Physics 1st Year Chapter 4 Notes

• Work done by a constant force
• Work done by a variable force
• Work done by a gravitational field:  The space around the Earth in which its gravitational force acts on a body is called the gravitational field. When an object is moved in the gravitational field, the work is done by the gravitational force. If displacement is in the direction of gravitational force, the work is positive. If the displacement is against the gravitational force, the work is negative. Let us consider an object of mass m being displaced with constant velocity from point A to B along various paths in the presence of a gravitational force (Fig. 4.8). In this case the gravitational force is equal to the weight mg of the object.
• The work done by the gravitational force along the path ADB can be split into two parts. The work done along AD is zero, because the weight mg is perpendicular to this path, the work done along DB is (-mgh) because the direction of mg is opposite to that of the displacement i.e. e = 180°. Hence, the work done in displacing a body from A to B through path 1 is ADB=0+(-mgh) = - mgh If we consider the path ACB, the work done along AC is also (-mgh). Since the work done along CB is zero, therefore, WACB=mgh+0=-mgh.  Let us now consider path 3, i.e. a curved one. Imagine the curved path, to be broken down into a series of horizontal and vertical steps as shown in Fig. 4.9. There is no work done along the horizontal steps, because mg is perpendicular to the displacement for these steps. Work is done by the force of gravity only along the vertical displacements. as Hence, Was-mg(y+Ay₂ + + .. (Ay + A +A+.......+ y) =h WAB = -mgh +дул)  The net amount of work done along AB path is still (-mgh). We conclude from the above discussion that  Work done in the Earth's gravitational field is independent of the path followed. Can you prove that the work done along a closed path such as ACBA or ADBA (Fig. 4.8), in a gravitational field is zero? The field in which the work done be independent of the path followed or work done in a closed path be zero, is called a conservative field. 
• Important physics 1st year chapter 4 short questions from Punjab Textbook.
  
• The frictional force is a non-conservative force, because if an object is moved over a rough surface between two points along different paths, the work done against the frictional force certainly depends on the path followed.
• Definition of Power with formula: In the definition of work, it is not clear, whether the same amount of work is done in one second or in one hour. The rate, at which work is done, is often of interest in practical applications. Power is the measure of the rate at which work is being done. If work AW is done in a time interval Af, then the average power Pav during the interval Af is defined as P = W/t. If work is expressed as a function of time, the instantaneous power P at any instant is defined as P=Limit AW . Where AW is the work done in short interval of time At following the instant t.
• Important physics 1st year chapter 4 numericals from Punjab Textbook.
• Power and Velocity
• Energy and its type
• Kinetic energy and potential energy
• Work- Energy Principle
• Absolute Potential Energy
• Escape Velocity
• Interconservation of Potential energy and kinetic energy
• Conservation of energy
• Non-conservation energy sources
• Energy from Tides
• Energy from waves
• Solar energy
• Energy from biomass
• Energy Waste products
• Geothermal energy

Total Page  = 60 

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