Home / CIE AS & A Level Physics 4.1 Turning effects of forces Study Notes

CIE AS & A Level Physics 4.1 Turning effects of forces Study Notes

CIE AS & A Level Physics 4.1 Turning effects of forces Study Notes- 2025-2027 Syllabus

CIE AS & A Level Physics 4.1 Turning effects of forces Study Notes – New Syllabus

CIE AS & A Level Physics 4.1 Turning effects of forces Study Notes at  IITian Academy  focus on  specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Physics  Study Notes syllabus with Candidates should be able to:

  1. understand that the weight of an object may be taken as acting at a single point known as its centre of gravity
  2. define and apply the moment of a force
  3. understand that a couple is a pair of forces that acts to produce rotation only
  4. define and apply the torque of a couple

AS/A Level Physics Study Notes- All Topics

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4.1.1 understand that the weight of an object may be taken as acting at a single point known as its centre of
gravity

  • Consider n individual articles of an object of weight w present parallel to each other as shown below.

                                                                            

  • The total weight of the body if the sum of weights (W) of individual objects directed towards the center of Earth.

                                                                         

  • The centre of gravity (G) of a rigid body is the point through which the weight of the object appears to act.
  • The position of the centre of gravity is determined by (1) Distribution of mass and (2) Acceleration of free fall (g).

 

4.1.2 define and apply the moment of a force

  • The turning effect of force is called the moment of force.
  • Moment of force measures the tendency that rotates the body at a definite point.
  • Example, If you and your friend are pushing a door from both ends, it will create force but not moment of force. As long as you both are pushing, it will be classified as an equal force. However, once you opt out of pushing, your friend is the only force exerting party. As you leave from exerting any force, the door would twist and swing open. Here, the twist of the door’s swing openness is the moment of force. 
  • The moment of a force about a pivot is the product of that force and the perpendicular distance between the line of action of the force and the pivot.
  • The moment of a force can be worked out using the formula: Force Applied × Perpendicular distance from the pivot.

                                                                             

4.1.3 understand that a couple is a pair of forces that acts to produce rotation only

  • In physics, couple is a pair of equal parallel forces that are opposite in direction. Couples produce or prevent the turning of a body.
  • The forces used to turn the steering wheel of a car constitute a couple; each hand exerts a force, parallel but opposite in direction, yet they work together to achieve the same goal.
  • It tends to produce rotation only. There is no linear change in motion due to net/resultant force being zero. The pair of forces is non-concurrent. (i.e. their lines of actions do not pass through the a single common point)

4.1.4 define and apply the torque of a couple

  • When a driver turns a steering wheel, he exerts two equal but opposite forces on it. The two forces form a couple. 
  • The turning effect of a couple is the sum of moment of the two forces. The moment of a couple is called a torque.
  • The torque of a couple is the product of one of the forces and the perpendicular distance between their lines of action.
  • Torque is a measure of how much a force acting on an object causes that object to rotate.
  • Consider an object rotates about an axis, which we will call the pivot point.
                                                             
  • We will call the force ‘F’. The distance from the pivot point to the point where the force acts is called the moment arm, and is denoted by ‘r’. Note that this distance, ‘L’, is also a vector, and points from the axis of rotation to the point where the force acts.
  • Torque is calculated using the equation  τ = L x F = L F sinθ.
  • Using the right hand rule, we can find the direction of the torque vector. If we put our fingers in the direction of r, and curl them to the direction of F, then the thumb points in the direction of the torque vector.

What is a Force?

  • We can define a FORCE as a push or a pull due to the interaction between objects which produces or tends to produce motion; stops or tends to stop motion; changes or tends to change motion.
  • There are various forces experienced in our daily life, for example, gravitational force, electrical force, magnetic force, normal reaction, tension, friction, viscous force and upthrust.

Types of Forces

  •  Gravitational Force, Electric Force, Magnetic Force and Weak force.
  •  Four types of fundamental forces govern the physical universe. They are gravitational force, electromagnetic force, nuclear force and weak force.

The forces on mass and charge in uniform gravitational and electric fields.

Field

  •  In Physics, a field refers to a region of space within which a force is experienced.
  • There are several different types of forces that act on different types of “objects”. For all these types of forces, Newton’s laws of motion still apply.
  • A gravitational field due to a mass is a region of space within which a gravitational force is experienced by another mass. An electric field due a charge is a region of space within which an electric force is experienced by another charge. A magnetic field is a region of space within which a magnetic force is experienced by a moving charge.

                                   

Description

  •  Attractive force that acts between any two masses.
  •  Direction of force always in the direction of the external gravitation field strength

                   

  • Attractive or repulsive force that acts between any two electric charges.
  • For positive charges, direction of force is always in the direction of an external electric field
  •  For negative charges, direction of force is always opposite to the direction of an external electric field

     

  •  If the conductor is placed parallel to the magnetic field, no force is experienced.
  •  The direction of magnetic force is perpendicular to the magnetic field and is given by Fleming’s Left Hand Rule.

The origin of the upthrust acting on a body in a fluid

Upthrust acting on a body in a fluid

• A fluid will exert a force upward on a body if it is partly or wholly submerged within it. This is because the deeper into a fluid you go, the greater the weight of it and so the greater the
pressure.
• This difference in pressure between the top and the bottom of the object produces an upward force on it. This is called Upthrust.

Using the equation p = ρgh

A fluid is a collection of molecules that are randomly arranged and held together by weak cohesive forces and by forces exerted by the walls of a container i.e. it is a substance which can flow. Both liquids and gases are fluids.

Fluid pressure increases with depth. Water pressure increases with depth. Likewise, atmospheric pressure decreases with increasing altitude (height).
Consider a cylinder of fluid with height $h$ as shown in Figure 4,
The fluid pressure $p$ (at the base of container) is given by

                                               

where:

  • $\quad p$ is the pressure due to the fluid (liquid or gas) at depth $h$, (unit: $\mathrm{Pa}$ )
  •  $\rho$ is the density of the fluid, (unit: $\mathrm{kg} \mathrm{m}^{-3}$ )
  •  $g$ is the acceleration of free fall, (unit: $\mathrm{m} \mathrm{s}^{-2}$ )
  •  $\quad h$ is the depth of the fluid, (unit: m)

Note:

1. Pressure is a scalar quantity.
2. The fluid density $\rho$ is assumed to be constant (i.e. the fluid is incompressible)
3. The fluid pressure $p$ is equal (acting in all directions) at all points having the same depth, independent of the shape of the container.
4. The equation $p=\rho g h$ allows the pressure due to the fluid to be calculated. It should be remembered that the actual pressure at depth $h$ in a liquid would be given by

Sample Problem

Consider a uniform cylinder of cross-sectional area $A$ and height $h$ totally immersed in a liquid
The top and bottom face of the cylinder are at depth of $h_1$ and $h_2$ respectively from the surface of the liquid.
Pressure at the top face $=h_1 \rho g$
Force at the top face $=h_1 \rho g A$
Pressure at the bottom face $=h_2 \rho g$
Force at the bottom face $=h_2 \rho g A$
As different forces act on the two faces, a resultant force known as the upthrust is exerted on the cylinder.
Hence upthrust $=F_2-F_1$
$=h_2 \rho g A-h_1 \rho g A$
$=\rho g A\left(h_2-h_1\right)$
$=\rho g V$ where $V$ is the volume of the cylinder
$=m g$ where $m$ is the mass of liquid displaced by volume $V$
The upthrust on an object immersed in a liquid is equal to the weight of the liquid displaced by that object. This statement is commonly remembered as Archimedes’ Principle.

  •  If the weight of an object is greater than the upthrust, it will sink (Fig 5.3a).

                                       

If the weight of an object is equal to the upthrust, it will float (Fig 5.3b : This is known as the principle of floatation. A ship made of steel can float because its internal hollow volume displaces a large amount of water which in turn generates sufficient upthrust to keep the ship in equilibrium. A submarine can rise or sink at will because it contains ballast tanks that can expel or take in water. The variable upthrust produced controls the up and down motion of the submarine.

Fig 5.3 Object remains in equilibrium (floats) only when $U=W$

Frictional forces and Viscous forces

Frictional Forces

Friction is a force that opposes relative motion. It acts along the common surface in contact between the two bodies.
Frictional force arises in part from
(1) one peak physically blocking the motion of a peak from the opposing surface, even for surfaces which are apparently very smooth, as shown in the magnified view in Figure 8;
(2) chemical bonding of opposing points as they come into contact.

                     

Frictional forces are dissipative in nature as energy (which can otherwise be used to do useful work) is required to overcome them. This work done in overcoming friction is “wasted” in the form of thermal energy (heat) produced, which causes a rise in temperature. That is why your hands feel warm when rubbed together.

Note:
With all other factors kept constant,
1. a larger normal contact force will result in a larger frictional force (only for static friction).
2. the frictional force between any 2 surfaces in contact also depends on the nature of the surface (e.g. roughness of surface).
3. the frictional force between any 2 surfaces in contact is independent of their contact area.

Static Friction & Kinetic Friction

 

                                 

  •  There are two forms of friction, kinetic and static.
  • If you try to slide two objects past each other, a small amount of force will result in no motion. The force of friction is greater than the applied force. This is static friction.
  •  If you apply a little more force, the object “breaks free” and slides, although you still need to apply force to keep the object sliding. This is kinetic friction. You do not need to apply quite as much force to keep the object sliding as you needed to originally break free of static friction.

Viscous Force

Viscous force (Friction in fluids)
It is the frictional force acting on a body when the body moves through a fluid i.e. a liquid or gas and dissipative in nature. Examples of viscous force is air resistance acting on a moving car and object falling in air.
Viscous force arises in fluids because there are attractive forces between fluid molecules.
Viscous force increases proportionally with speed in laminar flow conditions (when each particle of the fluid follows a smooth path and the paths of each particle do not cross each other).

Above a critical speed, the fluid flow becomes irregular and turbulent and viscous force increases proportionally with the square of speed.

Comparison between Frictional Forces and Viscous Forces

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