In this unit students investigate forces and energy transfers in interactions between objects. The fundamental concept is that the evidence of an interaction is a change in the motion or energy of an object. Students compare examples of contact forces as pushes or pulls between interacting objects that are touching. They also identify gravitational, magnetic, and electrostatic interactions as non-contact forces. Since the emphasis in a conceptual physics course is on understanding fundamental ideas that explain everyday phenomena, activities that involve the concept should come before the formal introduction of the concept. Students have multiple opportunities to calculate the net force when two or more forces are acting on an object. When unbalanced forces are applied to an object, students qualitatively recognize that an acceleration occurs. This lead to an understanding of the direct relationship between net force and acceleration (F=ma). Next, students observe and measure action/reaction forces using paired force scales with students pulling away from each other or from a stable object like a wall. They then explain why these action/reaction pairs do not cancel each other out and prevent motion. Finally, students investigate energy transfer in interactions. They distinguish between and calculate the kinetic energy (energy of moving objects) and the potential energy (energy stored due to position or shape) of objects in various situations such as a person jumping or a bouncing ball. The back and forth conversion of kinetic and potential energies in this situation is also analyzed. At this point the scientific concept of work as energy transfer is also introduced.
P2.3x Frames of Reference
P2.3a Describe and compare the motion of an object using different reference frames.
STANDARD P3: FORCES AND MOTION Students identify interactions between objects either as being by direct contact (e.g., pushes or pulls, friction) or at a distance (e.g., gravity, electromagnetism), and to use forces to describe interactions between objects. They recognize that non-zero net forces always cause changes in motion (Newton’s fi rst law). These changes can be changes in speed, direction, or both. Students use Newton’s second law to summarize relationships among and solve problems involving net forces, masses, and changes in motion (using standard metric units). They explain that whenever one object exerts a force on another, a force equal in magnitude and opposite in direction is exerted back on it (Newton’s third law).
P3.1 Basic Forces in Nature
P3.1A Identify the force(s) acting between objects in “direct contact” or at a distance.
P3.1d Identify the basic forces in everyday interactions.
P3.2A Identify the magnitude and direction of everyday forces (e.g., wind, tension in ropes, pushes and pulls, weight).
P3.2B Compare work done in different situations.
P3.2C Calculate the net force acting on an object.
P3.3A Identify the action and reaction force from examples of forces in everyday situations (e.g., book on a table, walking across the fl oor, pushing open a door).
P3.4 Forces and Acceleration
P3.4A Predict the change in motion of an object acted on by several forces.
P3.4C Solve problems involving force, mass, and acceleration in linear motion (Newton's second law).
P4.1x Energy Transfer — Work
P4.1c Explain why work has a more precise scientific meaning than the meaning of work in everyday language.
P4.1d Calculate the amount of work done on an object that is moved from one position to another.
P4.1e Using the formula for work, derive a formula for change in potential energy of an object lifted a distance.
P4.3 Kinetic and Potential Energy
P4.3A Identify the form of energy in given situations (e.g., moving objects, stretched springs, rocks on cliffs, energy in food).
P4.3B Describe the transformation between potential and kinetic energy in simple mechanical systems (e.g., pendulums, roller coasters, ski lifts).
P4.3C Explain why all mechanical systems require an external energy source to maintain their motion.
P4.3x Kinetic and Potential Energy — Calculations
P4.3d Rank the amount of kinetic energy from highest to lowest of everyday examples of moving objects.
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- What is the evidence for the existence of forces?
- What is the difference between contact and non-contact forces?
- How do balanced and unbalanced forces affect an object?
- How are kinetic and gravitational potential energy related in energy transfers?
- How is work related to the transfer of energy?
- What is the role of friction in everyday interactions?
balanced and unbalanced forces
coefficient of friction
elastic potential energy
frame of reference
gravitational potential energy (GPE = mgh)
kinetic energy (KE = ½ mv2)
Newton’s 1st Law
Newton’s 2nd Law (F = ma)
Newton’s 3rd Law