Analyzing the Motion of a Thrown Box Using Newton's Laws of Motion

[alexdude777]

Homework Statement

A 41-kg box is thrown at a speed of 235 m/s and is brought to a halt in a collision that lasts for a time of 6.8 ms. What is the magnitude of the average net force that acts on the box during the collision?

Homework Equations

Sum of Forces = m*a

The Attempt at a Solution

Well I tried t=6.8, v(o) = 235 m/s, v = 0, , and tried to solve for acceleration, which i got -34.56. but putting that in the sum of forces = m*a is not giving me the correct solution. i suck at physics :(

 

[Karlx]

Hi alexdude777.
Is it the time 6.8 seconds or miliseconds ?

 

[FARADAY JR]

step one of solving physics problem is to draw a picture, so where's your drawing? and then show your work. plus you can never have a (-) acceleration the (-) sign is just to show the direction.

 

[alexdude777]

It's definitely 6.8 milliseconds

Well I did make a drawing but how would I post it here? I mean it was just a box that I labeled 41-kg, with a vector towards the wall labeling the direction and velocity(235m/s), and then hitting a wall. Then I had t=6.8ms on the side

I am terrible at the stuff...I am going to fail :(

 

[tiny-tim]

welcome to pf!

hi alexdude777! welcome to pf!

Well I tried t=6.8, v(o) = 235 m/s, v = 0, , and tried to solve for acceleration, which i got -34.56.

as Karlx points out, you won't get 34.56 if you use .0068

 

[alexdude777]

Then I get it to equal -34558.823 ... there is NO way that can be right right? I was using that v=v(o) + at for the equation... am i even close to on the right track or no?

 

[Karlx]

Hi alexdude777.
Why do you think it cannot be right?

 

[alexdude777]

HOLY CRAP it was right! Wow. Thanks so much. I really appreciate it guys. Can I ask a projectile motion question here if I show you guys all the work I have tried?

 

[alexdude777]

Here is the question

A quarterback claims that he can throw the football a horizontal distance of 183 m (200 yd). Furthermore, he claims that he can do this by launching the ball at the relatively low angle of 35° above the horizontal. To evaluate his claim, determine the speed with which this quarterback must throw the ball.

Well I drew a picture. The line is x=183m, with theta being 35degrees. I need to find v(o)

I had v(ox)=v(x) = ?
a(x) = 0
t = ?
x = 183

v(oy) = ?
v(y) = ?
a(y) = -9.8m/s^2
t = ?
and I tried finding y with some trig...is it 159.53, the max height of the ball?

I guess I am just stuck here because I don't know where to go next, the book is not doing that great of a job explaining this or at least with a problem similar to this. I know I need to use the 35degrees and use trig. maybe to find the initial velocity in both the x and y direction? but i don't know what the length of the sides of the triangle would be, because the only other thing i have is how long x is. Any advice on how to get started?

 

[Karlx]

You are on the rigth way, alexdude.
You must see the motion of the ball as a composition of two kinds of motion: one in the horinzontal axis and another in the vertical axis.
Which kind of motion on the x axis?
Which kind of motion on the y axis?
Try to write the equations of motion for each axis.
And from it, up to you...