Lesson 15 : Summary on Newton’s Laws of motion
- Newton’s First Law of Motion states that an object at rest remains at rest, and an object in motion continues in motion along a straight line with constant speed unless acted upon by an external force. This principle, also known as the Law of Inertia, highlights the inherent property of a body to resist changes to its state of motion.
- Inertia is quantified by an object’s mass—the greater the mass, the greater the inertia. There are three types of inertia: the inertia of rest, which keeps a body at rest; the inertia of motion, which maintains a body’s constant velocity; and the inertia of direction, which prevents a change in the direction of motion. This fundamental principle explains that without a net force, an object’s velocity remains unchanged.
- Several real-life examples illustrate Newton’s First Law. When a car makes a sharp turn, the passengers’ bodies tend to continue moving straight due to inertia. Similarly, seatbelts in cars help counteract inertia during sudden stops, preventing passengers from continuing forward. A rolling ball will keep moving unless friction or another force intervenes. Shaking a bottle of ketchup utilizes inertia to move the ketchup, while removing a tablecloth quickly without disturbing the dishes showcases the tendency of objects to remain in their state of rest.
- Newton’s Second Law of Motion builds on the first law, addressing what happens when an unbalanced force acts on an object. It states that the acceleration of an object is directly proportional to the net applied force and inversely proportional to its mass. Mathematically, this relationship is expressed as F=ma, where F is the net force, mmm is the mass, and a is the acceleration. This law explains how changes in force and mass affect an object’s acceleration: increasing the force increases the acceleration, while increasing the mass decreases the acceleration.
- Applications of Newton’s Second Law are ubiquitous. In vehicle dynamics, understanding the relationship between force, mass, and acceleration is crucial for safety and performance. For instance, increasing a vehicle’s mass requires more force to achieve the same acceleration. This principle is also vital in sports, where athletes manipulate force and mass to optimize performance, such as a sprinter accelerating off the starting blocks.
- The concept of momentum is inherently linked to Newton’s Second Law. Momentum, the product of an object’s mass and velocity, changes in response to applied forces. The rate of change of momentum is equal to the applied force, emphasizing the law’s impact on motion dynamics. This relationship is pivotal in collision analysis, where understanding the forces involved can predict outcomes and improve safety measures.
- In conclusion, Newton’s Laws of Motion provide a foundational framework for understanding and predicting the behavior of objects in motion. The first law, or the Law of Inertia, describes how objects resist changes to their state of motion. The second law quantitatively relates force, mass, and acceleration, offering a predictive tool for analyzing dynamics. Together, these laws are essential for studying mechanics, engineering applications, and daily phenomena, underscoring their enduring significance in physics.
- Newton’s Third Law of Motion
- Newton’s Third Law of Motion which is also known as the law of action and reaction states that every action has an equal and opposite reaction. One of the bodies of the two decides the magnitude and the direction of the body. In other words, every action has an equal and opposite reaction.
- Every interaction involves two forces operating on the two interacting objects.
- The forces exerting pressure on the first object are the same as those exerting pressure on the second object.
- When compared to the force on the second object, the force on the first object acts in the opposite direction.
- Action and reaction forces are always equal and in opposition to one another.
- The push or pull that occurs when one object interacts with another is referred to as a force.
- Frictional, tensional, and applied forces are examples of contact-related forces, whereas gravitational, electrical, and magnetic forces are examples of distance-related forces.
- If object A exerts a force (FA) on object B, then object B will exert a force (FB) of equal magnitude and opposite direction back on object A.
- Action-Reaction Pair
- Action Force: The action force is the initial external force applied to the body.
- Reaction Force: The force a body utilizes to react in the opposite direction to an action force is referred to as reaction force.
- If body A is exerting force on body B, then the force acting on body B is known as action and the opposite force is called reaction.
- If Fa = force exerted by Body A on body B
- Fb = force exerted by body B on A
- then,
- Fa = Action and Fb = reaction.
- Both bodies depend on each other for action and reaction.
- There is no cause-effect relationship between the bodies of action-reaction.
- Their action and reaction to different bodies cannot be canceled out.
- The action-reaction pair involves mutual forces which are opposite and equal between the bodies.
- Weight and the Gravitational Force
- When an object is dropped, it accelerates toward the center of the Earth.
- According to Newton’s second law, acceleration is the effect caused by a force.
- Therefore, a falling object, experiences a downward force known as weight of the object denoted by W
- The magnitude of the weight is the product of mass (m) and the value of acceleration due to gravity g.
- Weight is a vector whose direction is always down towards the center of the Earth
- Weight = (mass)(acceleration due to gravity)
- W=mg
- Near to the surface of the earth, the magnitude of g is 9.8.m/s2
- Normal Force
- Normal force is a type of contact force – two objects or surfaces have to touch for there to be a normal force.
- Since the book is in equilibrium, the net force acting on the book is zero. Therefore, on a level surface the normal force is equal to the weight.
- Fg = w =mg
- Note that as the name implies normal force is always normal (perpendicular) to the surface.
- Thus the weight and the normal force are equal only when the object is placed on a level surface.
- In most cases objects are placed on non-leveled surfaces such as on an inclined plane
- The normal force is less than the weight by a factor of cos θ.
- As θ is increased the normal force that supports the object is decreased and it will be zero when θ is 900 .
- When θ is 0 , the inclined plane becomes a level surface and obviously F = W cos θ = mg cosθ