Forces PDF

Chapter 12 chapter 12 chapter 12 f o rc e s f o rc e s f o rc e s  Force is measured in Newton (N) force CONTACT FORCES Non-CONTACT FORCES Push or pull on an object. Can be balanced or unbalanced fo...

Chapter 12 chapter 12 chapter 12 f o rc e s f o rc e s f o rc e s  Force is measured in Newton (N) force CONTACT FORCES Non-CONTACT FORCES Push or pull on an object. Can be balanced or unbalanced forces Act from a distance Objects must be in contact with each Exert an attraction or repulsion force other Objects are not in contact with each o Frictional force other o Applied force o Gravitational force (Weight) o Normal force o Magnetic force o Tension o Electrostatic force  Physical q uantities VECTOR SCALAR Quantity with both Quantity with magnitude magnitude and direction only Displacement Distance Velocity Speed Acceleration Time Momentum Mass Force Energy 3  speed Direction of motion bending stretching Squeezing/ squashing tearing turning THE EFFECT OF twisting CONTACT FORCES friction  Free-body diagrams are diagrams used to Free body show the relative magnitude and direction of all forces acting upon an object in a given diagrams situation. Use a ruler and a sharp pencil ALWAYS. The size of the arrow in a free-body diagram reflects the magnitude of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labelled to indicate the exact type of force. It is generally customary in a free-body diagram to represent the object by a DOT and to draw the force arrow from the centre of the DOT outward in the direction that the force is acting. 5  Types of forces contact 1. Applied force - Fapp The force that allows something to happen. A force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object The applied force is the force exerted on the desk by the person. 6  Types of forces contact 2. Tension – FT Force that acts through a solid object such as a rope or chain; directed along the rope and pulls equally on the objects on either end. The rope/string/chain must be pulled taut (tight). Pull on an object through a solid object. 7  Types of forces contact 3. Normal Force - FN The force that a surface exerts on an object that is in contact with that surface. Perpendicular to the surface 8  Types of forces contact 4. Frictional force - FF The force parallel to the surface that opposes the motion of the object. Friction acts in a direction opposite to the object's direction in motion, but parallel to the surface. Without friction, the object would continue to move at a constant speed forever 9  Types of forces contact 4. Frictional force - FF Friction (Ff)- contact force between two surfaces that always opposes motion. Static friction (Fs) = keeps an object from moving (must be overcome to move an object at rest Kinetic friction (Fk) =acts when an object is moving Kinetic friction is weaker than static friction Fk< Fs 10 Types of forces contact 4. Frictional force – Air resistance The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object (upwards). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas. 11  Types of forces contact 4. Frictional force - FF Breaking in a car = Break pads increase friction to slow down the car Thinking distance = distance travelled before breaking occurs Braking distance = distance the car takes to come to a rest Total stopping distance = TD + BD Factors that affect stopping distance = Speed and mass of car Tyre and road conditions Efficiency of breaks Alertness of driver 12  Types of forces non-contact 1. Force of gravity – Fg or W The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. It is the weight of the object. All objects on earth (which have mass) experience a force of gravity that is directed "downward" towards the centre of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation: W = mg Magnetic and or electrostatic Fg= mg non-contact g = gravitational acceleration = 9.8 m.s-2 forces will be done later. 13  Mass vs weight mass weight Mass is the measure of the amount of matter in Weight is the measure of the amount of force a body. acting on a mass due to the acceleration due to gravity. Symbol – m Symbol – W Unit - kg Unit – N ( Newton) All objects with mass accelerate towards the centre of the earth at an acceleration of 9.8 m.s- 2 – g ( gravitational acceleration) o W=mxg o Fg = m x g 14  combining forces Forces in the same direction are added together Forces in the opposite direction are subtracted from each other Net force (Fnet) is the overall force acting on an object – Fnet = F1 +F2 + F3…. 15  forces balanced unbalanced Remain at Position will Fnet = 0 Remain at rest constant Fnet = 0 Accelerate change velocity All the forces If an object is at If an object is Net force will The object will The objects acting on the rest it will moving at a equal the larger accelerate due position will object combine remain at rest if constant force minus the to the applied change as the to equal zero. all of the forces velocity, it will smaller force. force applied force acting on it is remain at that will cause a balanced. velocity if all of change in the forces motion. acting on it is balanced. 16  Ba l a n c e d f o rc e s An object is in equilibrium when it is stationary and when it is moving at a constant speed. Fnet = 0 The forces acting on the object are balanced. Presentation title 17  U n ba l a n c e d f o rc e s An object that is not moving will not start moving unless there is an unbalanced force acting on it. An object will not stop moving unless there is an opposing force acting on it. if F net ≠ 0 N or F net > 0 N Presentation title 18  U n ba l a n c e d f o rc e s To have unbalanced forces means that the force applied in one direction is greater than the force applied in the opposite direction. When unbalanced forces are acting on an object, there is a change in velocity/speed and/or direction. By applying an unbalanced force, you can change the motion of an object. Unbalanced forces can make an object at rest start moving, make a moving object stop, or change the direction and speed of the object. if F net ≠ 0 N or F net > 0 N 19  Class activity Calculate Fnet in the following examples 1. Ayanda is pushing a box from one side of the class to another with a force of 20N. The force of friction is 10N and it is acting in the opposite direction. 2. Bokang is being pulled in a wagon by Jordan at a force of 15N. Amo is also pushing the same wagon with a force of 10N. The force of friction is 10N and it is acting in the opposite direction. 3. Liv is ice skating with a force of 30N. There is no friction on the ice rink. Presentation title 20  Class activity Calculate Fnet in the following examples 1. Ayanda is pushing a box from one side of the class to another with a force of 20N. The force of friction is 10N and it is acting in the opposite direction. Fnet = 20 – 10 = 10N 2. Bokang is being pulled in a wagon by Jordan at a force of 15N. Amo is also pushing the same wagon with a force of 10N. The force of friction is 10N and it is acting in the opposite direction. Fnet = 15 + 10 – 10 = 15N 3. Liv is ice skating with a force of 30N. There is no friction on the ice rink. Fnet = 30N 21 N e w to n s f i r s t l aw  1 s t l aw – l aw o f i n e rt i a Newton's first law states that if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by an external force. Inertia is a property of mass (matter) where objects will tend to resists change in motion. 23  inertia The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion, so it has more INERTIA. The more mass an object has, the more difficult it is to: oAccelerate it oIncrease or decrease its velocity oChange its direction 24 Presentation title 25  N e w to n s s e c o n d l aw  newtons 2 nd law Newton's second law of motion pertains to the behaviour of objects for which all existing forces are not balanced. The acceleration (m.s-2) of an object is dependent upon two variables - the net force (N) acting upon the object and the mass (kg) of the object. Newton’s second law states that: The acceleration of an object is directly proportional to the net force acting upon the object, and inversely upon the mass of the object. Net force (N) = mass (kg) x acceleration (m.s-2) Fnet = ma 27  Fnet = ma As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased. Fnet = ma = F1 +F2 + F3… Fnet m a 28  calculations 1. Calculate Fnet How much force is needed to accelerate a 66 kg skier at travelling down a slope at 2 m/s2? Presentation title 29  calculations 2. Calculate acceleration What is the acceleration of a 50 kg object pushed left with a force of 500 newtons? 30  calculations 3. Calculate mass A force of 250 N is applied to an object that accelerates north at a rate of 5 m/s2. What is the mass of the object? Presentation title 31  calculations 4. Calculate Fnet without mass Jordan is pushing a friend down a hill on a skateboard, his friend is accelerating at 5m/s2 and weighs 500N. Calculate the force Jordan applied. 32  calculations 5. More than 1 force A 5 kg block is pulled across a table to the right by a horizontal force of 40 N with a frictional force of 8 N opposing the motion. Calculate the acceleration of the object. 33  calculations 6. More than 1 object A bowling ball with a force of 15 N accelerates at a rate of 3 m/s2 ; a second ball rolled with the same force accelerates 4 m/s2. What are the masses of the two balls? 34  calculations 7. Objects on ice A force of 200 N is exerted on an object of mass 40 kg that is located on a sheet of perfectly smooth ice. A. Calculate the acceleration of the object. B. If a second object identical to the first object is placed on top of the first object, what acceleration would the 200 N force produce? 35  calculations 8. More than 1 object on top of each other If a 60 kg person on a 15 kg sled is pushed with a force of 300 N, what will be person’s acceleration? 36  calculations 9. No Fnet given A box with a mass of 20kg is being pushed with an applied force of 30N. The frictional force acting on the box is 10N. Calculate the acceleration of the box. 37  calculations 10. Calculate other forces A man pushes a 50kg wagon with an applied force of 140N and it accelerates at 2m.s2. determine the magnitude of the frictional force. 38 N o n - c o n ta c t f o rc e s  Non-contact forces Gravitational force(weight) Electrostatic force Magnetic force Act at a distance and are associated with a field Gravitationalfield Electrostatic/electric field Magnetic field Presentation title 40  electrostatics Electrostatic field is a region in space where a charged object experiences an electrostatic force. Electrostatics is about an imbalance of charge within a material. All materials contain both positive and negative charge because they are made up of atoms which contain both protons and electrons. A charged object has an imbalance of charge – positive objects have less electrons than protons; negative objects have more electrons than protons. Presentation title 41  electrons Only electrons are transferred between materials to cause an imbalance of charge (or to charge the object). Presentation title 42 How do objects become charged? 1. Friction 2. Conduction 3. Induction Presentation title 43  friction Charge can be separated by friction if a rough surface is rubbed against a smooth (or rough) surface. Electrons can be transferred from one surface to the other by friction. When neutrally charged insulating materials rub against each other, they may become electrically charged. Electrons, which are negatively charged, may be ‘rubbed off’ one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with a positive charge. Presentation title 44  friction When a rod made of cellulose acetate is rubbed with a duster, the friction causes electrons to gain energy. Presentation title 45  friction Both the rod and the duster are made of insulating materials. Insulators prevent the electrons from moving and the charge remains static. Conductors, on the other hand, cannot hold the charge, as the electrons can move through them. Presentation title 46  Electrical forces A charged object will experience non-contact force from another charged object. The type of force will depend on the type of charge (positive or negative) on the two objects. The properties of attraction and repulsion are often used to show that an object is charged: a charged rod can pick up small pieces of paper a charged balloon can stick to the wall by attraction a charged rod can pull a stream of water towards it Presentation title 47 Presentation title 48