# Work

14 Feb 2022
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### Work

• 2. What is work? Work (W) Work is defined as a force acting upon an object to cause a displacement. It is expressed as the product of force and displacement in the direction of force. W=F x s. Work is done when a force produces motion. In the following picture you can see that a man is pushing the box with a certain force say 5N to cover a distance of 2m so his work done will be 10J. Joules is the SI unit of work = 1 KG m2/s2. Energy is capacity of a body to do work.
• 4. Work depends upon- The work done on a body depends upon two factors: Magnitude of the force (F), and. The displacement through which the body moves (s). We can now say that work done in moving body is equal to the product of the force exerted on the body and the distance moved by the body in the direction of force. When the body is moved on the ground by applying force, then the work is done against friction(which opposes motion of the body)
• 5. Unit of work Work is the product of force and distance. Now unit of force is newton(N) and distance is metre (m) so the unit of work is newton metre (Nm). When a force of 1 newton moves a body through a distance of 1m in its own direction, then work done is known as 1 joule. So, 1 joule = 1 newton 1 metre 1J = 1 Nm Thus the SI unit of work is joule(J). Work is a scalar quantity.
• 6. Work Done Against Gravity Climbing stairs and lifting objects is work in both the scientific and everyday sense—it is work done against the gravitational force. When there is work, there is a transformation of energy. The work done against the gravitational force goes into an important form of stored energy that we will explore in this section.
• 7. Let us calculate the work done in lifting an object of mass m through a height h, such as in Figure 1. If the object is lifted straight up at constant speed, then the force needed to lift it is equal to its weight mg. The work done on the mass is then W = Fd = mgh. We define this to be the gravitational potential energy (PEg ) put into (or gained by) the object-Earth system. This energy is associated with the state of separation between two objects that attract each other by the gravitational force. For convenience, we refer to this as the PEg gained by the object, recognizing that this is energy stored in the gravitational field of Earth.
• 8. Why do we use the word “system”? Potential energy is a property of a system rather than of a single object—due to its physical position. An object’s gravitational potential is due to its position relative to the surroundings within the Earth-object system. The force applied to the object is an external force, from outside the system. When it does positive work it increases the gravitational potential energy of the system. Because gravitational potential energy depends on relative position, we need a reference level at which to set the potential energy equal to 0. We usually choose this point to be Earth’s surface, but this point is arbitrary; what is important is the difference in gravitational potential energy, because this difference is what relates to the work done. The difference in gravitational potential energy of an object (in the Earth-object system) between two rungs of a ladder will be the same for the first two rungs as for the last two rungs.
• 9. W=MGH When the work is done against gravity. This formula will be used to take out work done when a body is being lifted
• 10. Question- The work done by a force acting obliquely is given by the formula: W = F cos θ ｘs.What will happen to the work done if angle θ between the direction of force and motion of the body is increased gradually? Will, it increases, decrease or remain constant? Solution- As we increase the force’s angle with respect to the direction of motion, less and less work is done along the direction that we are considering; and more and more work is being done in another, perpendicular, the direction of motion.