30 according to newton's second law of motion, if a rigid body of unchanging mass is observed accelerating, what must be happening?

a. a force is being applied to the mass

b. the mass must be changing

c. no force was or is being applied to the mass

d. a force was once applied but is now not being applied.

Answer is: A. A force is being applied to the mass.

Newton's second law of motion: F = m · a.

F -  force applied.

m - mass of the object receiving the force.

a - the acceleration of the object.

One newton is the force needed to accelerate one kilogram of mass at the rate of one metre per second squared in direction of the applied force.

The weight of an object is the amount of force acting on the object due to gravity. SI unit for weight is newton (N).

a mercury barometer is a device that uses the physical properties of mercury in a tube to measure barometric pressure. mercury barometers are used for : -

1. measurement of atmospheric pressure  

2. determining the altitude of a place

3. weather forecasting

4. calibration and checking of aneroid barometers

5. application in fluid mechanics

6. measurement of pressure in aircrafts

7. surface weather analysis

8. preparation of barographs

9. usage in physical sciences like physics, astronomy & chemistry

10. preparation of aircraft altimeters

characteristics of mercury barometer : -

1.used to define and measure atmospheric pressure.

2.on the average at sea level the column of mercury rises to a height of about 760 mm.

3.this quantity is equal to 1 atmosphere.

4.it is also known as standard atmospheric pressure.

5.a barometer in which the weight of a column of mercury in a glass tube with a sealed top is balanced against that of the atmosphere pressing on an exposed cistern of mercury at the base of the mercury column, the height of the column varying with atmospheric pressure.

< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >