PS Ch. 12 - Forces and Motion
It's hard to believe, but everything in the world is in motion, all the time. Even things that look perfectly still are packed with atoms that are vibrating with energy. Understanding how motion works was one of the great milestones of science and it's credited to the brilliant English physicist Sir Isaac Newton. His laws of motion, written over 300 years ago, were so well stated that scientists still use them in most situations today. The basic idea Newton taught us is that motion is caused by forces—which is easy enough to understand: kick a ball (the force) and it flies into the air (the motion). But forces don't always make things move: a bridge has lots of forces acting on it, but it doesn't go anywhere. Also, the "motion" forces produce is sometimes a shift in the direction in which something is moving or a change in its shape. So what exactly are forces and how they do they produce these different kinds of motion? It's time to take a closer look at the science of moving things!
12.1 Forces
A force can cause a resting object to move, or it can accelerate a moving object by changing the object's speed or direction. When the forces on an object are balanced, there is not change in the object's motion. When an unbalanced force acts on an object, the object accelerates. There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction. Earth's gravity acts downward toward the center of Earth. Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration. The combination of initial forward velocity and downward vertical force of gravity cause a projectile to follow a curved path.
Chapter 12.1 - Forces.mp4ONLINE RESOURCES
BrainPOP - Forces (5:33)
BrainPOP - Acceleration (2:51)
The Physics Classroom - Types of Forces website
12.2 Newton's First and Second Laws of Motion
According to Newton's first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero. According to Newton's second law of motion, the acceleration of an object is equal to the net force acting on it divided by the objects mass. Acceleration = net force/mass. Weight = mass x acceleration due to gravity. Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.
ONLINE RESOURCES
BrainPOP - Isaac Newton (4:32)
BrainPOP - Newton's Laws of Motion (3:03)
Mass, Force, Friction simulation
PhET: Force and Motion Basics
Investigating Frictional Forces Lab (Honors only)
Chapter 12.2 - Newton's 1st and 2nd laws.mp412.3 Newton's Third Law of Motion and Momentum
According to Newton's third law of motion, forces exist as equal and opposite force pairs. Momentum = mass x velocity. An object has a large momentum if the product of its mass and velocity is large. Momentum is conserved in a closed system.
12.4 Universal Forces
Electric and magnetic forces are the only forces that can both attract and repel. The strong and weak nuclear forces hold the nucleus together. Newton's law of gravitation states that every object in the universe attracts every other object.