Q1.
But can we apply these laws to two or more objects connected together?
Answer
Yes. Two or more connected objects can be treated as a single system and Newton's laws applied to that system as a whole.
Forces inside the system (internal forces) — e.g. the tension T in the connecting string — need not be considered
Only forces from outside the system (external forces) matter — e.g. the applied force F
a = F ÷ (mass of the system) = F ÷ (m1 + m2) … Eq. 6.4
Only forces from outside the system (external forces) matter — e.g. the applied force F
a = F ÷ (mass of the system) = F ÷ (m1 + m2) … Eq. 6.4
Why internal forces can be ignored: by Newton's third law they always come in equal and opposite pairs — the string pulls Box 1 backwards and Box 2 forwards with the same T. Added over the whole system, each pair cancels, so they cannot change the motion of the system as a whole. The two boxes then accelerate exactly like one object of mass m1 + m2.
Tip: your own body is a system too. While walking, your arms and legs move in a complicated way, yet your overall motion can be studied by treating your body as a single object. Science often becomes simpler when we stop looking at parts and start looking at the whole.