1. CHS Physics Work and Kinetic Energy

2. WORK An external net force can cause a change in an object’s speed. An external net force can cause a change in an object’s speed. Speed up or slow down as a result of the force Speed up or slow down as a result of the force The Kinetic Energy of the object depends upon speed. The Kinetic Energy of the object depends upon speed. Thus the Kinetic Energy of the object changes. Thus the Kinetic Energy of the object changes. Increase in speed yields an increase in energy Increase in speed yields an increase in energy Decrease in speed yields a decrease in energy Decrease in speed yields a decrease in energy

3. Transfer of Energy Increase in Energy Increase in Energy Energy is transferred from the source of the force to the object. Energy is transferred from the source of the force to the object. Decrease in Energy Decrease in Energy Energy has been transferred from the object to the source of the force. Energy has been transferred from the object to the source of the force. When Energy is transferred by the application of a force, Work has been done on the object. When Energy is transferred by the application of a force, Work has been done on the object.

4. Definition of Work Work W is energy transferred to or from an object by means of a force acting on the object, Energy transferred to the object is positive work, and energy transferred form the object is negative work. Work W is energy transferred to or from an object by means of a force acting on the object, Energy transferred to the object is positive work, and energy transferred form the object is negative work. Note that work can be positive or negative. Note that work can be positive or negative. Work is transferred energy. Work is transferred energy. Doing work is the act of the transfer. Doing work is the act of the transfer.

5. SI Units Work is transferred Energy, thus Work has the same units. Work is transferred Energy, thus Work has the same units. Joules Joules Work is a Scalar Quantity Work is a Scalar Quantity Magnitude only! No Direction. Magnitude only! No Direction. Positive or Negative indicate the “direction” of energy transfer. Positive or Negative indicate the “direction” of energy transfer.

6. Work and Kinetic Energy Work is Transferred Energy Work is Transferred Energy This transfer can be a many forms of energy. This transfer can be a many forms of energy. We will limit discussion to changes in Kinetic Energy for the time being. We will limit discussion to changes in Kinetic Energy for the time being. The First step is to calculate the work done by the force. The First step is to calculate the work done by the force.

7. Calculating Work Figure 7-2 Figure 7-2 A bead on a frictionless horizontal wire being acted upon by an external force. A bead on a frictionless horizontal wire being acted upon by an external force. Begin by applying Newton’s second law to the bead. Begin by applying Newton’s second law to the bead. The bead is limited to the horizontal dimension. The bead is limited to the horizontal dimension.

8. Calculating Work As the bead moves along the wire under the constant force: As the bead moves along the wire under the constant force: It moves some distance d It moves some distance d Speed changes from v 0 to v Speed changes from v 0 to v We can calculate a We can calculate a Relate these using a Kinematic Equation: Relate these using a Kinematic Equation:

9. Combine Equations Solve the second law for acceleration Solve the second law for acceleration Substitute the result in to the kinematic equation. Substitute the result in to the kinematic equation. Do some algebra. Do some algebra.

10. Transfer of Energy The left side of the equation is the transfer of kinetic energy due to the force. The left side of the equation is the transfer of kinetic energy due to the force. But a transfer of energy is Work! But a transfer of energy is Work!

11. The Work Done The right side of the Equation is the work done by the force. The right side of the Equation is the work done by the force.

12. General Statement of Work To calculate the work done on an object by a force during a displacement, we use only the force component along the object’s displacement. The force component perpendicular to the displacement does no work. To calculate the work done on an object by a force during a displacement, we use only the force component along the object’s displacement. The force component perpendicular to the displacement does no work.

13. General Equation for Work The component of the force in the direction of the displacement is The component of the force in the direction of the displacement is The Work done is then The Work done is then

14. Work as a Scalar Product Note that the general statement of work is the scalar product of the applied force and the displacement. Note that the general statement of work is the scalar product of the applied force and the displacement. These statements are only valid for constant force. These statements are only valid for constant force.

15. Work-Kinetic Energy Theorem We can generalize the previous situations again by dealing strictly with net quantities. We can generalize the previous situations again by dealing strictly with net quantities. The left side of the original equation is the change in kinetic energy. The left side of the original equation is the change in kinetic energy. The right side is the Net Work done on the object. The right side is the Net Work done on the object.