A particle of mass m moving with velocity u makes an elastic one-dimensional collision with a stationary particle of mass m. They are in contact for a very short time T. Their force of interaction increases from zero to F0 linearly in time T/2 and decreases linearly to zero in further time T/2. The magnitude of F0 is:
1. mu/T
2. 2mu/T
3. mu/2T
4. None of these