(BQ) Part 2 book "Mechanical engineering principles" has contents: Force, mass and acceleration; work, energy and power; friction, motion in a circle, simple harmonic motion, simple machines, heat energy and transfer, thermal expansion,.and other contents. | 13 Force, mass and acceleration At the end of this chapter you should be able to: • define force and state its unit • appreciate ‘gravitational force’ • state Newton’s three laws of motion • perform calculations F = ma involving force • define ‘centripetal acceleration’ • perform calculations involving centripetal mv 2 force = r • define ‘mass moment of inertia’ Introduction When an object is pushed or pulled, a force is applied to the object. This force is measured in newtons (N). The effects of pushing or pulling an object are: (i) (ii) to cause a change in the motion of the object, and to cause a change in the shape of the object. If a change occurs in the motion of the object, that is, its velocity changes from u to v, then the object accelerates. Thus, it follows that acceleration results from a force being applied to an object. If a force is applied to an object and it does not move, then the object changes shape, that is, deformation of the object takes place. Usually the change in shape is so small that it cannot be detected by just watching the object. However, when very sensitive measuring instruments are used, very small changes in dimensions can be detected. A force of attraction exists between all objects. The factors governing the size of this force F are the masses of the objects and the distances between their centres: m1 m2 F ∝ d2 Thus, if a person is taken as one object and the Earth as a second object, a force of attraction exists between the person and the Earth. This force is called the gravitational force and is the force that gives a person a certain weight when standing on the Earth’s surface. It is also this force that gives freely falling objects a constant acceleration in the absence of other forces. Newton’s laws of motion To make a stationary object move or to change the direction in which the object is moving requires a force to be applied externally to the object. This concept is known as Newton’s first law of .
