Forces JPJC Notes

1 JURONG PIONEER JUNIOR COLLEGE 9749 H2 PHYSICS FORCES Content  Types of force  Centre of gravity  Turning effect of forces  Equilibrium of forces  Upthrust Learning Outcomes Students 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 forces and viscous forces including air resistance (no treatment of the coefficients of fri ction 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 force of upthrust acting on a body in a fluid. (m) state that upthrust is equal in magnitude and opposite in direction 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 in magnitude and opposite in direction to the weight of the object to new situations or to solve related problems.

2 1 INTRODUCTION A force is any interaction between two objects.  It must be acted by one object on another, and can either be a push or a pull.  It is a vector; it has both magnitude and direction.  The S.I. unit of force is the newton (N).  A combination of multiple forces acting on a body can change its motion, cause it to rotate or change its shape and size by stretching, compressing or twisting. In this topic, we will cover the common types of forces, the turning effect of forces as well as forces in equilibrium. The next topic Dynamics will cover how forces affect the motion of objects in detail. 2 TYPES OF FORCES At present, there are four known fundamental forces: 1. gravitational force – acts between all particles, and is the dominant force for shaping the large scale structure of galaxies, stars, etc, 2. electromagnetic force – acts between charged particles and accounts for the bonding energy of atoms and molecules, 3. weak force – responsible for radioactive decay, and 4. strong force – holds neutrons and prot