Ch. 12 Forces & Motion

TODAY’S WARM-UP Answer these questions in your own words… What is a force? How can you measure force? Are there different types of forces? (and don’t say it’s what a Jedi uses to fight the dark side )

Ch. 12 Standards Newton’s Third Law P 3.3A Identify the action and reaction force from examples of forces in everyday situations (e.g., book on a table, walking across the floor, pushing open a door). P3.3b Predict how the change in velocity of a small mass compared to the change in the velocity of a large mass when the object interact ( e.g., collide).

Ch. 12 Standards Continued Gravitational Interactions P 3.6A Explain earth-moon interactions (orbital motion) in terms of forces. P3.6B Predict how the gravitational forces between objects change when the distance between them change. P3.6C Explain how your weight on earth could be different from your weight on another planet.

Force Force: a push or pull that acts on an object. Force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. There are 4 different types of forces we will discuss:

4 types of Forces… 1. Gravitational Force (FG) 2. Normal force (FN) The force that pulls two objects together. (we are PULLED down towards the center of the Earth) 2. Normal force (FN) This is the force that pushes back against gravitational pull. THESE TWO FORCES ACT ON RESTING OBJECTS

4 types of Forces… (cont) 3. Applied Force (FA) This is the force that causes an object to move. (This can be a push or a pull) 4. Frictional Force (Ff) This force works against applied forces or gravitational forces. THESE TWO FORCES ACT ON AN OBJECT IN MOTION

Measuring Force Use a spring scale to measure. The SI unit of force is the Newton. 1N = 1kg.m/s2 1N = 0.225lbs

Combining Forces Net force is the overall force acting on an object after all forces are combined. Balanced Forces When the forces on an object are balanced , the net force is zero and there is no change in the objects motion. Unbalanced Forces When an unbalanced force acts on an object, the object accelerates.

Friction Friction: a force that opposes the motion of objects that touch as they move past each other. All objects are subject to friction. Without friction everything would be slippery. There are 4 types of friction:

Types of Friction 1. Static Friction Force that acts on objects that are not moving. Always acts in the direction opposite to that of the applied force. Ex: walking- with each step static friction keeps your shoe from sliding.

Types of Friction 2. Sliding Friction Force that opposes the direction of motion of an object as it slides over a surface. Less force is required to keep an object moving than to start it. Ex: sliding your book across the table.

Types of Friction 3. Rolling Friction Force that acts on rolling objects. Friction is greatly reduced because rolling friction replaces sliding friction. Ex: ball bearings, skates, cars

Types of Friction 4. Fluid Friction Force on objects through a fluid. Water and a mixture of a gases such as air are known as fluids. Ex: stirring cake batter. Air Resistance

QUESTION OF THE DAY… What is the difference between the weight of an object, and the mass of an object??? (You have 1 minute to discuss this with the other person at your table) MAKE A T-CHART TO SHOW THE DIFFERENCE

Weight and Mass Weight- force of gravity acting on an object. Weight = mass x acceleration due to gravity Mass is a measure of the inertia of an object

Gravity Gravity: a force that acts between any two masses. Gravity is an attractive force that pulls objects together. Earth’s gravity acts downward toward the center of the earth. Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite and reduces acceleration.

12.2 Newton’s 1st & 2nd Laws of Motion

Newton’s First Law of Motion AKA Law of Inertia The state of motion of an object does not change as long as the net force acting on an object is zero. Inertia- an object at rest remains at rest, and an object in motion remains in motion. Ex: Front end collision in a car. Your body would keep moving even when the car has stopped.

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 = Force Mass A = F m How would you solve for acceleration? F A m

Practice problem A 100kg crate, sliding on the floor, is brought to a stop by 25N of force. What is the acceleration of the crate? F A m

12.3 Newton’s Third Law of Motion and Momentum When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. Action and reaction forces.

Momentum Momentum- is the product of an objects mass and its velocity. An object with a large momentum is hard to stop. Mass is directly proportional to momentum Velocity is also directly proportional to momentum. Ex: football, skiing, race cars, pool

Conservation of Momentum If no net force acts on the system, then the total momentum of the system does not change. In a closed system momentum is conserved. The momentum from one object is transferred to the second object. Ex: p.376 Train cars

12.4 Universal Forces There are 3 forces that are in the universe: Electromagnetic Nuclear forces (strong and weak) Gravitational forces

Electromagnetic Forces Charged particles. Electric force and magnetic force are the only forces that can both attract and repel. Electric forces- act between charged particles such as electrons and protons (example clothes in a dryer). Magnetic forces- act on certain metals, on poles of magnets, and on moving charges.

Nuclear Forces Strong nuclear Force -acts only on protons and neutrons in the nucleus of an atom. Weak nuclear force - Also, a force within the nucleus of an atom, but it acts only over a short range.

Gravitational Forces Attractive force that acts between any two masses. The weakest universal force. The greater the mass, the greater the gravitational force. Gravitational force decreases with the square of the distance between the objects.

The Earth & Moon How is the moon kept in orbit around the earth? Moon has inertia- should move along a straight path unless acted on by a force. That force would be earth’s gravitational force, which keeps the moon in a nearly circular orbit around the earth.