Good Morning Everyone!

Hope your holidays have been great. I just had a small question, which may seem easy, but I haven't been able to find a simple answer for it. It is: *Why is the force that is exerted by the Sun on the Earth the same as the force exerted by the Earth on the Sun?*. I understand why the accelerations of both objects towards one another is different, but I still can't get my head around the meaning as to why the **force** is the same.

Thanks a lot!

Assuming you aren't just talking about struggling mathematically with the problem. I'll try to explain a couple different ways.

Newton's laws of motion are just facts, definitions and postulates taken about physical observations and experiments. Newton's 3rd law is just a consequence of observational evidence. It is what we see when we do the experiments. Newton's 3rd law is basically just the conservation of momentum, which may may it easier to see what kind of basic experiments allow us to infer this. This is why Newton's 3rd law is the way it is; because this is how we observe the universe acting.

Try to think about what would happen if the 3rd law was not true.

To ask the meaning of why the forces are the same though is a deep question. One way to try and look at this would be to understand why conservation of momentum is true (since N 3rd law is a direct consequence of it). To do this you would look at an idea called Noether's Theorem (outside the scope of VCE Physics), which states that general conservation laws (eg. conservation of momentum, conservation of energy, conservation of angular momentum) are results of symmetries of physical systems. If our system has translational symmetry (think shifting a graph eg. g(x) = f(x) +

**a**, where a is the shift), then the conserved quantity of the system is momentum. And translational symmetry is a consequence of the

*homogeneity of space*, which just means that space is the same everywhere. The length of something doesn't change if you put it in a different place. Not sure how to tie it in directly to your question, but examples like this may help explain why the universe acts the way it does.