# AP Physics 1 review of Forces and Newton's Laws | Physics | Khan Academy

TLDRThe video script delves into Newton's three laws of motion, explaining their principles and applications in real-world scenarios. Newton's first law discusses the state of constant velocity or rest in the absence of net force, while the second law links acceleration to net force and mass. The third law highlights the mutual forces between two objects. The script further explores concepts like gravity, normal force, tension, and friction, and how they influence motion on inclined planes and within systems of objects.

###### Takeaways

- ๐ Newton's First Law: Objects maintain constant velocity or remain at rest unless acted upon by an unbalanced force.
- ๐ Example of Newton's First Law: An elevator moving upward at a constant velocity has balanced forces between cable tension and gravity.
- ๐ Newton's Second Law: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
- ๐ Newton's Third Law: For every action, there is an equal and opposite reaction between two interacting objects.
- ๐ Gravitational Force: The force of gravity on an object near Earth is directed downward and is calculated as mass times the acceleration due to gravity (weight = m * g).
- ๐ Normal Force: The force exerted by a surface that is perpendicular and outward, countering gravity when an object is at rest on a horizontal surface.
- ๐ Tension Force: The pulling force exerted by a string, rope, or similar object, always directed away from the object it's attached to.
- ๐๏ธ Kinetic Friction: The force resisting the sliding motion between two surfaces, proportional to the normal force and the coefficient of kinetic friction.
- ๐ช Static Friction: The force that prevents two surfaces from slipping until the applied force exceeds the maximum static frictional force.
- ๐ Inclines: Forces on an incline are analyzed by breaking them into components perpendicular and parallel to the surface, with net force being parallel component (mg * sin(theta)).
- ๐ Systems as Single Objects: When multiple objects move with the same acceleration, treat them as a single system to simplify calculations by considering only external forces and total mass.

###### Q & A

### What does Newton's first law state?

-Newton's first law, also known as the law of inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

### How does Newton's first law apply to a system of objects?

-Newton's first law applies to a system of objects by considering the center of mass of the system. If there are no external unbalanced forces acting on the system, the center of mass will remain at rest or in constant motion. Internal forces within the system cancel each other out and do not affect the motion of the center of mass.

### What is the relationship between force, mass, and acceleration according to Newton's second law?

-Newton's second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, it is expressed as a = \frac{F_{net}}{m}, where a is the acceleration, F_{net} is the net force, and m is the mass of the object.

### How do you determine the acceleration in a specific direction using Newton's second law?

-To determine the acceleration in a specific direction, you consider only the net force in that direction and apply Newton's second law. The acceleration in a particular direction is equal to the net force in that direction divided by the object's mass.

### What does Newton's third law state and what are the implications of it?

-Newton's third law states that for every action, there is an equal and opposite reaction. This means that any force exerted by object A on object B is met with an equal and opposite force by object B on object A. The law applies regardless of the sizes of the objects or their states of motion.

### How do you identify a Newton's third law force pair?

-A Newton's third law force pair consists of two forces that are equal in magnitude and opposite in direction, acting on two different objects. To identify such a pair, you reverse the order of the objects and check if the described force matches the reaction force according to Newton's third law.

### What is the force of gravity, and how is it related to an object's weight?

-The force of gravity on an object is the attractive force exerted by the Earth on the object, directed toward the center of the Earth. It is calculated as the product of the object's mass and the acceleration due to gravity (F_g = m \cdot g). An object's weight is the force of gravity acting on it, not its mass.

### How does the weight of an object change when it is taken from Earth to the Moon?

-The weight of an object changes when it is taken from Earth to the Moon because the gravitational pull on the Moon is weaker than on Earth. While the mass of the object remains constant, the weight (which is the force of gravity) decreases due to the lower gravitational acceleration on the Moon.

### What is the normal force and how is it determined?

-The normal force is the force exerted by a surface that supports an object, acting perpendicular to the surface. It is not calculated with a specific formula but is determined using Newton's second law, where it is often one of the unknowns to be solved for, based on the forces acting on the object and its state of motion.

### How does the normal force on an object differ when it is on an incline compared to a horizontal surface?

-On an incline, the normal force on an object is different from that on a horizontal surface because it is only the component of the gravitational force that is perpendicular to the incline. The normal force on an incline is given by F_{normal} = m \cdot g \cdot cos(ฮธ), where m is the mass of the object, g is the acceleration due to gravity, and ฮธ is the angle of inclination.

### What is the force of tension and how is it represented in a force diagram?

-The force of tension is the pulling force exerted by a string, rope, cable, or similar object. In a force diagram, tension forces are always represented as directed away from the object the string is attached to, indicating the direction of the pull. Tension is a vector quantity and has units of Newtons.

### How do you calculate the magnitude of the force of static friction?

-The magnitude of the force of static friction is calculated based on the normal force between the two surfaces and the coefficient of static friction. It is given by F_{static} = ฮผ_s \cdot N, where ฮผ_s is the coefficient of static friction and N is the normal force between the surfaces.

### What happens to the force of kinetic friction when the velocity of the object changes?

-The force of kinetic friction does not depend on the velocity of the object. It remains constant as long as the normal force and the coefficient of kinetic friction between the two surfaces remain the same, regardless of how fast or slow the object is moving.

###### Outlines

##### ๐ Newton's Laws of Motion

This paragraph introduces Newton's three laws of motion, explaining their implications on the behavior of objects. Newton's first law, the law of inertia, states that an object will remain at rest or in uniform motion unless acted upon by an external force. The second law establishes the relationship between force, mass, and acceleration, stating that the acceleration of an object is directly proportional to the net force applied and inversely proportional to its mass. The third law discusses action and reaction, indicating that forces always occur in equal and opposite pairs. The paragraph also includes examples of how these laws apply in real-world scenarios, such as the motion of an elevator, a space rock, and the interaction between objects and forces like gravity and friction.

##### ๐ Gravity and its Effects

This section delves into the concept of gravity, particularly the force of gravity acting on objects near Earth, which is directed towards the center of the Earth and is calculated as mass times the acceleration due to gravity (weight). It distinguishes between mass and weight, emphasizing that while mass is constant, weight varies with the gravitational pull. The paragraph discusses the changes in an object's mass and weight when it is moved from Earth to the Moon. It also explains the normal force, a reaction force exerted by a surface, and provides an example of calculating the normal force when a box is pushed against a ceiling. The force of tension, exerted by string-like objects, is also covered, with an example of determining the tensions in ropes holding up a stationary box.

##### ๐ Forces in Motion: Kinetic Friction and Inclines

This paragraph explores the force of kinetic friction, which resists the sliding motion between two surfaces, and the force of static friction, which prevents initial slipping. It uses a car braking scenario to illustrate how changes in initial speed or the coefficient of friction affect the stopping distance. The concept of inclines is introduced, explaining how to break down forces into components perpendicular and parallel to the incline's surface. The net force and the role of friction on an incline are discussed, with an example of ranking net forces on a frictionless ramp at different points. The paragraph concludes with a discussion on treating systems of objects as a single entity for simplified analysis, using an example of two masses connected by a rope on a horizontal surface.

##### ๐ System Analysis: Treating Multiple Objects as One

The final paragraph focuses on the concept of treating a system of objects as a single entity for easier analysis, particularly when all objects within the system share the same acceleration. It explains that internal forces within the system cancel each other out, allowing for the simplification of calculations by considering only external forces and the total mass of the system. An example is provided where a mass is pulled across a table by another mass connected via a rope, and the external forces (gravity and friction) are used to derive an expression for the system's acceleration without the need for multiple equations.

###### Mindmap

###### Keywords

##### ๐กNewton's First Law

##### ๐กConstant Velocity

##### ๐กNet Force

##### ๐กAcceleration

##### ๐กMass

##### ๐กForce Pair

##### ๐กGravitational Force

##### ๐กNormal Force

##### ๐กTension

##### ๐กKinetic Friction

##### ๐กStatic Friction

###### Highlights

Newton's first law states that objects don't change their velocity unless there's an unbalanced force.

An object will continue moving with a constant velocity or remain at rest if there's no net force acting on it.

Newton's first law applies to systems of objects, where the center of mass remains at rest or in constant motion unless acted upon by an external unbalanced force.

In a system of objects, internal forces cancel out, and only external forces affect the motion of the system's center of mass.

Newton's second law states that the acceleration of an object is proportional to the net force and inversely proportional to its mass.

The equation for Newton's second law is acceleration equals the net force on an object divided by the object's mass.

Acceleration in a specific direction is calculated by dividing the net force in that direction by the object's mass.

Newton's third law states that for every action, there is an equal and opposite reaction.

The force of gravity on an object is calculated as mass times the acceleration due to gravity.

Weight is the force of gravity and is different from mass; weight is a vector with units of Newtons.

The normal force is the perpendicular force exerted by a surface and is calculated using Newton's second law.

The normal force on an incline is not equal to mg but is mg times cosine of the incline's angle.

Tension is a force exerted by a string or rope and can only pull on objects, not push.

Kinetic friction is the force between two sliding surfaces and is proportional to the normal force and the coefficient of kinetic friction.

Static friction matches the force trying to move an object until it reaches the maximum static frictional force.

Inclines are analyzed by breaking forces into components perpendicular and parallel to the surface.

Treating systems as a single object simplifies calculations by ignoring internal forces and focusing on external forces and the total mass.

###### Transcripts

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