Kinetics

A body at rest stays at rest and a body in motion stays in motion with the same speed and in the same direction unless the body is acted upon by an external force.

If resultant forces acting on a human body are zero, the body stays at rest or stays in motion with the same speed and in the same direction. Then the following holds true: If both the magnitude and the direction of a body’s velocity are the same, the vector sum of all forces acting on that body is zero. This hold true for all three directions of the Cartesian system of coordinates but in this textbook, to make things simpler, we will only work with two directions: vertical and horizontal.

Let us imagine that we are holding a barbell weighing 100 kg. With what force do we have to act on the barbell to keep it at rest? If we decide that the upward direction of the vertical axis will be marked as “positive”, then the gravitational force has negative direction and its magnitude is -981 N because it acts vertically downwards. To keep the barbell at rest we have to act with a reaction force of 981 N (in the opposite direction). At first sight it might seem odd that if we lift or drop the barbell with constant velocity, we still have to exert the force of 981 N with upward direction. If we wanted to lift the barbell with higher velocity, we would have to exert a force greater than 981 N. If we stopped exerting force on a barbell moving upwards, the barbell would start slowing down by 9,81 m/s². This slowing down is caused by gravitational force. If we wanted to slow the barbell down faster, we would have to act on the barbell with a force oriented downwards.