Football player kinematics in one dimension

[ladolce]

Homework Statement

A football player, starting from rest at the line of scrimmage, accelerates along a straight line for a time of 3 seconds. Then during a negligible amount of time, he changes the magnitude of his acceleration to a value of 1.1 m/secondssquared. With this acceleration, he continues in the same direction for another 2 seconds until he reaches a speed of 6.4 m/sec. What's the value of his acceleration (assumed to be constant) during the initial 3 sec period?

Homework Equations

kinematics

The Attempt at a Solution

I tried drawing a graph but to no avail. How would I do it with just equations?

 

[Dick]

You'll have to pick out an equation to use first. As you are given times, velocities and accelerations - something relating the three of those could be handy. Can you find one?

 

[ladolce]

v=vinital+at
but i don't understand what 1.1 m/s sqrd is for...is that the acceleration during the 3sec-5sec time period? i don't know what to plug in

 

[Dick]

Good equation choice and yes, that's what the 1.1m/sec^2 is. So first concentrate on the 3-5 sec period. You know the final velocity (at 5 sec) and you know the acceleration. Can you determine the velocity at 3 sec? BTW write the equation as v=vinitial+a*(t-tinitial) where tinitial is 3 sec for this time period.

 

[ladolce]

Ok, so I got that stuff plugged in but the acceleration isn't there...what would that be?

 

[ladolce]

Ohhh ok, so 1.1 is the magnitude of the acceleration so that means it IS the acceleration from 3-5 seconds right?

 

[Dick]

Right. You may find it easier to work with your equation in the form (change of v)=a*(change of t).

 

[ladolce]

Ok, so i got 1.4 m/s^2 but i don't know if that's the right answer? can anyone do it and see what they get/help me? =)

 

[Dick]

Yes, that's right.

 

[ladolce]

Yay, thank you so much for your help =)