H2_Forces_Lecture_Notes

9749 H2 PHYSICS Lecture Notes Nanyang Junior College 1 Chapter 4 FORCES Content  Types of force  Centre of gravity  Turning effects of forces  Equilibrium of forces  Upthrust Learning Outcomes Candidates should be able to: (a) recall and apply Hooke’s law ( F  kx, where k is the force constant) to new situations or to solve related problems (b) describe the forces on a mass, charge and current -carrying conductor in gravitational, electric and magnetic fields, as appropriate (c) show a qualitative understanding of normal contact forces, frictional forc es and viscous forces including air resistance (no treatment of the coefficients of friction and viscosity is required) (d) show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity (e) define and apply the moment of a force and the torque of a couple (f) show an understanding that a couple is a pair of forces which tends to produce rotation only (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 a vector triangle to represent forces in equilibrium (j) derive, from the definitions of pressure and density, the equation p  ρgh (k) solve problems using the equation p  ρgh (l) show an understanding of the origin of the upthrust acting on a body in a fluid (m) state that upthrust is equal to the weight of the fluid displaced by a submerged or floating object (n) calculate the upthrust in terms of the weight of the displaced fluid (o) recall and apply the principle that, for an object floating in equilibrium, the upthrust is equal to the weight of the object to new situations or to solve related problems.

9749 H2 PHYSICS Lecture Notes Nanyang Junior College 2 Types of Forces In general, forces can be categorized as either non-contact forces or contact forces. Non -contact Forces Gravitational Force Gravitational force is an attractive force exerted by one mass on another, 2 GMmF r Gravitational force is a force on a mass in a gravitational field, in the direction of the field, F = mg The weight (a.k.a. gravitational force exerted by earth on the object ) of an object placed in the gravitational field of the Earth is the gravitational force exerted by the Earth on the object. weight mass gravitational field strength W mg   The centre of gravity of a body is the point at which the entire weight (of body) appears to act. Electric Force Electric force is a force exerted by one charge on another. Like charges repel, unlike charges attract. 2 04 QqF r Electric forc e is a force on a charge in an electric field, in the direction of the field on a positive charge, opposite t he direction of the field on a negative charg