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Forces in Equilibrium
Forces in Equilibrium
What the forces are acting on the book?
TRK
Physics
SPM
Forces in Equilibrium
Although the book is lying stationary on the table,
that does not mean that there are no forces at work.
TRK
Physics
SPM
Forces in Equilibrium
The first force is the Earth’s gravitational pull.
This exerts a downward force, called the weight, on the book.
mg
Forces in Equilibrium
mg
For every action, there is an equal and opposite reaction.
Weight of book
Forces in Equilibrium
Therefore, the weight of the book acting downwards causes a reaction force acting upwards from the table.
mg
R
Forces in Equilibrium
Since these two forces are of equal magnitude and act in opposite directions, they balance each other.
mg
R
The forces are in equilibrium. There is no resultant force.
Weight of book
Forces in Equilibrium
Since these two forces are of equal magnitude and act in opposite directions, they balance each other.
mg
R
The forces are in equilibrium. There is no resultant force.
Weight of book
TRK
Physics
SPM
Forces in Equilibrium
Weight of girl
There is no resultant force.
The girl is in equilibrium.
Normal reaction of floor
Pearson Malaysia Sdn Bhd
Forces in Equilibrium
Since these two forces are of equal magnitude and act in opposite directions, they balance each other.
mg
R
Weight of book
Forces in Equilibrium
It is now moving across the table with uniform velocity.
What are the forces acting on the book?
Pulling force F
Forces in Equilibrium
mg
R
The resultant force along the vertical direction is zero.
Pulling force F
Forces in Equilibrium
A frictional force acts in the opposite direction of the pulling force.
mg
R
Weight of book
A frictional force exists between two surfaces in contact to oppose any motion.
Pulling force F
Forces in Equilibrium
As the book is moving with constant velocity, the resultant force in the horizontal direction is also zero.
mg
R
Pulling force F
Forces in Equilibrium
Now, if the book is moving on a table with smooth surface, we can assume that there is no friction.
mg
R
Pulling force F
Forces in Equilibrium
mg
R
Pulling force F
Forces in Equilibrium
Now, consider a wooden block resting on an inclined plane.
What are the forces acting on the wooden block?
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Forces in Equilibrium
We can resolve the force into two component forces.
mg
Forces in Equilibrium
We can resolve the force into two component forces.
mg
A force F can be resolved into two component forces which are perpendicular to each other.
Horizontal component
Vertical component
Forces in Equilibrium
We can resolve the force into two component forces.
mg
Suppose the angle the force F makes with the horizontal is .
F
Forces in Equilibrium
We can resolve the force into two component forces.
mg
Suppose the angle the force F makes with the horizontal is .
F
Forces in Equilibrium
mg
Pearson Malaysia Sdn Bhd
Forces in Equilibrium
Suppose the angle of inclination of the inclined plane is .
So, the two component forces are mg cos and mg sin .
mg
Forces in Equilibrium
One component of the weight of the wooden block is balanced by the normal reaction R:
mg cos
mg sin
Reaction R
Forces in Equilibrium
The other component of the weight is balanced by the frictional force F:
mg cos
mg sin
Reaction R
Friction F
Forces in Equilibrium
Therefore, the forces acting on the stationary wooden block are in equilibrium.
mg cos
mg sin
Reaction R
Friction F
End of presentation