I've come across this question and I'd just like someone to please clarify if this is correct.
When a stationary car is hit from behind by another vehicle at moderate speed, headrests behind the occupants reduce the likelihood of injury. Explain in terms of Newton's laws how they do this.
Newton's First Law of Motion:
Newton's First Law of Motion states than an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result. The headrest prevents this jolt, and thus prevents injury.
Newton's Second Law of Motion:
Newton's Second Law of Motion states that the force is directly proportional to the product of the mass and acceleration. But I have no idea how this relates to the question.
Newton's Third Law of Motion:
Newton's Third Law of Motion states that every action has an opposite and equal reaction. The stationary car is hit, and the person jolts forwards. The opposite and equal reaction would be for the passenger to jolt backwards. The headrest limits how far back the passenger jolts backwards, and so prevents any injuries.
Could someone please go over this and help me. I'd also like to be assisted in how I could apply Newton's Laws to questions (i.e. how I should go about explaining it on exams/SACs).
Thanks
Correct me if I'm wrong here (which I likely am); but I don't think it's necessary to restate the law and their properties. Especially in exam conditions -- do you? Otherwise, I would've just used key terms (as my former physics teacher would suggest) to demonstrate your understanding. For example, I would have wrote Newton's First Law's explanation would be something like this:
"
Newton's First Law of Motion states than an object will remain at rest or continue at cruising altitude unless acted upon by a non-zero force. When the stationary car is hit, a non-zero force acts on the car and so the individual's head would jolt as a result. The headrest acts against the movement of the head as it is in a state of inertia.Thus it prevents injury. "
In another light, Newton's Second Law pertains to the behaviour of objects having a netforce when force is unbalanced. In this case, it's all about the net force. Thus, the 'head rest provides support against the movement of the head as it now has a force. Headrests absorb the force. (or something similar)