We will apply the concepts related to Newton's second law. At the same time we will convert everything to the system of international units.
[tex]m = 3500lb = 1587.57kg[/tex]
The values of the velocities are,
[tex]\text{Initial Velocity} = V_i = 0[/tex]
[tex]\text{Final Velocity} = V_f = 60mph = 26.822m/s[/tex]
We know that the acceleration is equivalent to the change of the speed in a certain time therefore
[tex]a = \frac{v_f-v_i}{t}[/tex]
[tex]a = \frac{26.822-0}{5}[/tex]
[tex]a = 5.36m/s^2[/tex]
Now applying the Newton's second law we have,
[tex]F= ma[/tex]
[tex]F = (1587.57)(5.36)[/tex]
[tex]F = 8516.36N[/tex]
Therefore the approximate magnitude is 8516.36N