Physics: Work and Energy

Physics: Work and Energy 

 

• Fluid appears to provide partial support to the objects placed in it.

• When a body is wholly or partially immersed in a fluid at rest, the fluid exerts pressure on the surface of the body in contact with the fluid.
• The pressure is greater on lower surfaces of the body than on the upper surfaces as pressure in a fluid increases with depth.
• The resultant of all the forces is an upward force called buoyant force.
• For totally immersed objects the volume of the fluid displaced by the object is equal to its own volume.
• If the density of the immersed object is more than that of the fluid, the object will sink as the weight of the body is more than the upward thrust.
• If the density of the object is less than that of the fluid, it floats in the fluid partially submerged.
• The apparent weight of the floating body is zero.
• Archimedes’ Principle can be summarized as; ‘the loss of weight of a body submerged (partially or fully) in a fluid is equal to the weight of the fluid displaced’.
• Buoyancy is also known as upward thrust. Swimming in water and Flying of bird or aero plane
• Factors which affect buoyancy is Volume of the object and Density of the fluid
• Buoyancy or upward thrust exerted by a fluid increases with the volume of the object immersed on it
• The buoyant force or upward thrust increases with increase in density of the fluid.
• Denser liquid exert more upward thrust.
• This is the cause that it is easier to swim in sea water rather than fresh water
• Salts dissolved in sea water increase the density and hence it exerts more upward thrust than fresh water
• Mass per unit volume of an object is called density or mass density.
• Density is denoted by Greek letter rho (ρ).
• Density(ρ) =Mass/Volume; Density(ρ) =m/V; ρ = m/ V. Where,

• m is mass of object and V is the
• volume of that object.

• SI unit of density (ρ) is kg m−3; ρ=kg/m3; (The SI unit of mass is kg and SI unit of volume is cubic meter)
• Relative density - when density of a substance is expressed in comparison with water, it is called

• relative density

• Relative density =Density of substance/Density of water
• Relative density has no unit, because it is the ratio of similar quantity.
• When the relative density of a substance is less than 1, it will float in water otherwise it will sink in water.
• The relative density of ice is 0. 91, which means 9/10th of part is submerged in water.
• 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, Here, W= work done on an object F = Force on the object; s = Displacement of the object
• The unit of Work is Newton metre (Nm) or joule (J).
• 1 Joule is defined as the amount of work done by force of 1 N when displacement is 1 m.
• When both the force and the displacement are in the same direction, positive work is done. W = F x s
• when force acts in a direction opposite to the direction of displacement, the work done is negative. W= − F x s
• Angle between force and displacement is 180o.
• If force and displacement are inclined at an angle less than 180o, then work done is given as: W= Fs cos θ
• If force and displacement act at an angle of 90° then work done is zero.

 

• Two conditions need to be satisfied for work to be done: - 1) Force should act on the object. 2) Object must be displaced.
• Work is not done when - 1) A coolie carrying some load on his head stands stationary. , 2) A man is applying force on a big rock.
• Some cases force and displacement make some angle with each other is called - oblique displacement.