Fig. 6. The System is the cart (brown dot). The forces acting on the System are the force of gravity , the normal force and the tension of the rope . Energy is transferred to the System by the action of the forces doing work on the System. Work is often said to be equal to a force multiplied by a distance . This is a poor definition of work. A much better definition of the work done by a constant force causing an object to move along a straight line is to use the idea of scalar (dot) product work is a scalar quantity where the angle is the angle between the two vectors and . The angle between the two vectors is always a positive quantity and is always less than or equal to 180o hence . Work is a scalar quantity. However, its numerical value can be zero, positive or negative. If > 0, work is done on the object and if < 0, then W W work is done by the object. Work done by the gravitational force and by the normal force are zero because the angle between the force vectors and displacement vector is 90o (cos(90o) = 0).
Fig. 6. The System is the cart (brown dot). The forces acting on the System are the force of gravity , the normal force and the tension of the rope . Energy is transferred to the System by the action of the forces doing work on the System. Work is often said to be equal to a force multiplied by a distance . This is a poor definition of work. A much better definition of the work done by a constant force causing an object to move along a straight line is to use the idea of scalar (dot) product work is a scalar quantity where the angle is the angle between the two vectors and . The angle between the two vectors is always a positive quantity and is always less than or equal to 180o hence . Work is a scalar quantity. However, its numerical value can be zero, positive or negative. If W > 0, work is done on the object and if W < 0, then work is done by the object. Work done by the gravitational force and by the normal force are zero because the angle between the force vectors and displacement vector is 90o (cos(90o) = 0).