Question: You have an irregularly shaped object. How can you use buoyancy to help you work out its density? Remember to draw a diagram. Ch 10 P73 A force acts on a rectangular cabinet weighing 400N...

Question: You have an i
egularly shaped object. How can you use buoyancy to help you work out its
density? Remember to draw a diagram.
Answer
A simple experiment can be done to measure the buoyant force on a rock (or any object) with a spring
scale. Suppose you have a rock with a volume of 1,000 cubic centimeters and a mass of three kilograms.
In air, the scale shows the rock’s weight as 29.4 newtons. The rock is then gradually immersed in water,
ut not allowed to touch the bottom or sides of the container. As the rock enters the water, the reading
on the scale decreases. When the rock is completely submerged, the scale reads 19.6 newtons.
Now density can be calculated just dividing mass of the object to the volume of water it displaces.
Ch 10 P73 A force acts on a rectangular cabinet weighing 400N (Figure P10.73). Assume the cabinet
slides with a constant speed when F = 200N and h=0.400m
a) Find the coefficient of kinetic friction and the position of the resultant normal force
Answer
First Normal Reaction acting on the block will be = mg = 400N
The net downward force on the surface is
400 - 208sin37° = 274.8 N
The horizontal force is 208 cos37°= 166.1N

166.1 =274.8µ
µ= 166.1/274.8 = 0.60 (Answer)
) Take F=300N, find the value of h for which the cabinet just begins to tip.
Answer
From the equili
ium of moments principle we have

clockwise = counterclockwise

Center of rotation at lower right corner.

h(300cos37°) = 400 x30

h = (400 x30)/300cos37°
h= 50cm (Answer)
Ch 11 P21 After our sun exhausts its nuclear fuel, its ultimate fate may be to collapse to a white dwarf
state, in which it has approximately the same mass as now but a radius equal to the radius of the Earth.
Calculate
a) The average density of the white dwarf
Mass of the sun = 2.0 x 1030
Radius of earth = 6400km
So, its volume will be = 4
3
??R3 = 1097 x 1018 m3
So , its density will be = mass/Volume
= 1.82 x 1012 kg/m3 (Answer)
) The freefall acceleration at its surface
Free fall can be calculated as
g = GM/R2
g = 6.67 x 10-11*2 x10 30/6.4 x 106x6.4 x 106
g = 3.256 x 106m/s2 (Answer)
c) The gravitational potential energy associated with a 1.00kg object at its surface
Gravitational Potential Energy at its Surface will be
U = -GMm

U = -2.09 x 1013 J (Answer)
Ch 15 P 23 A cube of wood having an edge dimension of 20.0cm and a density of 650 kg/m3 floats on
water.
a) What is the distance from the horizontal top surface of the cube to the water level?
Edge of cube = 20cm = 0.2 m
Density given = 650kg/m3
Density of water = 1000kg/m3
Density of cube is 65% density of water hence only 35% part of it will be above the water level.
Hence 35% of 20cm will be = .35 x 20 = 7cm
= 7cm (Answer)
) What mass of lead should be placed on top of the cube so that its top will be just level with the
water?
Since volume and density of the cube is known we can calculate its mass
Mass = volume x density
Mass = 0.008 x 650
Mass = 5.2kg
Density of substance needed = 1000kg/m3
Hence additional mass of cube required = 1000 x 0.008
= 8Kg
Hence we need
= 8 – 5.2 = 2.8Kg of additional mass of lead to be kept on top of cube so that the cube can float on water
surface
Mass of lead Required = 2.8kg (Answer)
Tute 5
Ch 19 P5 Three point charges are located at the corners of an equilateral triangle (Figure P19.5)
Calculate the resultant electrical force on the 7.00 microC charge.
Answe
Force acting on 7microC of charge is as shown in the diagram above,
From the superposition principle, we known
If we resolve the two forces in to X and Y component we get,
Hence Resultant force can be calculated as
Hence Net Resultant Force acting on the 7microC of charge will be 0.872N and will be directed below
the axis by 300.
Ch 20 P73 A...