In Figure 10-1, if the force exerted on a 3.0-kg backpack that is initally at rest is 20.0 N and the distance it acts over is 0.
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Answer:
The moment is -78.4 N-m (clockwise).
Explanation:
Given:
Mass of the object (m) = 20 kg
Distance of the object from the knee joint (d) = 0.4 m
Weight of leg is not considered.
Acceleration due to gravity (g) = 9.8 m/s²
Now, weight of the object is equal to the product of its mass and acceleration due to gravity. So,
Weight = Mass × Acceleration due to gravity
=
We know that, moment of a force about a point is defined as the product of force applied and the perpendicular distance between the point and the line of application of force.
Moment of the given weight about the knee joint is given as:
Moment about knee joint = Weight × Distance from knee joint to weight
Moment about knee joint = 196 × 0.4 = 78.4 Nm
Now, from the diagram below, we can observe that, the weight acts vertically down and thus the sense of rotation about the knee joint at point O is clockwise. So, moment is negative.
Therefore, the moment is -78.4 N-m (clockwise).
