Presentation on theme: "Chapter 9 Objectives Calculate the torque created by a force."— Presentation transcript:
1 Chapter 9 Objectives Calculate the torque created by a force. Define the center of mass of an object.Describe a technique for finding the center of mass of an irregularly shaped object.Describe the relationship between torque, angular acceleration, and rotational inertia.
2 Chapter 9 Vocabulary Terms torquecenter of massangular accelerationrotational inertiarotationTranslationCenter of rotationRotational equilibriumlever armcenter of gravitymoment of inertialine of actionCentripetal forceCentrifugal Force
3 9.1 TorqueKey Question:How does force create rotation?
4 9.1 Torque A torque is an action that causes objects to rotate. Torque is not the same thing as force.For rotational motion, the torque is what is most directly related to the motion, not the force.
5 9.1 TorqueMotion in which an entire object moves is called translation.Motion in which an object spins is called rotation.The point or line about which an object turns is its center of rotation.An object can rotate and translate.
6 9.1 TorqueTorque is created when the line of action of a force does not pass through the center of rotation.The line of action is an imaginary line that follows the direction of a force and passes though its point of application.
7 9.1 TorqueTo get the maximum torque, the force should be applied in a direction that creates the greatest lever arm.The lever arm is the perpendicular distance between the line of action of the force and the center of rotation
9 9.1 TorqueLever arm length (m)t = r x FTorque (N.m)Force (N)
10 9.1 Calculate a torqueA force of 50 newtons is applied to a wrench that is 30 centimeters long.1) You are asked to find the torque.2) You are given the force and lever arm.3) The formula that applies is τ = rF.4) Solve: τ = (-50 N)(0.3 m) = -15 N.mCalculate the torque if the force is applied perpendicular to the wrench so the lever arm is 30 cm.
11 9.1 Rotational Equilibrium When an object is in rotational equilibrium, the net torque applied to it is zero.Rotational equilibrium is often used to determine unknown forces.
12 9.1 When the force and lever arm are NOT perpendicular
13 9.2 Center of MassKey Question:How do objects balance?
14 9.2 Center of MassThere are three different axes about which an object will naturally spin.The point at which the three axes intersect is called the center of mass.
15 9.2 Finding the center of mass If an object is irregularly shaped, the center of mass can be found by spinning the object and finding the intersection of the three spin axes.There is not always material at an object’s center of mass.
17 9.2 Finding the center of gravity The center of gravity of an irregularly shaped object can be found by suspending it from two or more points.For very tall objects, such as skyscrapers, the acceleration due to gravity may be slightly different at points throughout the object.
18 9.2 Balance and center of mass For an object to remain upright, its center of gravity must be above its area of support.The area of support includes the entire region surrounded by the actual supports.An object will topple over if its center of mass is not above its area of support.
20 9.3 Rotational Inertia Key Question: Does mass resist rotation the way it resists acceleration?
21 9.3 Rotational InertiaInertia is the name for an object’s resistance to a change in its motion (or lack of motion).Rotational inertia is the term used to describe an object’s resistance to a change in its rotational motion.An object’s rotational inertia depends not only on the total mass, but also on the way mass is distributed.
22 9.3 Linear and Angular Acceleration Angular acceleration (kg)Linearacceleration(m/sec2)a = a rRadius of motion(m)
23 9.3 Rotational InertiaTo put the equation into rotational motion variables, the force is replaced by the torque about the center of rotation.The linear acceleration is replaced by the angular acceleration.
24 9.3 Rotational InertiaA rotating mass on a rod can be described with variables from linear or rotational motion.
25 9.3 Rotational InertiaThe product of mass × radius squared (mr2) is the rotational inertia for a point mass where r is measured from the axis of rotation.
26 9.3 Moment of InertiaThe sum of mr2 for all the particles of mass in a solid is called the moment of inertia (I).A solid object contains mass distributed at different distances from the center of rotation.Because rotational inertia depends on the square of the radius, the distribution of mass makes a big difference for solid objects.
27 9.3 Moment of InertiaThe moment of inertia of some simple shapes rotated around axes that pass through their centers.
28 9.3 Rotation and Newton's 2nd Law If you apply a torque to a wheel, it will spin in the direction of the torque.The greater the torque, the greater the angular acceleration.