AP Physics Section 4-1 to 4-6 Forces

Force A force is a push or pull on an object with mass. Forces have the ability to make objects accelerate — speed up, slow down, or change direction. The derived SI unit of force is the newton (N) 1 N = 1 kg•m/s2 A force is a push or pull on an object with mass. Forces have the ability to make objects accelerate — speed up, slow down, or change direction. The derived SI unit of force is the newton (N) 1 N = 1 kg•m/s2 A force is a push or pull on an object with mass. Forces have the ability to make objects accelerate — speed up, slow down, or change direction. The derived SI unit of force is the newton (N) 1 N = 1 kg•m/s2 A force is a push or pull on an object with mass. Forces have the ability to make objects accelerate — speed up, slow down, or change direction. The derived SI unit of force is the newton (N) 1 N = 1 kg•m/s2 A force is a push or pull on an object with mass. Forces have the ability to make objects accelerate — speed up, slow down, or change direction. The derived SI unit of force is the newton (N) 1 N = 1 kg•m/s2 A force is a push or pull on an object with mass. Forces have the ability to make objects accelerate — speed up, slow down, or change direction. The derived SI unit of force is the newton (N) 1 N = 1 kg•m/s2 Forces are often measured with a spring scale. Home bathroom scales are a type of spring scale.

Force vectors Force is a vector quantity. The push or pull is in a specific direction. Several force vectors can act on an object simultaneously, and usually do. Force is a vector quantity. The push or pull is in a specific direction. Several force vectors can act on an object simultaneously, and usually do. Lift

Motion and inertia Galileo and others investigated inertia. Inertia is the tendency of an object with mass to resist changes in motion. Before Galileo, the Aristotelian view was that all objects naturally come to rest if there is no motive force. Galileo explained that objects in constant motion would remain in motion if nothing disturbed the object. Galileo and others investigated inertia. Inertia is the tendency of an object with mass to resist changes in motion. Before Galileo, the Aristotelian view was that all objects naturally come to rest if there is no motive force. Galileo explained that objects in constant motion would remain in motion if nothing disturbed the object. Galileo and others investigated inertia. Inertia is the tendency of an object with mass to resist changes in motion. Before Galileo, the Aristotelian view was that all objects naturally come to rest if there is no motive force. Galileo explained that objects in constant motion would remain in motion if nothing disturbed the object. “A body moving on a level surface will continue in the same direction at a constant speed unless disturbed.”

Newton’s first law Newton’s first law is called the law of inertia. Objects at rest remain at rest, and objects in straight line motion remain in motion, if the net force on the object is zero. Newton’s first law is called the law of inertia. Objects at rest remain at rest, and objects in straight line motion remain in motion, if the net force on the object is zero. Newton’s first law is called the law of inertia. Objects at rest remain at rest, and objects in straight line motion remain in motion, if the net force on the object is zero. Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare. Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

Force and acceleration Newton actually examined the relationship between force and what he called “motion.” Today we call this momentum. Momentum is the product of mass and the velocity vector. However, if mass is constant we will see that the force applied and the resulting acceleration plot as a straight line. So if a certain force is applied to a mass the resulting acceleration is, a = F/m. Newton actually examined the relationship between force and what he called “motion.” Today we call this momentum. Momentum is the product of mass and the velocity vector. However, if mass is constant we will see that the force applied and the resulting acceleration plot as a straight line. So if a certain force is applied to a mass the resulting acceleration is, a = F/m. Newton actually examined the relationship between force and what he called “motion.” Today we call this momentum. Momentum is the product of mass and the velocity vector. However, if mass is constant we will see that the force applied and the resulting acceleration plot as a straight line. So if a certain force is applied to a mass the resulting acceleration is, a = F/m. Newton actually examined the relationship between force and what he called “motion.” Today we call this momentum. Momentum is the product of mass and the velocity vector. However, if mass is constant we will see that the force applied and the resulting acceleration plot as a straight line. So if a certain force is applied to a mass the resulting acceleration is, a = F/m. Newton actually examined the relationship between force and what he called “motion.” Today we call this momentum. Momentum is the product of mass and the velocity vector. However, if mass is constant we will see that the force applied and the resulting acceleration plot as a straight line. So if a certain force is applied to a mass the resulting acceleration is, a = F/m.

a = F/m or a = F 1 1 is the slope m m Since slope is 0.151, the mass = 6.62 kg What if the mass was 3.31 kg? 13.2 kg? What if the mass was 3.31 kg? 13.2 kg? What if the mass was 3.31 kg? 13.2 kg?

Which will have lower acceleration for the same force applied?

Newton’s second law The relationship between force and acceleration is usually given as: F = ma This is Newton’s second law. The vectors must be in the same direction. Lex II: Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur. Law II: The alteration of motion is ever proportional to the motive force impress'd; and is made in the direction of the right line in which that force is impress'd.

mass It should be obvious from this equation that the force on an object, and the object’s mass are different. Mass can be defined in two ways in terms of Newton’s second law: using inertial changes or the force of gravity. Inertial mass is a measure of the object’s resistance to changing its state of motion when a force is applied. With the same force applied, less massive objects will accelerate faster. mi = F/a Inertial mass is a measure of the object’s resistance to changing its state of motion when a force is applied. With the same force applied, less massive objects will accelerate faster. mi = F/a Inertial mass is a measure of the object’s resistance to changing its state of motion when a force is applied. With the same force applied, less massive objects will accelerate faster. mi = F/a Inertial mass is a measure of the object’s resistance to changing its state of motion when a force is applied. With the same force applied, less massive objects will accelerate faster. mi = F/a Gravitational mass is a measure of the strength of an object’s interaction with a gravity field. Since all objects accelerate at the same rate, more massive objects experience more force. mg = Fg/g Gravitational mass is a measure of the strength of an object’s interaction with a gravity field. Since all objects accelerate at the same rate, more massive objects experience more force. mg = Fg/g Gravitational mass is a measure of the strength of an object’s interaction with a gravity field. Since all objects accelerate at the same rate, more massive objects experience more force. mg = Fg/g Gravitational mass is a measure of the strength of an object’s interaction with a gravity field. Since all objects accelerate at the same rate, more massive objects experience more force. mg = Fg/g Gravitational mass is a measure of the strength of an object’s interaction with a gravity field. Since all objects accelerate at the same rate, more massive objects experience more force. mg = Fg/g

The sum of forces Newton’s second law applies to the net force, Fnet. This is the sum of all forces on an object. So we can write: This also reaffirms the first law, since if the net force equals zero, there is no acceleration, and the object’s velocity will remain constant. Newton’s second law applies to the net force, Fnet. This is the sum of all forces on an object. So we can write: This also reaffirms the first law, since if the net force equals zero, there is no acceleration, and the object’s velocity will remain constant. Newton’s second law applies to the net force, Fnet. This is the sum of all forces on an object. So we can write: This also reaffirms the first law, since if the net force equals zero, there is no acceleration, and the object’s velocity will remain constant. Fnet = ΣF = ma

Newton’s third law To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. This action-reaction law applies whether the objects are accelerating or not. No force ever exists without a corresponding reaction force of the same type on another body. To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. This action-reaction law applies whether the objects are accelerating or not. No force ever exists without a corresponding reaction force of the same type on another body. To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. This action-reaction law applies whether the objects are accelerating or not. No force ever exists without a corresponding reaction force of the same type on another body.

Will the forces applied be equal? YES Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi. Law III: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. Will the forces applied be equal? YES Will the girls accelerate at the same rate? NO

Weight Weight is the force exerted on a body by gravity. Fg = mg We actually experience weight (and measure weight) by means of the reaction force called the normal force. It’s called “normal” because it always acts perpendicular to a surface. Weight is the force exerted on a body by gravity. Fg = mg We actually experience weight (and measure weight) by means of the reaction force called the normal force. It’s called “normal” because it always acts perpendicular to a surface. Weight is the force exerted on a body by gravity. Fg = mg We actually experience weight (and measure weight) by means of the reaction force called the normal force. It’s called “normal” because it always acts perpendicular to a surface. Weight is the force exerted on a body by gravity. Fg = mg We actually experience weight (and measure weight) by means of the reaction force called the normal force. It’s called “normal” because it always acts perpendicular to a surface.

Apparent weight The weight you experience (or see on a scale) may not equal the true force of gravity on you. In an accelerating frame of reference, or under the influence of buoyancy, the apparent weight will be greater or less than the true gravitational force. The weight you experience (or see on a scale) may not equal the true force of gravity on you. In an accelerating frame of reference, or under the influence of buoyancy, the apparent weight will be greater or less than the true gravitational force. The weight you experience (or see on a scale) may not equal the true force of gravity on you. In an accelerating frame of reference, or under the influence of buoyancy, the apparent weight will be greater or less than the true gravitational force. N N N N

Drag and terminal velocity Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity. Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity. Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity. Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity. Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity. Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity. Drag is a friction force present when an object moves through a fluid such as air or water. Drag increases with greater speed. As an object falling through a fluid increases in speed, the drag (fluid resistance) increases and will eventually equal Fg. When drag equals Fg, the net force is zero, and the object will no longer accelerate. The speed when this occurs is known as terminal velocity.