From our everyday experience, we know that we get more exhausted while walking up the stairs compared to going down. Similarly, while walking uphill we become more tired if the path is a steeper one. Is there any relation between these two?
Our earth attracts everything and it pulls all objects towards its centre. This feature is known as the force of gravity of the earth. In fact, this attractive force towards the centre of earth keeps everything in its place on the surface of earth. In the absence of force of gravity, we would have been floating in the atmosphere of earth. So to move away from the surface of the earth, we have to overcome this attraction. In other words, we have to use our force against the force of gravity.
While walking uphill, to carry the weight of the body, our muscles do extra work against the force of gravity. To support the effort of the body, heart pumps more blood to the cells. More carbon dioxide is generated from faster breathing. Lungs have to do additional work to push out the carbon dioxide and absorb more oxygen from outside. Hence during steep climb, we breathe more quickly and deeply.
Now, suppose there are two routes to climb a hill. One steeper compared to the other. We feel more fatigue when we walk by the steeper route. Compared to walking in a horizontal plane, more energy is to be spent for walking in a steeper route. The extra energy required is the weight of the body multiplied by the height to be climbed. When we walk down a hill, energy to be spent is less as the body automatically gets attracted by the gravitational pull of earth. Hence muscles are more relaxed and they do only less work.
Now, another question. Why do we have to exert more to lift a heavy article compared to a light one? Yes, your answer is correct. The gravitational pull is more in the case of a heavy object and hence more work is to be done to lift a heavy object.