for calculating initial velocity is:
v0 = (vf - at)
where v0 is the initial velocity, vf is the final velocity, a is the acceleration, and t is the time.

3. What units are used for initial velocity?

Initial velocity is generally measured in meters per second (m/s) or feet per second (ft/s), depending on the unit system being used.

4. Can initial velocity be negative?

Yes, initial velocity can be negative if the object is moving in the opposite direction of its chosen positive direction. For example, if a car is initially moving north at 20 m/s and then starts moving south at 10 m/s, the initial velocity would be -20 m/s.

5. What factors can affect the calculation of initial velocity?

The calculation of initial velocity can be affected by factors such as air resistance, friction, and external forces acting on the object. These factors can cause the acceleration to vary and therefore impact the initial velocity.

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