A weight lifter lifts a barbell from the ground, raises it at a constat velocity and holds it motionless over his head. How does the force of the lifter compare with the wieght of the barbell at different stages of the lift?
w=F*d
d= 0m
0J....no work
when the barbell is being lifted its weight is different than when it is stationary
A weight lifter lifts a 550 N weight 1.25 m over his head in 2.5 seconds. What is the power of the weight lifter?
To understand how the force of the lifter compares with the weight of the barbell at different stages of the lift, we need to consider Newton's laws of motion.
1. When the lifter lifts the barbell from the ground:
- At this stage, the lifter exerts an upward force on the barbell, which exceeds the weight of the barbell.
- According to Newton's third law of motion, the barbell exerts an equal and opposite force downward on the lifter.
- The lifter must exert a force greater than the weight of the barbell to overcome gravity and lift it off the ground.
2. When the lifter raises the barbell at a constant velocity:
- Once the barbell is lifted off the ground, the lifter needs to maintain a constant velocity to prevent it from falling back down.
- At this stage, the force exerted by the lifter is equal to the weight of the barbell.
- This is because the lifter is supporting the barbell against the downward force of gravity, but since it is not accelerating, the net force is zero.
3. When the lifter holds the barbell motionless over his head:
- Finally, when the lifter holds the barbell motionless over his head, the force exerted by the lifter still equals the weight of the barbell.
- However, now the lifter is countering the force of gravity and preventing the barbell from falling to the ground.
- As long as the lifter maintains the barbell's position, the force they exert will match the weight of the barbell.
In summary, when the lifter lifts the barbell from the ground, the force exerted by the lifter is greater than the weight of the barbell. However, once the barbell is raised and held motionless, the lifter exerts a force equal to the weight of the barbell.