CHAPTER-14 MORE ABOUT FORCES
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A force is defined as:
A push or a pull that acts on an object due to the interaction with another object
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Forces can have an effect on bodies in several different ways:
o They can change their speed
o They can change their direction
o They can change their shape
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Some examples of these changes are:
o An engine can increase the speed of a vehicle due to a force called thrust
o A comet’s direction can be affected by gravitational attraction
o A spring can have its shape changed by the force from a heavy load
Diagram showing the effects of forces on different objects
Types of Forces
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There are many types of force. Some examples include:
o Gravitational (or weight) – the force between any two objects with mass (like
the Earth and the Moon)
o Electrostatic – the force between any two objects with charge (like a proton
and an electron)
o Thrust – the force pushing a vehicle (like the push from rocket engines on the
shuttle)
o Upthrust – the upward force on any object in a fluid (like a boat on the
surface of a river)
o Air resistance (or drag) – the force of friction between objects falling through
the air (like a skydiver in freefall)
o Compression – forces that squeeze an object (like squeezing a spring)
o Tension – forces that stretch an object (like two teams in a tug-of-war)
o Reaction force – the force between any two objects in contact (like the
upwards force from a table on a book)
Several types of forces acting on different objects
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All forces can be categorised into one of two types:
o Contact forces
o Non-contact forces
Contact Forces
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A contact force is defined as:
A force which acts between objects that are physically touching
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Examples of contact forces include:
o Friction
o Air resistance
o Tension
o Reaction force / Normal force
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Friction:
o Is a force that opposes motion
o Occurs when objects rub against each
Air resistance:
o Is a type of friction
o Occurs when an object moves through air
Tension:
o Is a force that pulls two objects connected by a length, such as a string or rope
o Occurs when a force is applied to the length
Reaction force / Normal force:
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Is a force that pushes touching objects apart
Occurs when objects are supported by a surface
Examples of contact forces
Non-Contact Forces
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A non-contact force is defined as:
A force which acts at a distance, without any contact between bodies, due to the action
of a field
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Examples of non-contact forces include:
o Gravitational force
o Electrostatic force
o Magnetic force
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Gravitational attraction:
o The attractive force experienced by two objects with mass
o For example, the force between a planet and a comet
Electrostatic force:
o A force experienced by charged objects which can be attractive or repulsive
o For example, the attraction between a proton and an electron
Magnetic force:
o A force experienced between magnetic poles that can be attractive or repulsive
o For example, the attraction between the North and South poles of magnets
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Examples of non-contact forces
Scalar & Vector Quantities
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All quantities can be one of two types:
o A scalar
o A vector
Scalars
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Scalars are quantities that have only a magnitude
For example, mass is a scalar quantity since it is a quantity that has no direction to it
Vectors
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Vectors have both a magnitude and a direction
Velocity is a vector quantity since it is described with both a magnitude and a
direction
Examples of Scalars & Vectors
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The table below lists some common examples of scalar and vector quantities:
Newton’s Third Law
For every action (force), there is an equal and opposite reaction(force).
Forces exists in pairs.
When first object exerts a force on another object, the another object will exert an equal and
opposite force on the first object.
Examples:
Rest your elbow on the table. What is the pair of contact forces?
The force of elbow on the table = The force of table on the elbow.
What are the forces when you walk or run?
The force of the feet on the Earth = The force of the Earth on the feet
(moves the earth slightly backward) = (moves you forward)
The block resting on the table
First pair of forces
The contact force of block on the table= The contact force of table on the block
Second pair of forces
The force of gravity of the earth pulling down on the block=The force of the gravity of the
block pulling up the Earth.
Get a plastic water rocket and let the compressed air force out the water.
The force of the rocket on the water (downwards)= The force of the water on the rocket
(upwards)
Balloon in air
Force of air on the balloon(downwards) = force of balloon on the air (upwards)
Recoil of a gun
The force of a gun on the bullet (in forward direction) = The force of bullet on the gun (in
backward direction)
Resultant Forces
Calculating Resultant Force
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A resultant force is a single force that describes all of the forces operating on a body
When many forces are applied to an object they can be combined (added) to produce
one final force which describes the combined action of all of the forces
This single resultant force determines:
o The direction in which the object will move as a result of all of the forces
o The magnitude of the final force experienced by the object
The resultant force is sometimes called the net force
Resultant forces can be calculated by adding or subtracting all of the forces acting on
the object
o Forces working in opposite directions are subtracted from each other
o Forces working in the same direction are added together
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If the forces acting in opposite directions are equal in size, then there will be no
resultant force – the forces are said to be balanced
Diagram showing the resultant forces on three different objects
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Imagine the forces on the boxes as two people pushing on either side
o In the first scenario, the two people are evenly matched – the box doesn’t
move
o In the second scenario, the two people are pushing on the same side of the
box, it moves to the right with their combined strength
o In the third scenario, the two people are pushing against each other and are not
evenly matched, so there is a resultant force to the left
Friction
The force which opposes the motion of an object
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Frictional forces always act in the opposite direction to the object’s motion
Friction emerges when two (or more) surfaces rub against each other:
o At a molecular level, both surfaces contain imperfections – i.e. they are not
perfectly smooth
o These imperfections tend to push against each other
Air resistance (sometimes called drag) is a form of friction caused by a body moving
through the air
Friction (including air resistance) results in energy loss due to the transfer of energy
from kinetic to internal (heat)
The image below shows the force of friction acting on a downhill sledder:
Friction is a force which opposes an objects motion, acting in the opposite direction to it
Ways in which friction is helpful.
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Friction helps us to walk on the floor
Friction allows us to pick up this book with our hands.
When the driver of a car applies brakes, friction of the brake pads against the brake
drum slows the car.
Friction between the wheel and the ground keeps us from spinning the wheels, when
we ride a scooter.
Air friction (drag) slows down the parachute of a falling man so that he can land
safely.
Knots in string and the threads in your clothes are held together by friction
Nails and screws are held in wood by friction
Ways in which friction is harmful.
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Friction wears away the rubber on car tyres.
Due to friction holes are formed in the soles of the shoes.
Friction between wind and soil can cause soil erosion.
Reducing friction
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The slide in the park is polished smooth so that you can slide down easily.
Friction can be reduced by lubricating with oil
Another way of reducing friction is to separate the surfaces by air. This is how a
hovercraft works.
A fourth way of reducing friction is to have the object rolling instead of sliding. This
is what happens with ball bearings.
A fifth way: boats, cars, planes and rockets are streamlined to reduce friction with
water or air.
Unbalanced Forces
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Forces can combine to produce
o Balanced forces
o Unbalanced forces
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Balanced forces mean that the forces have combined in such a way that they cancel
each other out and no resultant force acts on the body
o For example, the weight of a book on a desk is balanced by the normal force
of the desk
o As a result, no resultant force is experienced by the book, the book and the
table are equal and balanced
A book resting on a table is an example of balanced forces
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Unbalanced forces mean that the forces have combined in such a way that they do
not cancel out completely and there is a resultant force on the object
o For example, imagine two people playing a game of tug-of-war, working
against each other on opposite sides of the rope
o If person A pulls with 80 N to the left and person B pulls with 100 N to the right,
these forces do not cancel each other out completely
o Since person B pulled with more force than person A the forces will be
unbalanced, and the rope will experience a resultant force of 20 N to the right
A tug-of-war is an example of when forces can become unbalanced
Unbalanced Forces, Mass & Acceleration
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When forces combine on an object in such a way that they do not cancel out, there is
a resultant force on the object
This resultant force causes the object to accelerate (i.e., change its velocity)
o The object might speed up
o The object might slow down
o The object might change direction
The relationship between resultant force, mass and acceleration is given by the
equation:
F=m×a
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Where:
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F = resultant force, measured in Newtons (N)
m = mass, measured in kilograms (kg)
a = acceleration, measured in metres per second squared (m/s2)
Weight
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Weight is defined as:
The force acting on an object due to gravitational attraction
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Planets have strong gravitational fields
o Hence, they attract nearby masses with a strong gravitational force
Because of weight:
o Objects stay firmly on the ground
o Objects will always fall to the ground
o Satellites are kept in orbit
Some of the phenomena associated with gravitational attraction and the weight force
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Weight, mass and gravitational field strength are related using the equation:
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This can also be written as:
W=m×g
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g is known as the acceleration due to gravity or the gravitational field strength
o On Earth, this is equal to 9.81 m/s2 (or 10 N/kg)
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The weight that an object experiences depends on:
o The object’s mass
o The mass of the planet attracting it
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Mass (measured in kilograms, kg) is related to the amount of matter in an object
Weight (measured in newtons, N) is the force of gravity on a mass
o The weight of an object and the mass of an object are directly proportional
o The size of this force depends on the gravitational field strength (often
called gravity, g, for short)
Free Body Diagrams
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Free body diagrams are useful for modelling the forces that are acting on an object
Each force is represented as a vector arrow, where each arrow:
o Is scaled to the magnitude of the force it represents
o Points in the direction that the force acts
o Is labelled with the name of the force it represents
Free body diagrams can be used:
o To identify which forces act in which plane
o To resolve the net force in a particular direction
Free body diagrams can be used to show the various forces acting on objects
Draw free-body diagrams for the following scenarios:
a) A picture frame hanging from a nail
Part (a)
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The size of the arrows should be such that the 3 forces would make a closed
triangle as they are balanced
Part (b) A helicopter moving up and forward (accelerating )
When the total force on the plane is in one direction, the force is called unbalanced.
An unbalanced force changes the motion of the plane. For instance, when thrust is greater than
drag, it is the unbalanced force that causes the plane to speed up or accelerate. Thrust > drag
In addition, as the velocity of the plane increases, the lift force increases and becomes the
unbalanced force that causes the plane to fly. Lift > weight
Part (c) A man fishing in a stationary boat
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As the boat is not moving, the size of both arrows must be the same
Upthrust = weight
Part (d) A car accelerating along a road
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As the car is accelerating, the size of the thrust must be larger than the size of
the friction force
As in part (c), the upwards and downwards forces must be equal
Terminal Velocity
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Falling objects experience two forces:
o Weight (due to gravity)
o Air resistance (due to friction)
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The force of air resistance increases with speed. This is illustrated in the image below:
Debbie initially accelerates downwards due to her weight. The upwards air resistance
increases as she falls until it eventually grows big enough to balance the weight force
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Initially, the upwards air resistance is very small because the skydiver isn’t falling very
quickly
o Therefore, there are unbalanced forces on the skydiver initially
As the skydiver speeds up, air resistance increases, eventually growing large enough
to balance the downwards weight force
Once air resistance equals weight, the forces are balanced
o This means there is no longer any resultant force
Therefore, the skydiver’s acceleration is zero – they now travel at a constant speed
This speed is called their terminal velocity
The weight of an object is the force of gravity which acts on it.
When something falls, initially it accelerates.
The faster it falls, the larger the force of friction which acts on it.
Eventually it falls at a steady speed when the force of friction equals the force of
gravity acting on it.