Seesaws
We 2.1 are going to see how turning is different than going straight!
Recap: Work and Energy Work: transfer of energy
W=F d= energy transferred
Energy: the capacity to do work
Ugravity=m g h=gravitational potential energy
energy form 1
energy 2 energy 3 physics work= force distance
Do work to transfer from one form to another. Amount lost from 1 = amount gained by 2 and 3. Energy transferred = amount of work done If know energy change, know work and vice-versa!
You already know what it means for energy to be conserved!
Put money in into bank in one form (quarters). Get out money in other form ($20 bills). $ in = $ out
Why really useful to know that money is conserved going in and out of bank?
We dont have to keep track of details!
Ideas for today
Rotation Rotational inertia Torque
Things rotating and moving at the same time look complicated!!
But we can separate the two kinds of motion
Juggling clubs
The club has a center-of-mass that follows the same path as any other thrown object (like a ball).
On an extended object, gravity acts as if it is applied at the center of mass of the object.
Center of Mass (c.o.m)
The point about which an objects mass balances A free object rotates about its center of mass, while its center of mass follows the path of a falling object
Why does Mike look like hes floating?
Ever see his free-throw line dunk? (back in the day 1988)
Why does Mike look like hes floating?
Ever see his free-throw line dunk? (back in the day 1988)
The c.o.m. follows the trajectory of an object in free fall with an initial velocity: lifting his legs makes it possible to keep the head at constant hight!
Ballet dancers use the same trick!
The club is also rotating
Rotates around the center-ofmass Has angular speed and direction Have to exert a force away from the center-ofmass to get it spinning
Physical Quantities
Angular Position an objects orientation Angular Velocity its change in angular position with time Torque a twist or spin
Observations About Seesaws
A balanced seesaw rocks back and forth easily Two equal-weight children balance a seesaw Two unequal-weight children dont balance But moving the heavy child inward helps
Physics Concept
A force can produce a torque A torque produce can a force Torque = Lever Arm Force (where the lever arm is perpendicular to the force)
=rF
Another example of mechanical advantage
Clicker question
You are trying to open a door that is stuck by pulling on the doorknob in a direction perpendicular to the door. If you instead tie a rope to the doorknob and then pull with the same force, is the torque you exert increased?
A) Yes
B) No
All that matters is the force perpendicular to the lever arm and the length of the lever arm.
How do we explain why a car rotates when you brake hard?
Newtons First Law of Rotational Motion
A rigid object thats not wobbling and that is free of outside torques rotates at a constant angular velocity.
Physics Concept
Rotational Inertia
A body at rest tends to remain at rest. A body thats rotating tends to continue rotating.
Moment of inertia (rotational inertia) is big when mass is far from axis of rotation
Physical Quantities
Angular Position an objects orientation Angular Velocity its change in angular position with time Torque a twist or spin Angular Acceleration its change in angular velocity with time Moment of Inertia measure of its rotational inertia
The torque exerted on an object is equal to the product of that objects moment of inertia times its angular acceleration. The direction of the angular acceleration and the torque a...