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 protons together in a nucleus. Watch this… https://www.youtube.com/watch?v=a-6skWBuHaE This video from YouTube gives a summary of the four fundamental forces. Note: The weak and strong forces will not be covered in this syllabus. The gravitational and electromagnetic forces will be covered in greater detail in future chapters. The table below shows the two main types of forces that we covered in A Level Physics: Non-contact force (influences over distance) Contact force (requires contact to exert a force) Gravitational force Pull / Push Electric force Tension / Compression Magnetic force Frictional force / Viscous Forces Normal contact force Upthrust

3 2.1 Concept of field (b) describe the forces on a mass, charge and current-carrying conductor in gravitational, electric and magnetic fields, as appropriate. A mass can exert a force on another mass, even though there may not be any physical contact between the two masses. The same can be observed for two charges or two magnets. To explain this phenomenon where two bodies can interact with each other at a distance, the concept of field can be used. A field is a region in which a body experiences a force.  A mass p roduces a gravitational field in which another mass will always experience an attractive gravitational force (refer to Gravitational Field). o For example, the Moon experiences a gravitational force by the Earth because it is in the gravitational field of the Earth, and vice versa.  A charge p roduces an electric field in which another charge will experience a n electric force (refer to Electric Fields). o Like charges repel, while unlike charges attract. Watch this… https://www.youtube.com/watch?v=rPbx_XrrKLQ This video from YouTube gives a summary of the electric field. gravitational force Earth Moon Fig. 1 Gravitational forces exerted by the Earth and Moon on each other - Fig. 2 Attractive electric forces between two unlike charges + + Fig. 3 Repulsive electric forces between two like charges + gravitational force electric force electric force electric force electric force

4  Each pole of a magnet produces a magnetic field in which another magnetic pole will experience a magnetic force. o Two poles of the same polarity will repel each other, while those of unlike polarity will attract each other.  A current-carrying conductor or a moving charge in a m agnetic field will also experience a magnetic force (refer to Electromagnetism). The direction of the force is perpendicular to the directions of both the current or moving charge and the magnetic field, as shown in Fig. 4. 2.2 Contact forces When atoms of one object are too close to the atoms of another object, there will be a contact force between them.  All contact forces are electromagnetic forces in nature.  Examples of contact forces are pull, push, tension , compression, frictional forces, normal contact forces, viscous forces. Watch this… https://www.youtube.com/watch?v=yE8rkG9Dw4s This video from YouTube aims to address the question on when we touch an object, are we really in contact with that object? field current force current field force Fig. 4 Fleming’s Left-Hand Rule representing the magnetic force acting on a current-carrying conductor in a magnetic field

5 2.2.1 Tension and compression Under the influence of applied forces, every material deforms to some extent. The shape or volume of a material changes when external forces act on it. Hooke’s Law (a) recall and apply Hooke’s law (F = kx, where k is the force constant) to new situations or to solve related problems. Hooke’s Law states that the extensi on (or compression) of a material is directly proportional to the force required to extend (or compress) it, provided the limit of proportionality is not exceeded. Mathematically, it is written as Fx or F kx where F: load or force applied to the material x: extension in material k: proportionality constant, or force constant (S.I. unit: N m−1) F1 F1 bar bar (a) F2 F2 (b) Fig. 5 (a) A bar in tension, and (b) the bar under compression Fig. 6 A spring loaded vertically x 2x

6 The behaviour of stretching and compressing a material can be represented using a force-extension graph, as shown in Fig. 7. Note:  Hooke’s Law applies only within the limit of proportionality, indicated by the point P in Fig. 7. o The material extends linearly and elastically with increasing force within this limit. The material returns to zero extension when the force or load is removed. o Beyond this limit, force is no l onger proportional to extension, i.e. the material does not extend linearly. Howe