Final answer:
The net force acting upon a weightlifter holding a stationary barbell is zero because the forces are in equilibrium. The upward force exerted by the weightlifter matches the downward gravitational force, resulting in no acceleration.
Step-by-step explanation:
The student has asked about the net force acting upon a weightlifter who is holding a barbell. To answer this, we first need to understand that when the weightlifter is holding the barbell stationary above his head, the forces acting on him are in equilibrium; this means that the net force is zero. The weightlifter has to exert a force upwards equal to the gravitational force pulling the combined mass of himself and the barbell downwards. This is because the net force on an object is equal to the mass of the object multiplied by its acceleration (F = ma), and since the weightlifter and the barbell are not accelerating, the acceleration is zero, which makes the net force zero as well.
The force due to gravity can be calculated using the formula F = mg, where m is the mass and g is the acceleration due to gravity (9.81 m/s² on Earth). The total mass is the sum of the weightlifter's mass and the barbell's mass, which is 77 kg + 120 kg = 197 kg. Therefore, the force due to gravity is F = 197 kg × 9.81 m/s² = 1933.57 N. To counteract this force and keep the barbell stationary, the weightlifter must apply an equal and opposite force of 1933.57 N upwards. As these two forces are equal and opposite, the net force is indeed zero.