Why Is Initial Velocity Not Always Zero?

[askor]

There is something I don't understand. Isn't every object, if thrown, the initial velocity is zero? Please take a look at below example. Why the initial velocity in the below example is 98 ms^-1? Please explain. I also would like to ask what textbook contain a lot of basic physics example for better understanding? Please notice that I don't like Schaum's series.

 

[Nugatory]

Isn't every object, if thrown, the initial velocity is zero?

Say you throw something. At the moment that the object leaves your hand, it is moving at the same speed as your hand. How fast is that?

 

[kuruman]

There is something I don't understand. Isn't every object, if thrown, the initial velocity is zero? Please take a look at below example. Why the initial velocity in the below example is 98 ms^-1? Please explain. I also would like to ask what textbook contain a lot of basic physics example for better understanding? Please notice that I don't like Schaum's series.

If all thrown objects had zero initial velocity, what distinguishes an object that is thrown "hard" from another object that is thrown "less hard"? "Hardness of throw" in this context has a numerical value which is called "initial velocity."

 

[Mister T]

There is something I don't I also would like to ask what textbook contain a lot of basic physics example for better understanding? Please notice that I don't like Schaum's series.

Shaums is bad. Any college library should have something better. Just look for something you like.

 

[Delta2]

It depends on how you define the "initial" in the initial velocity. If you define it as the moment that the object is lying at rest in some inertial frame then yes usually all objects start from rest in some inertial frame. However in the context of the problem, the term "initial" is defined in a different way. There initial means the moment at the end of a small acceleration phase during which the ball accelerates from 0 to 98ms^-1. This phase is not even mentioned in the problem because we don't care about it, the only thing we care about it is the velocity at the end of this phase, which we consider to be the "initial" velocity, though in fact is the final velocity with regards to this small acceleration phase. This small acceleration phase might be for example when we take the ball from a table and we accelerate it with our hand to some final velocity. The initial velocity of this ball is at the moment is resting at the table and it is therefore zero, however for the purpose of the problem we consider as "initial" velocity the final velocity at which the ball leaves our hand.

 

[jbriggs444]

I think of initial velocity to be the velocity an object has when we start looking at it. If we start looking at time t=0 then it is based on the situation at or after t=0.

That is, it is the situation when the baseball is in the air, not when it was sitting on the ground prior to being picked up.

More formally, it is the one-sided limit of the ratio of displacement (x) to time (t):$$v(0) = lim_{t \to 0+} \frac{x(t) - x(0)}{t}$$

 

[kuruman]

I think of initial velocity to be the velocity an object has when we start looking at it. If we start looking at time t=0 then it is based on the situation at or after t=0.

That is, it is the situation when the baseball is in the air, not when it was sitting on the ground prior to being picked up.

More formally, it is the one-sided limit of the ratio of displacement (x) to time (t):$$v(0) = lim_{t \to 0+} \frac{x(t) - x(0)}{t}$$

I think of initial velocity somewhat differently. One can look at a moving object whenever one wants and for as long as one wants. If, however, one starts a clock (real or imaginary) that puts time stamps on the object's position, the velocity of the object at the instant the clock starts is the initial velocity.

 

[jbriggs444]

I think of initial velocity somewhat differently. One can look at a moving object whenever one wants and for as long as one wants. If, however, one starts a clock (real or imaginary) that puts time stamps on the object's position, the velocity of the object at the instant the clock starts is the initial velocity.

Yes, I agree. A key is that we put our blinders on and ignore everything prior to when we start looking.

If the prior velocity was something different, we care not.