# Homework Help: Acceleration HELP

1. Mar 2, 2010

### 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 dont 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.

Last edited: Mar 2, 2010
2. Mar 2, 2010

### Gunthi

Re: Acceleration HELP!!!

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

3. Mar 2, 2010

### ckaybabii

Re: Acceleration HELP!!!

what is F and m? sorry i havent learned that just yet.

4. Mar 2, 2010

### Char. Limit

Re: Acceleration HELP!!!

F is force, and m is mass.

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

5. Mar 2, 2010

### Gunthi

Re: Acceleration HELP!!!

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

6. Mar 2, 2010

### ckaybabii

Re: Acceleration HELP!!!

Okay but im not looking for Force or Mass though.

7. Mar 2, 2010

### collinsmark

Re: Acceleration HELP!!!

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.)

: 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.

Last edited: Mar 2, 2010
8. Mar 2, 2010

### Gunthi

Re: Acceleration HELP!!!

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.

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

9. Mar 2, 2010

### ckaybabii

Re: Acceleration HELP!!!

Okay thank you every 1 I will keep trying.