A 8.4-kg block is lifted vertically 9.0 m at constant speed by a boy. The work done by the boy is about:
work = force * distance
force = mg
To calculate the work done by the boy in lifting the block, we can use the formula:
Work = force * distance * cos(theta)
Here, force is the force exerted by the boy to lift the block, distance is the vertical distance the block is lifted, and theta is the angle between the force and the direction of motion (which is 0 degrees in this case because the force is exerted vertically).
In this scenario, the force exerted by the boy is equal to the weight of the block, which can be calculated using the formula:
Weight = mass * gravity
where mass is the mass of the block and gravity is the acceleration due to gravity (approximately 9.8 m/s^2).
So, the weight of the block would be:
Weight = 8.4 kg * 9.8 m/s^2 = 82.32 N
Now, we can use the formula for work:
Work = Force * Distance * cos(theta)
Work = 82.32 N * 9.0 m * cos(0°)
Since the angle between the force and the direction of motion is 0 degrees, cos(0°) is equal to 1.
Work = 82.32 N * 9.0 m * 1
Work = 741.84 Joules
Therefore, the work done by the boy in lifting the block vertically 9.0 m at a constant speed is approximately 741.84 Joules.