- Since, there is relative motion between the block and the surface,
- **Friction force** $f=\mu_k N$ $=\mu_k m g$
- $F=f=\mu_k m g$
- **4 forces acting on block**
- a) mg acting downwards.
- b) N =m g, normal reaction, acting upwards.
- c) $F=\mu_k m g$, applied in the forward direction.
- d) $f=\mu_k m g$, acting in the -x direction.
![image](https://temp-public-img-folder.s3.ap-south-1.amazonaws.com/sathee.prutor.images/subject-images/iitpal/image/physics-class-11-unit-06-chapter-01-work-energy-concepts-variable-forceskinetic-energy-l-1_6-8wv0qlipa48-09.jpg)