151
B: NEWTON’S LAWS
Unit 2: Free body diagrams
Unit 3: Newton’s laws of motion
Unit 1: Forces
Doc Scientia - IEB Senior Physics Textbook and Workbook Book 1
UNIT 1: Forces
1.1 Gravitational force 1.6.1 Factors
1.2 Normal force 1.6.2 Coefficient
1.3 Applied force 1.6.3 Reduced
1.4 Tension 1.6.4 Static friction
1.5 Air resistance 1.6.5 Kinetic friction
1.6 Friction 1.6.6 Application
Unit 1: Forces
Someone accidentally knocks over a tower of food tins in a shop. A force is exerted on the previouslystationary tins and they now move. A tin hits the floor and dents due to the force of the floor acting on the tin. A force:• is a pull or a push;• is a vector (note that the direction in which the force acts is very important);• has the symbol F;• is measured in newton (N);• can be represented by an arrow like any other vector quantity. - The length of the arrow represents the size, or magnitude, of the force. - The arrowhead points in the direction in which the force acts.
Contact and non-contact forces Forces
Non-contact:act over a distance.
Examples:1. Magnetic force2. Electrostatic force3. Gravitational force (Fg)
Contact: objects are in contact with each other.
Examples:1. Applied force2. Friction3. Normal force4. Tension5. Air friction6. Compression
If a unit that has been named after a person is written in words, only lower case letters are used, e.g. newton. When it is abbreviated, we use a capital letter, e.g. N.
QUICK FACTS
NEWTON’S LAWSB
Doc Scientia - IEB Senior Physics Textbook and Workbook Book 1152
The normal is always perpendicular to the surface, even if the object is on an inclined surface. It acts on the object resting on or against it.The normal force is a supporting force. It is equal in magnitude, but in the opposite direction to the force of the object acting on the contact surface.
1.1 Gravitational force
Weight is calculated as follows:Fg = mgg = gravitational accelerationClose to Earth’s surface, the approximate value of g = 9,8 m⋅s-2.
1.2 Normal force (FN)
1.3 Applied force
A person or object exerts a force on another object/person.The applied force is:• in the same line – for example forwards or backwards – as the direction of movement; or• at an angle to the direction of the movement. The components of the force can be calculated in the
direction of movement.
Weight (Fg) is the gravitational force that Earth exerts on any object on or near its surface.
GRAVITATIONAL FORCE
Normal force (FN) is the perpendicular force exerted by a surface on an object in contact with it.
NORMAL FORCE
Fg
FT = T
FN
f
An object must be in contact with a surface to experience a normal force.Objects falling through the air do not experience normal force.
QUICK FACTS
Components of a force acting atan angle to the horizontal.
Fx = F cosθFy = F sinθ
REMEMBER
Fx
Fy
Components of the gravitational force parallel and perpendicularto the slope:
Fg⊥ = Fg cosθFgll = Fg sinθ
REMEMBER
Fgll
Fg⊥
1Unit
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FORCES
1.4 Tension (FT or T)
When a cable or rope is pulled, tension is exerted on it.The tension in a rope is constant throughout, but the direction is not only in one direction.The mass of any rope/cable will be considered to be negligible, unless there is an instruction to take itinto consideration.
If you need to take the mass into consideration, the gravitational force will act on the centre of gravity of the rope.
1.5 Air resistance (Fair or Ff)
A resistant force exerted by the collisions of air particles on a moving object.Air friction always acts in the opposite direction to movement.
1.6 Friction force (f or Ff)
When an object moves, or tries to move on a surface. The contact surface exerts a frictional force on the object.
The force is always:• in the opposite direction to the movement, or attempted
movement;• parallel to the surface on which the object makes contact.
Friction is a contact force.It occurs when two objects touch each other and try to move over each other.A surface which looks smooth, has uneven particles at microscopic level.The surface of solids is fairly rough.When two objects move over each other (or try to move over each other), these rough parts of the surfaces get hooked onto each other. Friction acts against movement.
Friction (Ff) due to a surface is the force that opposes the motion of an object and acts parallel to the surface with which the object is in contact.
FRICTION FORCE
Frictional forces act parallel to the plane of motion, but are always in the opposite direction to the motion.
QUICK FACTS
Friction force
Static friction
Only for two objects at restrelative to one another.
Kinetic friction
Only for two objects in motionrelative to one another.
NEWTON’S LAWSB
Doc Scientia - IEB Senior Physics Textbook and Workbook Book 1154
1.6.1 Factors that influence the magnitude of the frictional force
Factors that influence friction:• normal force;• surface type.
1. The normal forceThe larger the forces that the two surfaces exert on each other perpendicular to the surface, the larger the friction.The normal is an indication of this. To calculate friction, we need to know what the normal is. The frictional force is directly proportional to the normal force ∴ f ∝ FN.
The frictional force is independent of the velocity at which the object moves.
QUICK FACTS
Examples
Rajesh pushes a crate across a rough surface, as indicated in the diagram. The applied force is labelled FRajesh and the friction is labelled f. At first, his force is not large enough to overcome thefriction, so he keeps increasing it.
Study the following table to understand how the friction varies:
Magnitude of appliedforce FRajesh (N) Crate Magnitude of
friction force f (N)Type of
friction force
40 At rest. 40 Static
60 At rest. 60 Static
80At the point of starting to move.
80 Static
82Accelerates to the right.
60 Kinetic
85Accelerates to the right.
60 Kinetic
Initially, the static frictional force is equal to the applied force and the crate is not moving.As the applied force increases, the static friction increases equally, to resist the movement.From the table it is clear that the static friction force reaches a maximum magnitude of 80 N.When Rajesh applies a force larger than the maximum frictional force, the crate starts to move and accelerate. The kinetic friction is now smaller than the maximum value of the static friction.As soon as the crate is in motion, the frictional force is smaller.
FN
FRajesh
Fg= 98 N
10 kg f