Susan pushes her dad, David, on an ice rink with a force of 30 N. She weighs 45 kg and her dad weighs 100 kg. What are the accelerations of Susan and David?

a(Susan) = 0.67 m/s², a(dad) = 0.3 m/s²

Explanation:

Assuming no other forces acting on both bodies in the horizontal direction (neglecting friction due to they are standing on ice), the interactions between them must obey Newton's 3rd law, so the magnitude of the force exerted by Susan on her dad (30N) must be equal to the one exerted by her dad on her.

So we can find the accelerations, just applying to their masses Newton's 2nd Law, as follows:

Fs = ms*as = 30 N

⇒ as =

Fd = md*ad = 30 N