Someone give me a hint been at it for hrs

[sjcc]

Homework Statement

A ball is kicked from a location < 7, 0, -6 > (on the ground) with initial velocity < -9, 15, -3 > m/s. The ball's speed is low enough that air resistance is negligible.
What is the velocity of the ball 0.4 seconds after being kicked? (Use the Momentum Principle!)

Homework Equations

The Attempt at a Solution

 

[Pengwuino]

What are your thoughts on the problem? :)

 

[sjcc]

i've tried dividing the location of the ball by the 0.4 seconds. it was wrong. I thought of next to multiply the location by 0.4. but I am not sure if that is correct. i think there is something that needs to be done about "The ball's speed is low enough that air resistance is negligible".

 

[Pengwuino]

The fact that air resistance is negligible simply states that there is no friction. Dividing a location by a time is meaningless, as is multiplying a location by a time. Your velocity is of the form $$(v_x,v_y,v_z)$$ where the vertical direction is y. If your only under the influence of gravity, how does $$v_y$$ change over time?

 

[sjcc]

LaTeX Code: v_y should decrease over time.

 

[Pengwuino]

Yes, and in a gravitational field, you know it decreases by 9.8m/s^2. Thus, you can determine what the $$v_y$$ velocity is after the time given.

The question might want a different method though, what does the question mean by momentum principle?

 

[sjcc]

pfinal = pintial + the net force * the change in time.

 

[Pengwuino]

Well, that seems a little silly to use that method but ok. Remember, $$p = mv$$ so $$p_f = mv_i + F_{net} * \Delta t$$. You know the force of gravity on an object is $$F = mg$$ where g = -9.8m/s^2. Thus, you can easily determine the final momentum and thus, final velocity for the y-direction. Of course, the x and z velocities do not change as they are not affected by gravity.

 

[sjcc]

in the problem, there is no M stated.

 

[Pengwuino]

Just say m = M, it will cancel out since the mass does not change

 

[sjcc]

thanks. i was able to figure out the prob. j/w can you clear me up on graphs of motion? i am a bit confused on how to read them.

 

[Pengwuino]

What are you having problems with inregards to them?