Page 109 -

Physics
Finding the balance: Equilibrium and moments

If several forces act on a lever, each force will contribute
Person B Person A
to its state of equilibrium, especially if they happen on
different sides of the pivot.

A force acting on a lever can rotate it in a clockwise
or counter-clockwise direction. And to keep a lever in
textbooks nis edu kz
equilibrium, the moments of force in a clockwise direction 2 m 1 m
should be equal to the moments of force in a counter- 500 N 1000 N
clockwise direction . This is called the rule of moments. pivot
M 1 = M 2

F 1 ∙ d 1 = F 2 ∙ d 2

Using the image above, we can calculate the clockwise and anticlockwise moments.
M1 = 500 N x 2 m = 1000 N · m

M2 = 1000 N x 1 m = 1000 N · m

As the moments have equal magnitude and opposite directions, these moments of force are
in balance. This means the seesaw is in static equilibrium and will not move. Of course, any
shift in the distance from the pivot will make the seesaw move, which is why it is so hard to
balance on a seesaw!

INV Determine the conditions for a lever’s equilibrium using a ruler, a wooden wedge pivot
and masses.

Can you find at least 4 different positions along the ruler that can be placed in static
equilibrium, with masses on either side and with different distances from the pivot? Record all
your attempts.

THINK ABOUT IT!

Give me a point to lean on and I can move the Earth.

Archimedes’ idea was as follows: The longer the lever, the greater
the force it can supply. Using a very long lever, a great force can
be obtained. So, could Archimedes lift the Earth? Would this be
possible? If so, how? Archimedes’ lever

OVER TO YOU!

1. Explain why door handles are attached 3. 10 N and 5 N vertical forces are applied to the ends
far away from the door’s hinge rather of a lever. The lever is 15 cm long and its mass can be
than in the middle of the door. disregarded. Find the position of the pivot if the lever is
in equilibrium.
2. Two equal forces F₁ and F₂ are acting on
a weightless lever. Turning force of F₁ 4. A 10 m rail weighing 9 kN is being lifted horizontally by
equals 2 Nm; turning force of F₂ equals two parallel ropes. Find the tension in the ropes if one
1 Nm. Will this lever be in equilibrium? rope is attached to one end of the rail and the other
rope is attached 1 m away from the other end of the rail.
109