Energy, Momentum, and Impulse – Terms to Know

1. Energy - the ability to do work. Measured in Joules. (S)

2. Work - the product of the distance times the component of the APPLIED force in the direction of the force. (Defining equation: W = F d cosq where F is the applied force, d is the distance and q is the angle between the applied force and the displacement; measured in joules.) (S)

3. Work-Energy Theorem - the NET work done on an object is equal to its change in kinetic energy.

4. Law of Conservation of Energy - the total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one body to another, but the total amount remains constant.

5. Kinetic energy - the energy of motion of an object. (Defining equation: K.E. = ¹/₂mv² with units in Joules). (S)

6. Potential Energy - stored energy. Energy that is available for use later; measured in joules. (S)

7. Gravitational Potential Energy - the potential energy a mass has due to its position in a gravitational field. Defining equation: P.E. = mgh where m is the object's mass, g is the gravitational field strength and h is the vertical displacement of the object; measured in joules. (Note: the above equation assumes that g is constant over the vertical displacement h. When dealing with astronomical problems the defining equation is P.E. = -GmM/R where G is Newton's universal gravitational constant, m and M are the masses of the two bodies and R is the distance between their centres). (S)

8. Joule - unit of work. 1 J = 1 N•m.

9. Power - the rate at which work is done or the rate at which energy is transformed from one kind to another. (Defining equation: P = W / t; measured in watts) (S)

10. Watt - unit of power: 1 W = 1 J/s.

11. Momentum - the product of an object's mass and velocity (Defining equation: p = m v). (V)

12. Impulse - the product of the net force acting on an object and time over which the force acts. (Defining equation: I = F ∆t). Note that the impulse is equal to the change in the object's momentum, or I = ∆ p. (V)

13. Newton's Second Law (reinterpreted!) - the rate of change of momentum of a body is proportional to the net force applied to it, (Defining equation: F_net = ∆p/ ∆t) Note there are two statements of Newton's second law - this one and F_net = ma.

14. Law of Conservation Of Momentum - the total momentum of an isolated system of bodies is constant.

15. Isolated system - system of bodies where the only forces present are those between interacting objects. (There are no external forces: friction, etc.)

16. Elastic collision - a collision where the total kinetic energy of the system is conserved.

17. Inelastic collision - a collision where the total kinetic energy of the system is not conserved. (Concerning collisions you should note that collisions between ordinary everyday objects are never elastic - some of the kinetic energy of the system is transformed into other forms of energy such as heat or sound. If a system is isolated the total momentum of the system is always conserved no matter which type of collision occurs. Note also that the expression "inelastic" does not mean the objects stick together! For example, a ball that is dropped onto a floor will not return to the same height (heat and sound are produced during the collision). Therefore such a collision is inelastic.

18. Completely inelastic collision - a collision where the colliding objects "stick together" after the collision.