p=mv where p is momentum in kgms-1, m is mass/kg and v is velocity/ms-1
Therefore to find the change in momentum we can just substitute the numbers in and find the difference.
At 5ms-1 the runner has a momentum of 5*40=200kgm/s
At 11ms-1 the runner has a momentum of 11*40=440kgm/s
Finding the difference, the change of momentum is 240kgm/s
