Describe the forces acting on a sliding block.

The forces acting on a sliding block are gravity, friction, and any applied force.

The first force acting on a sliding block is gravity. This is the force that pulls all objects towards the centre of the Earth. It acts vertically downwards and its magnitude is equal to the mass of the block multiplied by the acceleration due to gravity (g). This force is always present, regardless of whether the block is moving or not.

The second force is friction. When a block slides on a surface, there is a resistance to its motion due to the interaction between the block and the surface. This resistance is known as friction. The frictional force acts parallel to the surface and in the opposite direction to the motion of the block. The magnitude of the frictional force depends on two factors: the nature of the surfaces in contact (characterised by the coefficient of friction) and the normal force (which is the force exerted by a surface perpendicularly, or 'normally', to the object resting on it). In the case of a sliding block, the normal force is equal to the weight of the block, which is the force of gravity acting on it.

Lastly, there may be an applied force. This is any external force that is applied to the block, such as a push or a pull. The direction of the applied force can vary depending on the situation. For example, if you push a block along a surface, the applied force is in the same direction as the motion. If you pull the block, the applied force could be at an angle to the surface.

In summary, a sliding block experiences a force due to gravity pulling it downwards, a frictional force resisting its motion, and any applied force. The net force on the block is the vector sum of these forces. According to Newton's second law of motion, this net force is equal to the mass of the block multiplied by its acceleration. Therefore, the motion of the block can be predicted by analysing these forces.