Let $$v_{\text{AB}}$$ represent the velocity of A wrt B. For example, if a car is travelling at $$40 \, \mathrm{km/h}$$ towards the East, we can say that $$v_{\text{CE}} = 40 \, \mathrm{km/h}$$. Here, C is the Car and E is the Earth.
An important equation is $$v_{\text{AB}} = -v_{\text{BA}}$$. So, if A is moving at $$10 \, \mathrm{km/h}$$ towards B, B is moving at $$- 10 \, \mathrm{km/h}$$ towards A.
We said that the velocity of the Car wrt Earth, $$v_{\text{CE}}$$ is $$30 \, \mathrm{km/h}$$. What is $$v_{\text{EC}}$$?
• $$30 \, \mathrm{km/h}$$
• $$-30 \, \mathrm{km/h}$$
• $$0 \, \mathrm{km/h}$$
• $$60 \, \mathrm{km/h}$$