A LEVEL PHYSICS
Year 1
Forces in Equilibrium
A* •I can explain the effect of balanced
A
B
C
forces on an object (Grade D/E)
•I can resolve three forces acting on a
point. (Grade B)
•Understand the conditions needed for a
body to be in equilibrium (Grade A)
To start
(from last week)
Have
Calculate the components of the weight of the boy and his
sledge parallel and perpendicular to the slope.
To start
(from last week)
Free-body force diagrams
•Shows a single body on its own.
•Includes all the forces that act on the body but
not the forces it exerts on the rest of the world.
•You only need to deal with coplanar forces.
From KS3 we should know that...
Balanced forces
.................... forces
................. speed
(could be stationary)
Changing speed
(......................./decelerating)
From KS4 we should know that...
The resultant force of two balanced forces is....
The resultant force of two unbalanced forces is....
A tennis ball at rest on court
A car moving at constant velocity
Draw a free-body
force diagram for the
following situations.
An object falling at terminal velocity
What is equilibrium?
When all forces acting on an object are balanced and cancel
out the object is in equilibrium.
When only two forces act, the
object is in equilibrium if they are
equal and opposite.
Does not have to be stationary to be in equilibrium.
• At rest
• Moving at a constant velocity
What is equilibrium?
Have a go at writing a definition...
State of an object when at
rest or in uniform motion.
An object is in equilibrium if
all the forces acting on it
cancel each other out.
Force vectors in a closed loop
The forces acting in equilibrium form a closed loop when
drawn tip-to-tail – vector polygon.
You won’t ever need to
balance more than 3 forces
so you only need to worry
about vector triangles.
Resolving vectors on a slope
(or inclined plane)
Resolving forces on a slope is
very useful and will crop up a
lot in mechanics. The two
perpendicular components
of a vector do not affect each
other. This means you can
deal with the two directions
completely separately (when
a force only affects one
vector you can ignore the
other).
Weight always acts
vertically down
(towards the centre
of the earth)
Equilibrium Worksheet
Marked for homework
To Finish
Look at the system that has been set-up
It is in equilibrium
1. Draw a free body force diagram for point P
2. Show that the resultant of F1 and F2 is equal and opposite
to F3
Next Lesson: Introduction to
Moments
Lesson after that: Advanced
Moments