A –Level Physics:
Just a moment.
Objectives:
• Spec point 23: be able to use the equation for the moment of a
force, moment of force = Fx  where x is the perpendicular
distance between the line of action of the force and the axis of
rotation
• Spec point 24: be able to use the concept of centre of gravity of
an extended body and apply the principle of moments to an
extended body in equilibrium
Additional Skills Gained:
• Extra Skills Gained:
• Balancing moments
Logic Puzzle…
The Monty-Hall Problem
https://www.youtube.com/watch?v=9vRUxbzJZ9Y
Lesson Link: Fermi Question
An arrow is shot at 40o from the horizontal, how far does it go
both vertically and horizontally?
Now your turn…
Who reckons they are the strongest in
the class?! Also. A prize for anyone
who can determine who I am. You
really should know me…I’m pretty epic.
A little bit of history….
https://www.youtube.com/watch?v=0v
86Yk14rf8
10mins
A new geezer…
A prize for anyone who can determine
who I am. You really should know
me…I’m pretty epic.
A little bit of history….
https://www.youtube.com/watch?v=9s
dxiHyRrCs
10mins
A new geezer…
The class must decide on the
STRONGEST member of the class and
the WEAKEST member.
10mins
Just a moment..
• Will the strongest member of the class please step
forward. I propose a challenge.
With your little finger. Try to push the door
open. Your finger must be close to the hinge
Just a moment..
• Will the weakest member of the class please step
forward. I propose a similar challenge.
With your little finger. Try to push the door
open. Your finger must be close to the
handle
Moments
• A moment is the tendency of a force to cause
rotation. It is defined by the following equation
Moment (Nm) = Force (N) X Perpendicular distance (m)
(from the pivot to line of action of the force)
Force applied
Perpendicular Distance
To get the same rotation, why was it harder to push the
door from near the hinge?
Flashback
• “When forces on an object are balanced (in
equilibrium), the object is either…….”
Flashback
• “When forces on an object are balanced (in
equilibrium), the object is either…….”
• When the moments (clockwise and anticlockwise) on
an object then the object is either……
ROTATIONAL EQUILIBIRUM
Moments in action
After the demo, pick up a smaller version of the set up
and have a play. Use the knowledge you have gained to
keep it balanced. Then draw a diagram of one example
where it’s balanced taking necessary measurements
Practice Questions
A 2m plank (10kg) is placed upon a triangular pivot at
the 1m mark. On the left hand side a 6kg mass is
placed 0.2m away from the pivot. On the other a
mass is placed 0.8m away. The plank remains
balanced. Calculate the mass of on the right hand
side.
A
This time there are two masses each side. On the left
is a 1kg mass 0.8m away from the pivot and a 2kg
mass 0.4m away. On the right is a 2.5kg mass 0.3m
away and an unknown mass 0.8m away. Calculate the
value of the unknown mass if the plank balances.
Another 2m plank (10kg) is placed on a pivot as
shown. On the left, mass A is placed 0.4m away
from the pivot at the end of the plank. On the right,
a 1.8kg mass (mass B) is placed 1.2m away from the
pivot. Calculate the mass of mass A. The plank is now
balanced.
A
A
B
B
C
B
D
Extra Challenge
A 24cm spanner is used at 30o from the
horizontal with a force of 20N vertically
downward. What is the moment exerted on
the spanner?