Final answer:
According to Newton's third law of motion, for every action force, there is an equal and opposite reaction force. The downward gravitational and normal forces on the weightlifter and the barbell by the Earth have corresponding opposite forces: the weightlifter's gravitational pull on Earth, the Earth's normal force on the weightlifter, and the weightlifter's lift on the barbell.
Step-by-step explanation:
A weightlifter holding a 20 kg barbell above his head and standing on the Earth experiences various forces due to gravity and normal force. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This can be applied as follows:
- The downward gravitational force from the Earth on the weightlifter is the action force, whereas the reaction force is the upward gravitational force from the weightlifter on the Earth.
- The downward normal force from the weightlifter on the Earth, as he stands on the ground, is the action force, and the reaction force is the normal force from the Earth pushing back up against the weightlifter, preventing him from moving through the ground.
- When the weightlifter holds the barbell, there is a downward normal force from the barbell on the weightlifter due to the weight of the barbell. The reaction to this force is the upward force exerted by the weightlifter's arms on the barbell, supporting its weight.