What is a system in physics?

a predetermined collection of objects

a standardized operating procedure

something that must be destroyed

the tools needed for an experiment

How might replacing the bob of a pendulum with one of larger cross-sectional area but identical mass affect its period in air?

It could increase due to greater air resistance.

It would decrease due to increased aerodynamic efficiency.

It would stay constant as mass remains unchanged.

It could decrease as it might experience more buoyant force.

When a simplified model of a closed system fails to account for quantum effects at extremely low temperatures, what is the most likely reason for the deviation from expected behavior?

Quantum tunneling alters particle behavior in ways not accounted for by classical mechanics.

Thermal expansion coefficients become negative, contradicting the model's assumptions.

Air resistance becomes more significant even though it's a closed system.

Relativistic speeds are reached, requiring Einstein's theory of relativity.

The force required to bring a light cart traveling at a certain speed to rest is ____ the force required to bring a heavy cart traveling at the same speed to rest.

There is not enough information to determine.

How does adding salt affect boiling point for an open pot of water when considering colligative properties during cooking at sea level?

Boiling point elevation occurs since salt ions disrupt solvent-solute interactions and require higher temperatures for boiling phase transition.

Freezing point elevation happens as salt ions facilitate formation of ice crystals more readily at higher temperatures.

The boiling point remains unchanged since colligative properties depend solely on solvent characteristics without solute influence.

A reduction in boiling point ensues as dissolved salt ions cause increased evaporation rates lowering necessary heating threshold.

What happens to the internal energy of a system if heat is added to it and no substantial work is performed on or by the system?

It converts to exclusively kinetic energy.

In a closed system where two ice skaters push off from each other with no external forces, what determines their final velocities after separation?

The masses of the skaters and magnitude of forces during push-off.

Only the mass of each individual skater affects their velocity.

The initial speed of one skater before pushing off from another.

The total time for which they were in contact before separating.

In terms of forces, what must happen for an object to accelerate?

A net external force must be applied to it.

It must be completely isolated from any external forces.

All forces acting on it must cancel each other out perfectly.

It has to move at constant velocity without any forces acting on it.

or

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5.3 Open and Closed Systems: Momentum

4 min read•january 1, 2023

Kanya Shah

Daniella Garcia-Loos

Kanya Shah

Daniella Garcia-Loos

Attend a live cram event

Review all units live with expert teachers & students

Closed Systems vs. Open Systems

A closed system refers to a system that doesn’t lose mass, energy, charge, etc so conserved quantities are considered to be constant. On the other hand, an open system refers to exchanges of energy, charge, etc with the object(s) surroundings.

Open systems may seem to violate the conservation of mass, charge, matter, and energy since the amount of the previously stated quantities can increase or decrease without replacement.

Here are some key points about closed and open systems in the context of momentum:

In a closed system, the total momentum of all objects within the system is conserved. This means that the total momentum of the system before and after an interaction remains the same.
In an open system, the total momentum of all objects within the system is not necessarily conserved. This is because an open system can exchange momentum with objects outside of the system.
In an open system, the total momentum of the system can be calculated by taking into account the momentum of all objects within the system as well as the momentum of objects outside the system that interact with the system.
In an open system, the total momentum of the system may change due to the transfer of momentum between the system and objects outside the system.
In order to analyze the momentum of an open system, it is important to consider all interactions with objects outside the system and how they affect the total momentum of the system.

Image Credit: physics.usyd.edu

Momentum of a System

In terms of momentum of a system, internal forces, like all forces, always occur in action reaction pairs. Since the forces in action reaction pairs are equal and opposite due to Newton’s 3rd law, internal forces must always sum to zero. If the internal forces cancel, that means that the sum of the total forces must simply be all of the external forces. External forces could sum to zero but that won’t always be the case.

The change in net momentum is directly related to the net external force. If the net external force acting on a system is equal to zero, then its net momentum is conserved so pinitial = pfinal. This only applies to the net momentum of a system, not individual momenta of each object.

Example Problem #1:

A car of mass 1000 kg is traveling at a velocity of 50 m/s. The car has a passenger of mass 75 kg traveling with it. What is the total momentum of the car and the passenger?

Solution:

The momentum of the car is given by the formula: momentum = mass * velocity

The mass of the car is 1000 kg, and its velocity is 50 m/s.

Therefore, the momentum of the car is: momentum = 1000 kg * 50 m/s = 50000 kg*m/s

The momentum of the passenger is given by the formula: momentum = mass * velocity

The mass of the passenger is 75 kg, and its velocity is 50 m/s (since it is traveling with the car).

Therefore, the momentum of the passenger is: momentum = 75 kg * 50 m/s = 3750 kg*m/s

The total momentum of the car and the passenger is therefore: 50000 kgm/s + 3750 kgm/s = 53750 kg*m/s

Example Problem #2:

A spaceship of mass 1000 kg is traveling at a velocity of 50 m/s. The spaceship has a cargo of mass 500 kg traveling with it. The cargo has a velocity of 25 m/s relative to the spaceship. What is the total momentum of the spaceship and the cargo?

Solution:

The momentum of the spaceship is given by the formula: momentum = mass * velocity

The mass of the spaceship is 1000 kg, and its velocity is 50 m/s.

Therefore, the momentum of the spaceship is: momentum = 1000 kg * 50 m/s = 50000 kg*m/s

The momentum of the cargo is given by the formula: momentum = mass * velocity

The mass of the cargo is 500 kg, and its velocity relative to the spaceship is 25 m/s.

Therefore, the momentum of the cargo is: momentum = 500 kg * 25 m/s = 12500 kg*m/s

The total momentum of the spaceship and the cargo is therefore: 50000 kgm/s + 12500 kgm/s = 62500 kg*m/s

Example Problem #3:

A train of mass 10000 kg is traveling at a velocity of 50 m/s. The train has 20 cars, each with a mass of 1000 kg and a velocity of 50 m/s. What is the total momentum of the train and the cars?

Solution:

The momentum of the train is given by the formula: momentum = mass * velocity

The mass of the train is 10000 kg, and its velocity is 50 m/s.

Therefore, the momentum of the train is: momentum = 10000 kg * 50 m/s = 500000 kg*m/s

The total momentum of the cars is given by the formula: momentum = mass * velocity

The mass of each car is 1000 kg, and the velocity of each car is 50 m/s.

There are 20 cars, so the total mass of the cars is: mass = 20 cars * 1000 kg/car = 20000 kg

The total velocity of the cars is: velocity = 20 cars * 50 m/s/car = 1000 m/s

Therefore, the total momentum of the cars is: momentum = 20000 kg * 1000 m/s = 2000000 kg*m/s

The total momentum of the train and the cars is therefore: 500000 kg*m/s + 2000000 kg

Key Terms to Review (8)

Closed System

: A closed system is a physical system that does not exchange matter with its surroundings, but can exchange energy.

Conservation of Charge

: The principle that states that the total electric charge in a closed system remains constant over time.

Conservation of Energy

: The principle that states that energy cannot be created or destroyed but can only be transferred or transformed from one form to another.

Conservation of Mass

: Conservation of mass states that mass cannot be created or destroyed in any chemical reaction. It remains constant before and after the reaction takes place.

Momentum Conservation

: The principle that states that the total momentum before an event involving interacting objects is equal to the total momentum after the event if no external forces act on them.

Net Momentum

: Net momentum refers to the total amount of momentum in a system, taking into account the combined effect of all objects involved. It is calculated by adding up the individual momenta of each object.

Newton's Third Law of Motion

: Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts a force of equal magnitude in the opposite direction on the first object.

Open System

: An open system is a physical system that can both exchange matter and energy with its surroundings.

5.3 Open and Closed Systems: Momentum

4 min read•january 1, 2023

Kanya Shah

Daniella Garcia-Loos

Kanya Shah

Daniella Garcia-Loos

Attend a live cram event

Review all units live with expert teachers & students

Closed Systems vs. Open Systems

A closed system refers to a system that doesn’t lose mass, energy, charge, etc so conserved quantities are considered to be constant. On the other hand, an open system refers to exchanges of energy, charge, etc with the object(s) surroundings.

Open systems may seem to violate the conservation of mass, charge, matter, and energy since the amount of the previously stated quantities can increase or decrease without replacement.

Here are some key points about closed and open systems in the context of momentum:

In a closed system, the total momentum of all objects within the system is conserved. This means that the total momentum of the system before and after an interaction remains the same.
In an open system, the total momentum of all objects within the system is not necessarily conserved. This is because an open system can exchange momentum with objects outside of the system.
In an open system, the total momentum of the system can be calculated by taking into account the momentum of all objects within the system as well as the momentum of objects outside the system that interact with the system.
In an open system, the total momentum of the system may change due to the transfer of momentum between the system and objects outside the system.
In order to analyze the momentum of an open system, it is important to consider all interactions with objects outside the system and how they affect the total momentum of the system.