How Does Torque Affect Angular Momentum in Circular Motion?

[EV33]

Homework Statement

A 1.8-kg particle moves in a circle of radius 3.4 m. As you look down on the plane of its orbit, the particle is initially moving clockwise. If we call the clockwise direction positive, the particle‘s angular momentum relative to the center of the circle varies with time according to L(t)=10(N m s)-(4 N m)t

(a) Find the magnitude and direction of the torque acting on the particle.

Homework Equations

T net ext z= dL sys Z/dt

The Attempt at a Solution

Using this equation I would assume that all that is needed to find the net ext torque would be to take the derivative of L(t), and that is =-4N m s.

My book gets an answer of -4.9 N m s.

 

[EV33]

What is wrong with the method I used to try and solve the problem?