Final answer:
The speed of the girder is constant. Therefore correct option is (3)
Step-by-step explanation:
The speed of the girder is constant.
When a heavy steel cable attached to a motor is lifting a girder, the tension in the cable is used to counteract the weight of the girder, allowing it to be lifted. As long as the motor maintains a constant tension in the cable, the speed of the girder will remain constant. This is because the tension in the cable is equal to the weight of the girder, and according to Newton's second law, if the net force on an object is zero, its acceleration (and therefore its speed) will be constant.