Rosary School - Forces and Shape Notes PDF

Rosary School – Marj Elhamam Chapter 2: Forces and Shape Name: _________________ Date: ____ /____ / 2024 Physics Summary Notes Grade: 9 ( ) Effects of Fo...

Rosary School – Marj Elhamam Chapter 2: Forces and Shape Name: _________________ Date: ____ /____ / 2024 Physics Summary Notes Grade: 9 ( ) Effects of Forces Forces can affect bodies in a variety of ways:  Changes in speed: forces can cause bodies to speed up or slow down.  Changes in direction: forces can cause bodies to change their direction of travel.  Changes in shape: forces can cause bodies to stretch, compress, or deform. The effects of forces on objects Types of Forces There are many types of force. Some examples include: 1)Gravitational (or weight) - the force between any two objects with mass (like the Earth and the Moon). The weight force always acts downwards to centre of the mass. 2) Electrostatic - the force between any two objects with charge (like a proton and an electron) 1 3) Thrust - the force pushing a vehicle (like the push from rocket engines on the shuttle) 4) Upthrust - the upward force on any object in a fluid (like a boat on the surface of a river). When an object floats, the upthrust is equal to the weight of the object. If the weight of the object is than the upthrust, it sinks. If the upthrust is bigger than the weight to the object it rises. 5) Drag - the force of friction between objects moving through the air (air resistance), like a skydiver in freefall, or between objects moving through a liquid (liquid resistance). 6) Compression - forces that squeeze an object (like squeezing a spring) 7)Tension - forces that stretch an object (like two teams in a tug-of-war) 8) Normal reaction force - the force between any two objects in contact (like the upwards force from a table on a book). Normal means that the force is always 90o to the surface. 2  9) Magnetic forces – the forces of attraction or repulsion between the poles of magnets. Forces as Vectors - Force is a vector quantity because it describes both magnitude and direction. - The length of the arrow represents the magnitude of the force. - The direction of the arrow indicates the direction of the force. - Not all forces are directed perfectly horizontally or vertically and thus need to have an angle described. It is useful to describe an angle with respect to the vertical or the horizontal. Example 1: Jan 2021 1PR A force called upthrust acts vertically upwards on the bubble. Draw two labelled arrows on the diagram to show the forces on the bubble as it is released. (3) Calculating Resultant Force - A resultant force is a single force that describes all the forces operating on a body. - When many forces are applied to an object they can be combined (added) to produce one final force which describes the combined action of all the forces. - This single resultant force determines: 3  1. The direction in which the object will move as a result of all of the forces. 2. The magnitude of the final force experienced by the object. - The resultant force is sometimes called the net force. - Resultant forces can be calculated by adding or subtracting all the forces acting on the object. - Forces working in opposite directions are subtracted from each other - Forces working in the same direction are added together. - If the forces acting in opposite directions are equal in size, then there will be no resultant force – the forces are said to be balanced. Example 2: Calculate the resultant forces for the following : - When calculating resultant forces, always remember to provide units for your answer and to state whether the force is to the right, to the left, up or down. Always provide your final answer as a description of the magnitude and the direction. - Balanced forces mean that the forces have combined in such a way that they cancel each other out and no resultant force acts on the body. - Unbalanced forces mean that the forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object. Friction: - Friction is defined as: The force which opposes the motion of an object and slows it down. It always acts in the opposite direction to object’s motion. - Friction emerges when two (or more) surfaces rub against each other: At a molecular level, both surfaces contain imperfections. These imperfections tend to push against each other. - Friction allows cars’ wheels to grip the road and makes objects accelerate. - Friction wears out surfaces and reduces efficiencies as energy will be wasted to the surrounding as heat. - Friction can be reduced on a moving object in the laboratory- using air-tracks- to a very low value. Such an object will continue to move in a straight line at a constant speed once set in motion even when the thrust force is no longer acting on it. 4 Forces and changing shape: - When some objects, such as springs or rubber bands, are stretched they will return to their original shape and length once the forces are removed. - Other materials, such as plastic, remain permanently deformed (stretched). - A change of shape is called a deformation and can either be: Elastic or Inelastic (plastic) Elastic deformation - Elastic deformation is defined as: The ability of a material to recover its original shape after the forces causing deformation have been removed. - Examples of materials that undergo elastic deformation are steel springs, rubber bands, fabrics. Inelastic (plastic) deformation Inelastic deformation is defined as: The permanent change in the shape of a material after the forces causing deformation have been removed. Examples of materials that undergo inelastic deformation are plastic, clay, glass. Hooke's Law: Springs change length when a force acts on them, and they return to their original length when the force is removed. The relationship between the extension of an elastic object and the applied force is defined by Hooke's Law.  Hooke's Law states that: The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality.  Directly proportional means that as more force is applied, the greater the extension (and vice versa) 5   The limit of proportionality is: o the point where if more force is added, the extension is no longer proportional to the force. o The elastic object starts to stretch more for each increase in the load force. o The extension beyond the limit of proportionality does not follow Hooke's law. o The elastic material can still return to its original length as you take the weights off until it reaches another point called the elastic limit.  If the elastic object was stretched beyond the elastic limit, it will not return to its original length as you take the weights off.  Hooke’s law applies to metal springs and wires. If a wire is stretched, the extension is proportional to the load up to a certain load, then some wires break, others extend plastically (narrowing /necking) before breaking. Hooke's Law states that a force applied to a spring will cause it to extend by an amount proportional to the force The Force-Extension Graph  Hooke’s law is the linear relationship between force (F/N) and extension (x/m).  This is represented by a straight line on a force-extension graph.  Any material beyond its limit of proportionality will have a non-linear relationship between force and extension.  Elastic bands are usually made of rubber. They are elastic material. When an elastic band stretches under load, the graph is not a straight line, showing that elastic bands don’t obey Hooke’s law. 6  The extension produced by a given load force is different when increasing the load force (loading) to when you decrease the load (unloading). The graph will look like an S shape. Question 9 page 27: 7 Note: 1.22 Practical: Investigate how extension varies with applied force for helical springs, metal wires and rubber bands. Refer to page 24 in student book, notes in copybook and relevant questions in Chapter 2 past papers worksheet. 8