ASRJC Physics Forces Notes

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 4-1 Additional Notes Topic 4: Forces Learning Outcomes: Candidates should be able to: Types of forces (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 friction and viscosity is required.) Centre of gravity (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. Turning effects of forces (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. Equilibrium of forces (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. Upthrust (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. 1. How Submarines Work. Retrieved from (http://science.howstuffworks.com/transport/engines-equipment/submarine1.htm). 2. Cartesian diver. Retrieved from (http://physics.org/interact/physics-to-go/cartesian-diver/). 3. Biomechanics & High Jump. Retrieved from (https://www.topendsports.com/sport/athletics/biomechanics-highjump.htm)

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 4-2 Additional Notes • A force is a push or a pull, that is, a force is an interaction between two bodies or between a body and its environment. Hence we always refer to the force that one body exerts on another. • From the topic of Dynamics, Newton’s 2 nd Law states that the resultant force acting on a body is proportional to the rate of change of momentum of the body and the change in momentum takes place in the direction of the force. • Force is a vector quantity and requires both magnitude and direction for it to be fully defined. The SI unit for force is newton (N) and one newton is equal to 1 kg m s -2. • In the analysis of forces and interactions, the terms objects and systems will be used in various discussions. Objects, can be treated as having no internal structure or an internal structure that can be ignored. A system, on the other hand, is a collection of objects with an internal structure which may need to be taken into account. A.1 Types of interaction • There are four fundamental forces within all atoms that dictate interactions between individual particles, and the large -scale behavior of all matter throughout the Universe. They are the strong and weak nuclear forces, the electromagnetic force and gravitational force. Name Relative Strength Comment Example Strong nuclear 1 Large short-ranged forces between nuclear particles. Basically attractive, but can be effectively repulsive in some circumstances. The holding together of nucleons in a nucleus. Electromagnetic ~ 10-2 It is long-ranged but much weaker than the strong force. It can be attractive or repulsive and acts only between pieces of matter carrying electrical charge. Electricity, magnetism, and light are all produced by this force. Weak nuclear ~ 10-14 It is very short-ranged and is very weak. It is responsible for - decay (the conversion of a neutron to a proton, an electron and an antineutrino). Gravitational ~ 10-40 It is long-ranged but very weak. It is always attractive, and acts between any two pieces of matter in the Universe since mass is its source. Pull of Earth on Moon and Moon on Earth. A Types of Forces Force is a physical quantity that can change the shape of an object, the direction of its motion and its acceleration.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 4-3 Additional Notes A.2 Field of Force • A physical body can exert a force on another similar body not in contact with it. Such interactions can be explained using the field concept. • Field concept can be used to explain a physical phenomenon in terms of a field and the manner in which it interacts with matter or with other fields. • A Field of Force is a region of space surrounding a body within which it can exert a force on another similar body not in contact with it. (i) Gravitational Force • A body of mass M establishes round itself a region of space in which it has the ability (or potential) to exert a gravitational force on another mass. • The strength of the gravitational field established by mass, M, at a point in the region of space is given by g, and the gravitational force F exerted on another body of mass, m, placed at that point is given by F = mg. The gravitational force F is always attractive and points towards the mass M. (ii) Electric Force • A body of charge Q establishes around itself a region of space in which it has the ability (or potential) to exert an electric force on another charge. • The two kinds of charges responsible for electric forces are the positive and negative charges. Unlike gravitational forces, electric forces can be attractive or repulsive, depending on the signs of the charges involved. • Like charges repel and unlike charges attract. (iii) Magnetic Force • A magnetic field may exist in a region of space as a result of the presence of either a permanent magnet or a moving charge or current–carrying conductor. They are thus sometimes known as non-contact forces.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 4-4 Additional Notes A.3 Some common types of Forces (i) Contact Force When a body is in contact with another surface, the surface exerts a contact force on the body. This contact force has two components, namely the normal contact force (F N) and the frictional force (Fr). Normal Contact Force, FN • Consider a stone resting on a road. If the contact between the road a nd the stone is examined closely, it can be seen that the two rough surfaces make close contact only at relatively few places. Where contact is made, the road will exert a force on the stone as shown in Fig 1(a). • The sum of all these forces is shown in Fig 1(b) and this single force is the normal contact force which the road exerts on the stone. The normal contact force acts perpendicular to the surface that the body is in contact with and is always directed away from the surface towards the body. Frictional Force, Fr • For stationary body, the frictional force acts along the surface in a direction to prevent the body from sliding (or to oppose the tendency of motion). • For surfaces in relative motion, the direction is to oppose the relative motion of the surfaces. • Using the example of the stone above, i f the stone is sliding across the road (e.g. to the left), Fig 1(a) then changes to Fig 2(a). • The contact force can be considered as being the vector sum of a horizontal component (the frictional force) and a vertical component (the normal contact force). road st