Quiz 5: Work and Energy

Physics & Astronomy

(a) Yes. The weight lifter does positive work on the barbell. Work done on an object is equal to the product of the force img applied on the object and the displacement img of the object. img Here, img is the angle between force and displacement. Here, in this case the weight lifter applied force on the barbell in the upward direction which is against the gravitational force and the displacement of the barbell is also in the direction of application of force. Thus, the weight lifter does positive work in this clean procedure. (b) Yes, the weight lifter had done work in lifting the bare bell above his head. Here, also the displacement of the Barbell is in the direction of application of force. Thus, the weight lifter does positive work. (c) No work is done by the weight lifter is holding the bare bell above his head. Holding the barbell at the position which is above his head implies there is no displacement of the barbell even though the force is applied. Thus, no work is done on it. Thus, the work done is holding the bare bell above the lifter's head is less when compared to the work done by the lifter in raising the bare bell above his head. (d) Yes, work is done on the bare bell when it is falling down. The gravitational force on the barbell does positive work because its displacement is in the direction gravitational force.

The work done is defined as the force times the displacement and it is expressed as follows: img Here, F is the net force acting on the system and x is the displacement. The expression for the work done on the suitcase is, img Here, F is the net force exerted on the suitcase and x is the displacement covered by the suitcase. Rearrange the equation img for F. img Substitute img for img and img for img . img Therefore, the force exerted on the suitcase is img . The force is pointed along the displacement vector.

Concept: Work is the scalar product of force and displacement. And the word done by constant force on an object is equal to the product of the magnitude of the displacement and the component of force. img Units of work are img or Joule. If we express force as product if mass and acceleration img Units of work done are img . Therefore the choice is ( d ).