Which law provoked a strong reaction from northerners because they worried it would force them to participate in the system of slavery?

The law is Fugitive Slave Act of 1850

Explanation:

The Fugitive Slave Act of 1850 was a law passed by Congress in 1850. It compelled citizens of free Northern states (where slavery had been already abolished) to capture and return any slave that escaped from southern states. 

Northerners feared that the act which mandated them to return escaped slaves back to their southern slave masters would force them (Northerners) to participate in the system of slavery. 

As a result, the law was met with strong reactions from Northerners who strongly resisted it. Some states like Wisconsin and Vermont even passed measures that nullified the law while anti-slavery activists redoubled their efforts in assisting runaway slaves.