RI Chap 2 Forces and Moments Lecture Notes

2 FORCES & MOMENTS H2 Physics 9478 Content Page 2.1 Type of Forces 2 2.2 Moment and torque 6 2.3 Translational and rotational equilibrium 8 2.4 Appendix 13 Learning Outcomes Candidates should be able to: (a) describe the forces on a mass, charge and current-carrying conductor in gravitational, electric and magnetic fields, as appropriate. (b) show a qualitative understanding of forces including normal force, frictional force, buoyant force (upthrust), and viscous force, e.g. air resistance (knowledge of the concepts of coefficients of friction and viscosity is not required). [Buoyant force (upthrust) not in H1 syllabus] (c) recall and apply Hooke’s law ( F = kx, where k is the force constant) to new situations or to solve related problems. (d) define and apply the moment of a force and the torque of a couple. (e) show an understanding that a couple is a pair of forces which tends to produce rotation only. (f) show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity. (g) apply the principle of moments to new situations or to solve related problems. (h) show an understanding that, when there is no resultant force and no resultant torque, a system is in equilibrium. (i) use free-body diagrams and vector triangles to represent forces on bodies that are in rotational and translational equilibrium.

RAFFLES INSTITUTION YEAR 56 PHYSICS DEPARTMENT Page| 2 2.1 Type of Forces To date, physicists recognize four fundamental forces in nature: 1. The gravitational force 2. The electromagnetic force 3. The strong nuclear force (not in syllabus) 4. The weak (nuclear) force All other forces we know can be derived from these four fundamental forces. Other than the gravitational force, most forces such as pushes, pulls and other contact forces like the normal reaction force and friction, are due to the electromagnetic force acting at the atomic level. In the H2 Physics syllabus, you will learn that a mass experiences a gravitational force in a gravitational field, a charge experiences an electric force in an electric field and a current carrying conductor experiences a magnetic force in a magnetic field. These forces will be covered in later topics.  Weight and Centre of Gravity For a body near the surface of the Earth, its weight W, is defined as the force experienced by a body of mass m in a gravitational field and can be expressed as =W mg where g is the gravitational field strength. This force acts vertically downwards towards the centre of the Earth through a single point on or outside the body known as its centre of gravity (C.G.). Fig. 2.1 The dot at the centre of the body is its centre of gravity. W W Definition The centre of gravity of a body is the point at which the weight of the body (or the resultant of the distributed gravitational attraction on the body) appears to act.

RAFFLES INSTITUTION Y