Presentation on theme: "Chapter 4 Forces in One Dimension"— Presentation transcript:
1 Chapter 4 Forces in One Dimension Newton’s Laws of Motion
2 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 Issac Newton described three Laws that govern motion.
3 Force is measured in Newtons (N) in the metric system. 1 N = 1 kgm/s2The English unit for force is a pound (lb).4.48 N = 1 lbForces are vector quantities and canbe denoted with arrows because youcan push in specific direction.
4 Force diagrams: Four types of forces can act on an object Fn = normal force.This is the support force that objects resting on each other exert.Fa= applied forceThis is the force that is being applied to the object. It always acts like it comes out of the center of massFf= frictional forceThis is a force that happens whenever two surfaces touch. This force will always oppose the applied force.Weight= Fg= Gravitational forceThis is the force created because of gravity pulling on the mass of the object.
5 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.INERTIAExample: The man pushed the cart in orderto make it roll.The bigger the cart theharder the man must push
6 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 aF = forcem = massa = accelerationF = m x aWhat is mass?The amount of stuff (atoms) in an object
7 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 Nm = Fa20 m/s2m = 10 N (kgm/s2)20 m/s2= 0.5kg?Fma
8 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 NRemember your vector rules10 kgNet Force= 50 N – 20 N= 30 N?a = Fm= 30N10kg= 3 m/s2
9 Remember what happens if all four forces are balanced? FnFfFgFaThe object is standing still or moving at a constant velocity!
10 Weight and GravityGravity is the rate at which all things fall to the earth.It is always 9.8 m/s2Weight 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= weightm = massg = gravityNotice that gravity is an acceleration valueFw= mgg = 9.8 m/s2Air 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!!!!
11 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.
12 Weight vs. MassYour 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.
13 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?EQUAL and OPPOSITE REACTIONSThey are standing still or moving at constant velocity.Action:The boy threw the ball forward with a certain amount of forceReaction:The boy rolled backward with the same amount of force
14 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.
15 You can calculate momentum using this formula: p = momentump = mv m = massv = velocity