Question

what is the effect of non - conservative forces like friction on energy? they do positive work. they have no effect on energy. they do negative work. they increase the total energy.

Explanation:

Non - conservative forces like friction act opposite to the direction of motion. Work is calculated as \(W = \vec{F}\cdot\vec{d}=Fd\cos\theta\), where \(\theta\) is the angle between the force \(\vec{F}\) and displacement \(\vec{d}\). For friction, \(\theta = 180^{\circ}\), and \(\cos(180^{\circ})=- 1\). So, \(W=-Fd\), which means they do negative work. Also, according to the work - energy theorem \(W_{total}=\Delta K\). Non - conservative forces remove mechanical energy from the system (convert it into non - mechanical forms like heat), which is consistent with negative work (as work done by the force is energy leaving the system in the context of mechanical energy).

Answer:

They do negative work.