L 10 torque and rotational motion
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L-10 Torque and Rotational Motion. Torque makes things spin!. What makes something rotate in the first place?. TORQUE. How do I apply a force to make the rod rotate about the axel? Not just anywhere!. AXEL. TORQUE. To make an object rotate, a force must be applied in the right place.

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L-10 Torque and Rotational Motion

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L-10 Torque and RotationalMotion

Torque makes things spin!

What makes something rotate in the first place?


How do I apply a force to make the rod rotate

about the axel? Not just anywhere!



  • To make an object rotate, a force must be applied in the right place.

  • the combination of force and point of application is called TORQUE

lever arm, L


Force, F

Torque = force times lever arm

Torque = F  L

measured in N m

Torque example

What is the torque on a bolt

applied with a wrench that

has a lever arm of 30 cm

with a force of 30 N?


Torque = F  L

= 30 N  0.30 m

= 9 N m


For the same force, you get more torque

with a bigger wrench  the job is easier!

Homer attempts to straighten out the leaning tower of Pisa



Net Force = 0 , Net Torque ≠ 0

10 N

10 N

  • > The net force = 0, since the forces are applied in

  • opposite directions so it will not accelerate.

  • > However, together these forces will make the rod

  • rotate in the clockwise direction.

Net torque = 0, net force ≠ 0

The rod will accelerate upward under these

two forces, but will not rotate.

Balancing torques

20 N

10 N

0.5 m

1 m

Left torque = 10 N x 1 m = 10 n m

Right torque = 20 N x 0.5 m = 10 N m


  • To ensure that an object does not accelerate or rotate two conditions must be met:

  •  net force = 0

  •  net torque = 0

  • this results in the practical 4-1 “ladder rule”

When is an object stable?

  • If you can tip it over a bit and it doesn’t fall

  • The object may wobble a bit but it eventually stops and settles down to its upright position.

Why things fall over

  • Every object has a special point called the center of gravity (CG). The CG is usually right smack in the center of the object.

  • if the center of gravity is supported, the object will not fall over.

  • You generally want a running back with a low CG then it’s harder to knock him down.

  • The lower the CG the more stable an object is. stable  not easy to knock over!


Condition for stability

If the CG is above

the edge, the object

will not fall



when does it fall over?

If the vertical line

extending down from

the CG is inside the

edge the object will

return to its upright

position  the torque

due to gravity brings

it back.



Stable and Unstable



torque due to gravity

pulls object back

torque due to gravity

pulls object down

Stable structures

Structures are

wider at their

base to lower their

center of gravity

Playing with your blocks


If the center of gravity

is supported, the blocks

do not fall over

Object with low CG

300 lb fullback who is

4 ft, 10 inches tall

and runs a 4-40

Stay low to the ground!

High Profile



As more and more stuff is loaded into a

semi, its center of gravity moves upward.

It could be susceptible to tipping over.

The shape of an object determines how easy or hard it is to spin


For objects of the same mass, the longer

one is tougher to spin  takes more torque

It matters where the hinge is

The stick with the hinge at the end takes 4 times

more torque to get it spinning than the stick with

the hinge in the center.

Rotational Inertia (moment of inertia)

  • Rotational inertia is a parameter that is used to quantify how much torque it takes to get a particular object rotating

  • it depends not only on the mass of the object, but where the mass is relative to the hinge or axis of rotation

  • the rotational inertia is bigger, if more mass is located farther from the axis.

How fast does it spin?

  • For spinning or rotational motion, the rotational inertia of an object plays the same role as ordinary mass for simple motion

  • For a given amount of torque applied to an object, its rotational inertia determines its rotational acceleration  the smaller the rotational inertia, the bigger the rotational acceleration

Same torque,


rotational inertia

Big rotational


Small rotational






rotational inertia - examples

Suppose we have a rod of mass 2 kg and length

1 meter with the axis through the center

Its moment of inertia is 2 units

Imagine now that we take the same rod and

stretch it out to 2 meters; its mass is, of course,

the same.

Its moment of inertia is 4 units

rotational inertia examples

Rods of equal mass and length

axes through center

Rotational inertia

of 1 unit

axes through end

Rotational inertia

of 4 units

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