Scope, Sequence, and Coordination

A Framework for High School Science Education

Based on the National Science Education Standards

Forces Acting on a Spring

Elastic and Frictional Forces: Electric Forces Between Atoms and Molecules
Between any two charged particles, electric force is vastly greater than the gravitational force. Most observable forces such as those exerted by a coiled spring or friction may be traced to electric forces acting between atoms and molecules.

Further Description:

We can compare the relative magnitudes of electrical and gravitational forces. This can be done by using Coulomb’s law and the law of universal gravitation for a pair of electrons or protons at some separation.

There are many interactions that can be reduced to electrical force upon analysis. Contact forces are the forces of repulsion of outer electrons in atoms and molecules of the interacting objects. Elastic forces involve electric forces associated with the distortion of molecules in such a way that the macroscopic effect is a force.

Elastic forces, as well as frictional forces, have associated heat transfers, so that force is not the only phenomenon involved. For most practical uses involving friction in elastic systems, we need consider only the nature and magnitude of the forces involved to solve problems. Since dynamics can be approached from the point of view of either forces or energy, we may solve problems using forces and acceleration without consideration of the thermal losses associated with frictional or elastic systems. The disadvantage, however, is that one must use vectors, whereas energy is a scalar and is much easier to use in calculations.

When the energy approach is followed, thermal energy becomes a major factor limiting the law of conservation of mechanical energy and requiring the use of the first and second laws of thermodynamics. At the macroscopic level, elastic systems require consideration of Hooke’s law, and frictional systems require the introduction of the coefficient of friction or viscosity in dynamics problems.

Concepts Needed:

Grade 9

Friction, fluid, viscosity, terminal velocity

Grade 10

Static friction, kinetic friction, coefficient of friction, Hooke=s law, spring Aconstant@

Grade 11

Vector nature of frictional forces

Grade 12

Nonconservative forces

Empirical Laws or Observed Relationships:

Simple observations show that something we call friction opposes the relative motion of two surfaces in contact. The frictional force depends on the nature of the surfaces. Two kinds of friction are observed, one static and the other kinetic; heat is observed; f < mN or f = mN for static friction and f = mN for kinetic friction (these relationships apply only approximately for dry friction and when the normal force is not too large); Hooke=s law, F = B kx for linear spring and t = B kq for linear torsion.

Theories or Models:

Microscopic view of friction and relation to electrical forces; microscopic view of elastic distortion in terms of changes in separation or orientation of charges in matter

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Micro-Unit Description:

Forces Acting on a Spring
Students should observe and graph the relationship between the force acting on a spring and its extension. They should recognize that the force of the spring has a direction opposite to its extension or compression.

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