Calculating Average Speed and Acceleration for a Fastball

[ckaybabii]

Acceleration HELP!

A fastball that traveled to home plate (20 meters) in .4seconds.

a.what is the average speed of the ball?
b.if the catcher allowed his mitt to recoil backwards 7cm while catching the ball, what was the acceleration of the ball while it was slowed down by the catcher?
c.what amount of time was used to slow the ball down?

ATTEMPT
d/t=ave velocity= 50m/s^2
Vo=50m/s
Vf=0
t=.4s
d=20m
.07/25=.0028=t


-I don't know if my calculations are all correct please help me. I have the answer to a. which is 50m, just not to b, and c.

 

[Gunthi]

a. average speed is defined as the time taken to travel a certain distance.
b. $$F=m\frac{dv}{dt}$$
c. see b

 

[ckaybabii]

what is F and m? sorry i haven't learned that just yet.

 

[Char. Limit]

F is force, and m is mass.

I don't see how that would help in this case, as you have none of either.

 

[Gunthi]

It's called the Impulse Theorem, or Newton's Second Law. F is the force, m is the mass of the body and dv/dt is the acceleration.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

 

[ckaybabii]

Okay but I am not looking for Force or Mass though.

 

[collinsmark]

There a couple of kinematics equations that will help you. I highly suggest memorizing these (there are a few others that definitely worth memorizing too).

Assuming a constant acceleration,

$$s = \frac{v_f + v_i}{2}t + s_i$$

and

$$s = \frac{1}{2}at^2 +v_it +s_i$$

(In many problems, $$s_i$$ is zero. And often times, either $$v_i$$ or $$v_f$$ is zero. But it doesn't hurt to memorize them in their full form.)

[Edit]: These equations might be in a slightly different form in your textbook (or your instructors notes). In order to stay consistent with the class material, I suggest using textbook's (or your instructor's) notation.

 

[Gunthi]

Oh, sorry I misread the question.

Because time isn't explicit you have to use some kinematic equations.
Try to get a time-independent relation between d, v and a from these relations.

$$d=v_it+\frac{1}{2}at^2$$
$$v_f=v_i+at$$

 

[ckaybabii]

Okay thank you every 1 I will keep trying.