Forces and Motion PDF Notes

FORCES and MOTION What is this called what we are actually doing? But what is a force? What are the effects of force? What is a Force? A force is a push or pull that acts on an object due to the interaction with another object. interaction = the objects experience an equal and...

FORCES and MOTION What is this called what we are actually doing? But what is a force? What are the effects of force? What is a Force? A force is a push or pull that acts on an object due to the interaction with another object. interaction = the objects experience an equal and opposing force on each other Force only exists as a result of an interaction What is a Force? A force is a vector quantity vector quantity = magnitude and direction Force is measured in Newtons (N) two main: contact forces non-contact forces What is a Force? Can you see a force? You can only see its effects Your task: Thinking Activity 1.1 – Effects of Forces Force How do we describe the motion of an object? … it has velocity Velocity the rate of change of the position of an object the speed of an object and the direction in which it is moving Speed describes only how fast an object is moving Force An object moving at constant velocity. it travels at the same speed in the same direction What happens when the car moves faster or slows down? change the motion of an object speed up or slow down change in velocity acceleration = ∆v/t Effects of Forces Forces can change in position and change the direction in which an object is moving. Forces can change the shape of an object deformation Scalar or Vector Quantities quantity is something that can be measured. Scalar quantities ONLY have magnitude. Vector quantities have both a magnitude and a direction. Force is a Vector Quantity 2 effects of an individual force upon an object: balanced force  Force is a Vector Quantity 2 effects of an individual force upon an object: unbalanced force = net force Force is a Vector Quantity 2 effects of an individual force upon an object: balanced force = net force = 0 Free Body Diagram (FBD) Free Body Diagrams in 1D Draw the Object & Surface all of the forces act in Add the forces the same line Construct a free body diagram showing the forces on a skydiver who has just opened their parachute. Free Body Diagram (FBD) Free Body Diagrams in 2D the forces will act over one plane A worker attempts to push a box across a factory floor. A static friction acts. Draw a FBD showing all of the forces. Draw a box is on a surface Add the forces Free Body Diagram (FBD) A frictional force can’t exist without a gravitational force and a gravitational force should always come with a normal force Pairs of Forces Pairs of Forces - 1 Dimension Opposing forces: net force = 10 N + (- 6 N) = 4 N, to the left Pairs of Forces F1 Same forces: F2 F1 F2 7N 10N Net force = 7 N + 10 N = 11 N, to the right Contact Forces Air resistance When an object moves through the air it experiences air resistance. This force acts against the direction of movement. The faster the object is travelling the greater the air resistance. Contact Forces Upthrust Upthrust is an upwards force that acts on an object when it is in a fluid (a liquid or gas). Contact Forces Thrust Thrust is a driving force exerted by an engine to make an object move. Normal reaction force When an object pushes on a surface like a table, wall or the ground, the surface pushes back on the object with a balancing force. The normal reaction force always acts at right angles to the surface. Friction can sometimes be useful and sometimes it can be a nuisance. Non-Contact Forces Non-contact forces act between two objects that are not physically touching. Magnetic force Magnetic force is experienced by a magnet or a magnetic material, eg iron, when placed in a magnetic field. This force can pull the two objects together or push them apart. Electrostatic force Electrostatic force is experienced by a charged particle in an electric field. This force can be either attractive or repulsive. Non-Contact Forces Gravitational force Gravitational force is experienced by a mass when it is sufficiently close to another mass. Gravity always pulls two objects towards each other. It never pushes them apart. In the diagram, Earth pulls the satellite and the satellite pulls Earth. The forces are equal in size and opposite in direction. A skydiver is falling from an aeroplane. Identify the two forces which are acting on her. (The gravitational force, or weight, acts downwards and air resistance acts upwards) Concept Check When a car turns around a corner, what force is causing the change of direction? (The change in direction is caused by friction between the tyres and the road). Which of the following is NOT a contact force? A. Tension B. Gravitational force C. Normal reaction force D. Friction Concept Check Which of the following statements is TRUE? A. Gravitational force only exists on Earth. B. Gravitational force can attract and repel. C. Gravitational force causes air resistance. D. Gravitational force is always attractive. In which situation is friction a useful force? A. On a playground slide B. In the moving parts of gears C. Car tyres gripping the road D. In a bicycle chain Concept Check What is a general name for a force that makes a vehicle move? A. Thrust B. Friction C. Power D. Air resistance In which direction does air resistance always act? A. Upwards B. Downwards C. Against the direction of movement D. Behind boat sails In which direction does the normal reaction force always act? A. At right angles to the surface C. to the left B. Upwards D. in a circle