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Adding forces
Adding forces
Consider the horizontal forces acting on the box in the
sketches below. (Remember the weight of the box is cancelled
by the upward normal force of the floor on the box.)
Consider a book lying on a table:
Force of book on Earth
book
Forces on object:
2N
2N
A
Weight Fg
3N
2N
B
Since the book is not moving, the forces are equal and thus
balanced.
3N
Resultant force on object
R=0N
A
R=1N
B
R=5N
C
The resultant of the 2 forces on the book is thus 0 N.
C
2N
The other pair of forces is the book on the table and the table on
the book.
3N
D
3N
R=2N
D
2N
In both cases – no resultant force.
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The inertia of an object is the tendency of
the object to resist a change in it’s state of
rest or of constant motion in a straight line.
Inertia is measured in kg.
The property of matter which maintains an
object’s state of rest, or its motion in a straight
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line, is called inertia. Law of inertia
Inertia
The beaker remains at rest since the force
due to friction between the paper and the
beaker does not overcome the inertia of the
beaker.
7 Inertia Demos
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Inertia & mass
A force is required to change the motion of an object.
Mass is the amount of matter contained by an object and
depends upon the number of atoms and their size.
This means that objects resist a change to their motion.
This ‘resistance to the change in motion’ of an object is known as
its inertia.
The bigger the mass of an object, the bigger its inertia.
The greater the inertia, the more difficult it is to accelerate the
object and the harder it is to stop it moving or change its
direction of motion.
If the baby and the baby seat are not properly
strapped in, the baby will continue to move
forward – as a result of its inertia – when the
brakes are applied sharply by the driver.
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Inertia is the tendency to resist change in an object’s motion.
Both mass and inertia are scalar quantities and are measured in
kg. They are equal to one another. Inertia & mass
A box having a mass of 5kg also has an inertia of 5kg.
This equal arm balance would be used to
measure the mass of an object.
However, the inertia would be measured by
establishing the acceleration an object would
have when a certain force is applied to the
object.
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Newton’s first law.
Definition:
Newton’s first law.
An object continues in a state of rest or uniform
(moving with constant) velocity unless it is acted
upon by an unbalanced (net or resultant) force.
Stationary:
2 important forces
– name them.
This means:
1.
If object has no forces acting on it, or if forces acting
on it are balanced (zero resultant) the object will
remain at rest forever.
2.
If object is moving and no resultant force acts on it, it
will continue to move in a straight line forever.
3.
If object moves at constant speed, the resultant
forces on the object are zero.
4 important forces –
name them.
Moving with constant velocity:
However, if a resultant force acts on the object, it will
accelerate in the direction of the resultant force.
Which forces are balanced?
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Newton's 1st law
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Newton’s first law.
Balanced & resultant forces
Force of table on box
Stationary:
For Newton’s first law there should be:
1. One, two or more pairs of balanced
forces acting on the object.
Fg
weight
2. The balanced forces only act on the
one object
Constant velocity:
Balanced & unbalanced forces
Force of table on box
3 N applied
3 N friction
Already moving
Fg
weight
Balanced forces on one object will cause
the object to remain at rest or move with
a constant velocity in a straight line.
Balanced forces
2N friction
Stationary:
Fg
weight
Force of table on box
2 N applied
Resultant force – acceleration:
3 N friction
Force of table on box
4 N applied
Fg
weight
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Newton’s first law.
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Newton’s second law of motion.
The Second law of motion is derived from the fact that
a resultant force produces an acceleration of an object.
N.B. Def:
Newton 2 experiment
Newton’s Second law of Motion:
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When a net force, Fnet, is applied
to an object of mass ,
m
m, it accelerates in the direction of the net force. The
acceleration, a, is directly proportional to the net force
and inversely proportional to the mass.
The shuttle ‘Atlantis’ is orbiting the Earth.
Discuss and explain the following questions:
1. Are the engines running?
Fnet∝ a
2. What forces are there on the shuttle?
3. What happens when the shuttle re-enters the
atmosphere?
and
a ∝ 1/m
i.e. Net or resultant force: Fnet = ma
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Newton's second law experiment Click here
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Newton’s second law of motion.
Fnet =
=
Alternative for Newton 2
This 2000 kg hot rod moves
from 20 m.s-1 to 30 m.s-1 in 5s.
Since
Find the resultant force applied
by the car engine.
and
v–u =a
t
then
Fnet = ma
ma = m(v f – vi)
t
2000 x (30 – 20) kg.m.s-1
5s
Fnet = ∆p = mv – mu
∆t
t
∆p = change
in momentum
Newton 2
When a resultant force acts on an object, the object
accelerates in the direction of the resultant force. The
acceleration is directly proportional to the resultant
force and inversely proportional to the mass.
= 4000 N in direction of motion.
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Newton’s Third Law
Force & free body diagrams
A box being pulled across the floor can be
shown as:
1. A force diagram – with arrows representing
applied
the force vectors:
friction
OR
weight
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Normal
2. A free body diagram. Object represented as a
dot with forces acting from the dot:
normal
applied
This is the free body
friction
diagram for the same
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weight
situation as above.
Newton’s Third Law
If object A exerts a 3 N
force on object B, then
A
B
Simultaneously object B exerts an equal and
opposite 3 N force on object A.
This is known as an action – reaction pair of
forces. Although they are equal and opposite,
they are not balanced forces – since they act
on different objects and not the same object.
Equal & opposite forces
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Newton’s Third Law
Consider a book on a table:
Consider this couple on ice skates.
If the man pushes on the lady with a force of
10 N, she pushes back on him with a force of
10 N.
Newton stated: To every action there is an equal but opposite
reaction.
If the book
exerts a 2 N
action force
on the table
then:
It is easier to explain and understand as follows:
N.B. Definition Newton’s Third Law:
The table exerts and equal and opposite
reaction force of 2 N on the book.
When object A exerts a force on object B, then object B
simultaneously exerts an oppositely directed force of equal
magnitude on object A.
Newton's third law
Newto 2 & 3
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Newton’s Third Law
Newton’s Third Law
The parachutist is in ‘free fall’.
A man pushes his vintage car
with a force of 400 N.
His weight is 1000 N (mass = 100 kg)
and the earth attracts him with a force
of 1000 N.
If this is the ‘action’ force, give
the size and direction of the
‘reaction’ force.
On what object would the reaction force act?
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If the maximum friction of the car is 300 N, will it accelerate?
Explain.
What prevents the man from being pushed backwards? Explain.
Action-reaction forces
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Newton’s Third Law
The parachutist also attracts the earth
with a force of 1000 N, but since the
earth is huge compared to the mass
of the man, the man is seen to fall
towards the earth.
Ignoring air friction, the parachutist falls towards the earth at a
rate of 9,8 m.s-2.
Terminal velocity
Here the Earth attracts the parachutist and simultaneously the
parachutist attracts the Earth with an equal and opposite
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reaction force.
Comparing Newton 1, 2 & 3
Newton 1
Newton 2
Newton 3
Space station orbiting
Earth pulls orbiting shuttle down.
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Shuttle pulls earth upwards.
If the weight of the shuttle is say 100 000 N,
this means that the earth pulls the shuttle
downwards with a force of 100 000 N.
At the same time the shuttle also exerts a
force of 100 000 N upwards on the earth.
Whether the objects move relative to one another, depends
upon the relative masses of the two objects. Here the mass of
the earth is huge relative to the shuttle – so the earth is not
seen to move up to the shuttle – rather the shuttle moves 21
down to the earth and that is why it stays in orbit.
Engine – 300 N
Engine – 400 N
Friction – 300 N
Friction – 300 N
One object –
One object –
forces Newton 1,2 & 3 resultant force
balanced, equal
– object must
and opposite
accelerate
Fnet = ma
Remain at rest or
Car must
move with
accelerate
constant velocity
Man on car –
200 N
Car on man –
200 N
2 objects – man
on car and equal
& opposite force
of car on man
Motion depends 22
on other forces
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