Forces and Newton's Laws of Motion

Forces

• A force is a push or pull that acts on an object that can cause a resting object to move
• It can accelerate a moving object by changing its speed or direction

Balanced Forces

• Balanced forces result in no change in motion with no acceleration

Unbalanced Forces

• Unbalanced forces result in a net force

Newton's Law of Motion

• 1st Law: An object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity unless acted upon by an unbalanced force
• 2nd Law: Force equals mass times acceleration
• 3rd Law: For every action, there is an equal and opposite reaction

Newton's First Law (Inertia)

• Inertia is the tendency of an object to resist any change in its motion unless acted upon by some external force

Newton's Second Law

• The net force of an object equals the product of its mass and acceleration, represented as F=ma
• Mass is measured in kilograms (kg)
• Acceleration is measured in meters per second squared (m/s²)
• Force measured in kg x m/s², is also known as Newtons (N)

Force Problems

• To calculate force (F), multiply mass (m) by acceleration (a); F = (25 kg)(15 m/sec²) = 375 N

Newton's Third Law

• Every action force has an equal and opposite reaction force

Measuring Forces

• Force is measured using a spring scale, where the stretch of the spring depends on the object's mass
• The unit of force is the Newton (N) and is equal to the force to accelerate 1 kg at 1 m/s²
• 1N = (kg•m) / s²

Representing Forces

• Arrows represent forces, where direction indicates the force's direction, and length indicates strength or magnitude

Combining Forces

• Forces in the same direction are added together
• Forces in the opposite direction are subtracted
• Net force is the overall force acting on an object

Balancing vs Unbalancing Forces

• Balanced forces combine to produce a net force of zero, causing no change in the object's motion
• Unbalanced forces net force is not zero and equals the larger force subtracted by the smaller force, causing the object to accelerate

Friction

• Friction is the force that opposes the motion of objects that touch as they move past each other
• Friction acts at the surface where objects are in contact
• There are four types of friction

Types of Friction

• Static friction is the force that acts on objects that are not moving and it always acts in the opposite direction of the applied force
• Sliding friction opposes the direction of motion of an object as it slides over a surface
• Rolling friction occurs when an object rolls across a surface and involves a change in shape at the point of rolling contact
• Fluid friction opposes the motion of an object through a fluid and increases with the speed of the object moving through the fluid, fluids include gases and liquids

Gravity

• Gravity is a force that acts between two masses
• It's an attractive force pulling objects together
• Earth's gravity is 9.8 m/s²
• Gravity acts downwards towards the center of the Earth
• The Law of Universal Gravitation states: every object in the universe attracts every other object
• The strength of the force of gravity between two objects depends on the objects mass and distance

Gravity and Falling Objects

• Terminal velocity is the constant velocity of a falling object when the force of resistance equals gravity