2020 Physics: Forces and Motion 

PH.PS2.1 Analyze and interpret data to support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.

PH.PS2.2 Use mathematical representations to support the claim that the total momentum of a system of objects is conserved when there is no net force on the system.

PH.PS2.3 Apply scientific and engineering ideas to design, evaluate, and refine a device that minimizes the force on a macroscopic object during a collision.*

PH.PS2.4 Use mathematical representations of Newton’s Law of Gravitation and Coulomb’s Law to describe and predict the gravitational and electrostatic forces between objects.

(Note: Only Newton’s Law of Gravitation and gravitational forces are addressed in this bundle. Coulomb’s law and electrostatic forces are addressed in the "Fundamental Forces, Electricity & Magnetism" bundle.) 

In a Nutshell

Students can analyze and interpret data to support the claim that Newton’s Second Law of motion relates an object’s acceleration to both the net (total) force acting on it and the object’s mass. When objects interact, their motion is affected. The mass of the objects involved in the collision can affect how much the motion changes as a result of the collision. Momentum is determined by the mass of an object and its velocity (speed and direction). Students use mathematical representations to support a claim that the total momentum of a system of objects is the same before and after a collision as long as there is no net force acting on the system. If an external force is applied to a system, the total momentum can change. Minimizing the forces on objects involved in collisions is important for safety and health considerations in many fields. Many devices exist to help minimize forces during collisions. Sports helmets, seat belts, airbags, guard rails, and bubble wrap all work to minimize the forces applied to objects during collisions. Students apply scientific and engineering ideas to design, evaluate and refine a device that will use the balance of forces to minimize the effects of a change in momentum on an object. Gravity is a universal force. Forces at a distance can be explained by gravitational fields permeating space. Students will use mathematical representations to describe Newton’s Law of Universal Gravitation and predict the effects of the gravitational force between distant objects.

Student Actions

Teacher Actions 

• Use models to organize, analyze and interpret data to determine the effect of mass and acceleration on net force.

• Analyze and interpret data to support a claim that describes how the mass of an object affects its motion.

• Analyze data to describe the relationship between the mass of an object, the net force and its acceleration (a = F_net/m). 

• Use a mathematical model to illustrate the proportional relationship between force, mass and acceleration.

• Use data as evidence to support the claim that in a system of two objects momentum lost by object 1 is equal to the momentum gained by object 2.

• Use data as evidence to show that a net force causes an object to accelerate and an object's acceleration is correlated to its mass.

• Use mathematical/descriptive representations to illustrate that the total momentum of a system of objects is conserved when there is no net force on the system.

• Apply engineering ideas to evaluate and refine a device that demonstrates how an increase of time over which a collision occurs will decrease the force acting on an object.

• Apply engineering ideas to evaluate and refine a device that demonstrates how a decrease in mass of an object within a collision will decrease the force acting on the object.

• Apply scientific ideas to solve a design problem relating to minimizing the force on an object during a collision while remaining in the societal constraints of risk and cost. 

• Use mathematical representations of Newton's universal law of gravitation to describe the patterns (of distance and mass) that affect gravitational force between two objects.

• Use mathematical representations to predict the gravitational force between two objects in a system. 

• Provide tasks where students will use models to organize, analyze and interpret data that allows them to determine the relationship between mass, acceleration, and net force.

• Assist students in the use of mathematical models to illustrate the proportional relationship between force, mass and acceleration (a = F_net/m).

• Support students in using data as empirical evidence to show that a net force causes a macroscopic object to accelerate and an object's acceleration is correlated to its mass.

• Support students in the use of qualitative descriptions to illustrate that the total momentum of a system of objects is conserved when there is no net force on the system. 

• Support students in the use of mathematical representations to illustrate that the total momentum of a system of objects is conserved when there is no net force on the system. 

• Provide opportunities for students to use and connect data that supports an overall claim that in a system of two objects momentum lost by object 1 is equal to the momentum gained by object 2.

• Provide opportunities for students to design and evaluate the success of a device that minimizes the force on a macroscopic object during a collision.

• Pose purposeful questions that assist students in refining a device that minimizes the force on a macroscopic object during a collision.

• Support students in the use of mathematical representations to describe and predict that the force of gravitational attraction is directly dependent upon the masses of two objects.

• Support students in the use of mathematical representations to describe and predict that the force of gravitational attraction is inversely proportional to the square of the distance that separates the center of two masses.  

Key Concepts 

Misconceptions 

Forces and Motion

• Newton’s Second Law accurately predicts changes in the motion of macroscopic objects.

• Momentum is defined for a particular frame of reference; it is the mass times the velocity of the object.

• If a system interacts with objects outside itself, the total momentum of the system can change; however, any such change is balanced by change in the momentum of objects outside the system.

Types of Interactions

• Newton’s Law of Universal Gravitation provides the mathematical model to describe and predict the effects of the gravitational force between distant objects.

Defining and Delimiting Engineering Problems

• Criteria and constraints also include satisfying any requirements set by society, such as taking issues of risk mitigation into account, and they should be quantified to the extent possible and stated in such a way that one can tell if a given design meets them.

• Acceleration is the same as speed.

• Acceleration always means that an object is speeding up.

• The forces exerted on an object are unbalanced when the object moves with constant velocity.

• When a force acts on an object in the direction of the object's motion, the speed of the object will increase for a while and then level off at the higher speed 

• Constant force produces constant speed. Under the influence of a constant force, objects move with constant velocity.

• In a two-object system, the more massive object exerts a greater force on the object.

• Momentum is a scalar physical quantity.

• Momentum is the same as a force.

• ­Momentum is conserved for an individual object, rather than a system.

• ­Momentum depends on the acceleration of an object rather than its velocity at a particular moment.

• Momentum is conserved for each object involved in a collision.

• ­There is no change in momentum to an extremely large or heavy object involved in a collision. 

• Gravitational forces only apply to objects near the surface of a planet or between planets.

Instructional Resources 

Unit 1: Forces and Motion