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Dynamics
1.
Apply Newton's Third Law to identify action-reaction pairs acting on two interacting
bodies (stating of Newton's Laws is not required)
●
Note carefully !
Action force and Reaction force always act on two ​different​ bodies.
Example 1
The earth exerts a force (Action force) on the man,
so the man exerts
an equal but opposite force (Reaction force) on the earth.
Example 2
The rock exerts
a force (Action force) on the table,
so the table exerts
an equal but
opposite force (Reaction force)
on the rock.
Example 3
The runner exerts a force (Action force) on the ground,
so the ground exerts
an equal but opposite force (Reaction force) on the runner.
This reaction force pushes the runner forward.
Example 4
The gun exerts a force (Action force) on the bullet,
so the bullet exerts
an equal but opposite force (Reaction force) on the
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gun.
Thus the gun recoils.
Example 5
Magnet A exerts a force (Action force) on magnet B,
so magnet B exerts
an equal but opposite force (Reaction force)
magnet A.
on
Thus, they attract and move towards each
other.
2.
Explain the effects of friction on the motion of a body
●
●
Friction is a force that tries to stop materials sliding across each other.
For two given surfaces in contact, the friction between them has a certain maximum or
limiting value.
Once this limiting friction is exceeded, sliding starts.
● There is also friction whenever an object moves through a fluid (liquids and gases).
● For example, when a car is travelling fast or a parachutist is falling through the air, there
is an opposing frictional force called air resistance.
● Air resistance increases as speed through the air increases.
Thus, no motion means no air resistance.
3.
Identify forces acting on an object and draw free body diagram(s) representing
the forces acting on the object
(for cases involving forces acting in at most 2 dimensions)
Example 1
The figure shows a
This free body
at rest on the top of a
diagram represents
table.
rock
all the forces acting
on the rock.
Example 2
The figure shows an
Free body diagram
aircraft moving
showing all the forces
through the air with
acting on the
a constant velocity.
aircraft.
2
Example 3
An applied force on the object
Free body diagram
causes it to accelerate to the
showing all
right along the rough ground.
the forces
acting on the
object.
4.
a
Apply Newton's First Law to describe the effect of balanced and unbalanced forces on
body (stating of Newton's Laws is not required)
●
●
When the forces acting on an object are balanced, we say that the external resultant force
(or unbalanced force or net force) acting on it is zero.
When
stay at
there is no resultant force acting on an object at rest, it will
rest.
Example
The rock at rest on the table top remains at rest because the
the rock is balanced by the reaction force from the table.
Thus, resultant force on the rock is zero and it stays at rest.
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weight of
●
When there is no resultant force acting on a moving object, it will continue to move with a
constant velocity (in a straight line with the same speed).
Example
The aircraft is moving.
Its weight is balanced by the lifting force from the wings.
The engine thrust is also balanced by air resistance.
Thus, resultant force on the aircraft is zero and it
continues to move with a constant velocity.
●
When there is a resultant force acting on an object, it will have an acceleration.
Example
The applied force is greater than
friction.
Thus, a resultant force acts on the
object
and it accelerates to the right.
●
5.
Thus, only when an external resultant force acts on an object would its state of rest or
motion be changed.
Apply Newton's Second Law to describe the ways in which a force may change the
motion of a body (stating of Newton's Laws is not required)
●
Only when an external resultant force acts on an object would its state of rest or motion be
changed.
● Thus, a force (specifically, a resultant force) can
►
change the speed of an object
►
change the direction of motion of an object
►
change the acceleration of an object
6.
Recall and apply the relationship ​resultant force = mass x acceleration to new
situations
or to solve related problems
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​
F
​m
​a
= resultant force in newton, N
= mass in kg
= acceleration in m/s​2
Note very carefully that, when using the above equation, ​F is the ​resultant force
( or unbalanced force or net force ) acting on an object.
Example
A wooden block of mass 2.0 kg is moving on a horizontal surface with an acceleration
of
1.5 m/s​2​. Given that the friction between the block and the surface is 4.0 N, determine
the force applied to the block.
F
=
ma
​P − ​f =
ma
=
(2.0 x 1.5) + 4.0
=
7.0 N
Thus, applied force, ​P
7.
Describe the motion of bodies with constant weight falling with or without air
resistance,
including reference to terminal velocity
For a body falling through the air
Weight of body
Without
air resistance
With
air resistance
constant
constant
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Air resistance
Resultant
downward force
on body
Acceleration of body
Velocity of body
Example
no
increases
as speed increases
constant
decreases to zero
constant
(≈10 m/s​2​)
decreases to zero
increases
without
limit
increases to a
constant maximum value
(terminal velocity)
​Key​ :
The series of diagrams on the right show
a parachutist falling through the air where
air resistance is not negligible.
Are you able to explain, in terms of the
forces acting on him, how his velocity and
acceleration change as he falls.
( Use the table and the diagrams to help you. )
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