#"The sum of the momentums before the hammer was thrown."#
#v_a=0#
#m_a=70 kg#
#v_h=0#
#m_h=4kg#
#P_b=m_a*v_a+m_h*v_h#
#P_b=70*0+4*0=0#
#"The sum of the momentums after the hammer was thrown."#
#m_a=70kg#
#v_a=3 ms^-1#
#m_h=4kg#
#v_h=v=?#
#P_a=m_a*v_a+m_h*v_h#
#P_a=70*3+4v#
#P_a=210+4v#
#"Since the momentum is conserved, we can write the following equation."#
#P_b=P_a#
#0=210+4v#
#-210=4v#
#v=-210/4#
#v=-52.5 ms^-1" should be thrown in the opposite direction."#