Answer:

Explanation:
= Rate at which the distance between A and starting point of B is changing = -20 km/h
= Rate at which the distance of B is changing = 15 km/h
= Rate at which the distance between A and B is changing
Time after which the rate at which the distance between A and B is changing is 4 hours
Distance covered by A in 4 hours = 
a = Distance remaining to the start point of B = 
b = Distance covered by B in 4 hours = 
Distance between A and B after 4 hours


Differentiating with respect to time we get

The rate at which the distance between the ships is changing at 4 PM is
.
Answer:
The velocity with which the mass will hit the floor is 
Explanation:
If the tension in the string is
, for
we have
,
and for the mass

From these equations we solve for
and get:

The kinematic equation

gives the final velocity
of a particle, when its initial velocity was
, and has traveled a distance
while undergoing acceleration
.
In our case
(the initial velocity of the particles is zero)

which gives us



which is the velocity with which the mass
will hit the floor.
Answer:
D. Freezing water and burning coal
Explanation:
Answer: a. Place the object on one side of a lever at a known distance away from the fulcrum. Place known masses on the other side of the fulcrum so that they are also paced on the lever at known distance from the fulcrum. Move the known masses to a known distance such that the lever is in static equilibrium.
d. Place the object on the end of a vertically hanging spring with a known spring constant. Allow the spring to stretch to a new equilibrium position and measure the distance the spring is stretched from its original equilibrium position.
Explanation:
The options are:
a. Place the object on one side of a lever at a known distance away from the fulcrum. Place known masses on the other side of the fulcrum so that they are also paced on the lever at known distance from the fulcrum. Move the known masses to a known distance such that the lever is in static equilibrium.
b. Place the object on a surface of negligible friction and pull the object horizontally across the surface with a spring scale at a non constant speed such that a motion detector can measure how the objects speed as a function of time changes.
c. Place the object on a surface that provides friction between the object and the surface. Use a surface such that the coefficient of friction between the object and the surface is known. Pull the object horizontally across the surface with a spring scale at a nonconstant speed such that a motion detector can measure how the objects speed as a function of time changes.
d. Place the object on the end of a vertically hanging spring with a known spring constant. Allow the spring to stretch to a new equilibrium position and measure the distance the spring is stretched from its original equilibrium position.
Gravitational mass simply has to do with how the body responds to the force of gravity. From the options given, the correct options are A and D.
For option A, by balancing the torque, the mass can be calculated. Since the known mass and the distance has been given here, the unknown mass can be calculated.
For option D, here the gravitational force can be balanced by the spring force and hence the mass can be calculated.
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