Forces, The laws of Motion & Momentum

What is force? When two or more forces act on an object at the same time, the forces combine to form the net force Forces on an object that are equal in size and opposite in direction are called balanced forces

Force is the action which has the ability to change motion. Objects need a force applied to them in order to change their motion. A force is also called a push or a pull. Sir Isaac Newton described three Laws that govern motion.

What is force? Forces don’t always balance out:

Force is measured in Newtons (N) in the metric system. 1 N = 1 kgm/s2 The English unit for force is a pound (lb). 4.48 N = 1 lb Forces are vector quantities and can be denoted with arrows because you can push in specific direction.

Force diagrams: Four forces can act on an object Fn = normal force. This is the force that holds atoms together and makes things solid. Fa= applied force This is the force that is being applied to the object. It always comes out of the center of mass Ff= frictional force This is a force that happens whenever two surfaces touch. This force will always oppose the applied force. Weight= Fg= Gravitational force This is the force created because of gravity pulling on the mass of the object.

Football Physics Why are the most massive players lineman? Why are the least massive players receivers? Receivers want to change their motion, lineman want to hold their ground

What if forces are balanced? Newton’s First Law of Motion An object at rest stays at rest or an object in motion stays in motion (constant velocity) unless acted on by an unbalanced (net) force. It is also known as “the law of inertia” Inertia: an objects ability to resist a change in motion. Example: The man pushed the cart in order to make it roll. The bigger the cart the harder the man must push

Newton’s First Law of Motion An object at rest will stay at rest and an object in motion will continue in motion unless acted upon by an unbalanced force.

Newton’s 2nd Law of Motion Which wagon is going to be easier to pull? Which wagon needs a greater force to pull it? How can you increase the acceleration of wagon A? A B

Force on different masses Which one will be harder to push will the same force?

What if forces are unbalanced? Newton’s Second Law of Motion If an unbalanced (net) force is applied to a mass it will accelerate. The mathematical relationship between acceleration and mass is directly proportional and inversely proportional to force. A math formula can be written: F = m a F = force m = mass a = acceleration What is mass? The amount of stuff (atoms) in an object

Newton’s Second Law So if you double the force what happens to the acceleration? It also doubles If you apply the same force to twice the mass what happens to acceleration? Results in half the acceleration

The 2nd law says…. The more mass an object has, the more force it’s going to take to get it to accelerate.

You try! Example: The man pushed the cart with 10 N of force to make it accelerate 20 m/s2. What was the mass of the cart? See if you can calculate the mass. F = a = m = 10 N m = F a 20 m/s2 m = 10 N (kgm/s2) 20 m/s2 = 0.5 kg ? F m a

What if forces are unbalanced? Tracker A is pulling on the 10 kg block with a force of 50 N and tracker B is pulling on the block with 20 N. How fast is the block accelerating and in what direction? F = m = a = 30 N Remember your vector rules 10 kg Net Force= 50 N – 20 N= 30 N ? a = F m = 30N 10kg = 3 m/s2

Remember what happens if all four forces are balanced? Fn Ff Fg Fa The object is standing still or moving at a constant velocity!

Weight and Gravity Gravity is the rate at which all things fall to the earth. It is always 9.8 m/s2 Weight is a measure of how gravity pulls on your mass to create a downward force. Your weight can be calculated just like any other force is calculated. Fw= weight m = mass g = gravity Notice that gravity is an acceleration value Fw= mg g = 9.8 m/s2 Air resistance is friction for falling objects. Air resistance is why it seems like things fall at different rates. All things fall at this rate no matter what their size!!!!

Remember the things that effect gravity are: The size of the mass. The bigger the mass the greater the gravitational pull. The distance between the masses. The farther apart two masses, the less gravitational force is felt between them.

Weight vs. Mass Your mass is constant because it is a measure of the amount of atoms in your body. Your weight will change if the gravity of the planet changes. Since Mars is a smaller planet, it will have a smaller gravity and pull down on a mass with less force.

Mass vs. Weight Weight is... Mass is... A measure of the gravitational force on an object Varies depending on where you are in the universe Measured with a spring scale Expressed in newtons (N) Mass is... A measure of the amount of matter in an object Always constant no matter where in the universe Measured with a balance Expressed in kg

Newton’s 3rd Law of Motion For every action, there is an equal but opposite reaction

Forces come in pairs Newton’s Third Law of Motion For every action there is an equal and opposite reaction. These forces always occur in pairs and are often called action-reaction forces. Remember; if forces are equal, what is happening to the object? They are standing still or moving at constant velocity. Action: The boy threw the ball forward with a certain amount of force Reaction: The boy rolled backward with the same amount of force

Momentum: An object’s tendency to continue with a certain motion Momentum has to do with collisions and what happens to motion when objects collide. The Law of conservation of momentum states that the amount of momentum after a collision must equal the amount of momentum before the collision if no outside forces act on the objects.

You can calculate momentum using this formula: p = momentum p = mv m = mass v = velocity