Forces An Introduction

Dynamics In the first unit we studied Kinematics which is the study of how objects move without being concerned with why they move the way that they do In this unit, we will be looking at Dynamics which explain why objects move the way that they do One way to understand why an object moves is to study the forces acting on it These forces may cause the object to start moving, to speed it up, to slow it down or to cause it to remain stationary

Forces & Technology People use their cars almost every day to get to work or school, or just to get around. Many of us take automotive technology for granted and assume that the car we are driving is safe and reliable. Yet there are many different types of cars, and the technology and safety features in them are quite different.

Forces & Technology Some safety features of a car are the headrest at the back of each seat, the seats belts, and the airbags. Did you know that many people put their headrest at the incorrect height? Did you know that older-style seat belts actually caused injuries? How do airbags work?

Forces & Technology Automotive technology is not the only type of technology involving forces For example, why is it so easy to slide across ice and wet floors? How are forces involved with sports such as snowboarding, golf and hockey? These are just a sample of the type of questions that could be asked…..there are far too many application of technology to be covered in this unit alone

Overall expectations… By the end of this unit, students will: Analyse and propose improvement to technologies that apply concepts related to dynamics and Newton’s laws, and asses the technologies’ social and environmental impact; Investigate, in qualitative and quantitative terms, net forces, acceleration, and mass, and solve related problems Demonstrate an understanding of the relationship between changes in velocity and unbalanced force in one dimension

Big Ideas Long after this course, the hope is that you retain the following concepts: Forces can change the motion of an object Applications of Newton’s laws of motion have led to technological developments that affect society and the environment

Forces in Nature Forces are everywhere, acting on every object we see Engineers must consider these forces carefully when designing bridges and buildings Forces are involved in every sport

Forces in Nature In simple terms, a force (F) is the push or pull (and thus a vector) Forces can cause objects to change their motion Forces can also distort matter by compressing, stretching, or twisting them

Forces in Nature Isaac Newton discovered many of the concepts in this unit. For this reason the unit of force, the Newton (N) is a derived SI unit equal to 1 kg∙m/s2 To measure force in the lab, you can either use a spring scale (mechanical) or a force sensor (electronic)

Force Force (F) A push or pull copy Force (F) A push or pull Causes objects to change their motion and/or shape Is a vector Is measured in newtons (N) 1 N = 1 kg∙m/s2

Try it out…. Mini Investigation: page 113 Two spring scales One 100 g object One 200 g object

The Four Fundamental Forces There may seem to be many different types of forces around us, but physicists have found that they are able to understand how objects interact with one another by classifying forces into only four categories: Gravitational force Electromagnetic force Strong nuclear force Weak nuclear force

Four Types of Forces copy 1. Gravitational Force - known as “force of gravity” or “weight” - attraction ONLY - acts between all objects in the universe - weak force but long range Electromagnetic Force - caused by electric charges - most common force (i.e. light, electricity, magnetic attraction…) - strong force and long range Strong Nuclear Force - holds protons and neutrons together in the nucleus of an atom - strongest force but very short range 4. Weak Nuclear Force - responsible for radioactive decay - strong force but very short range In the nucleus of an atom, the positively charged protons are very close and repel each other. They do not fly apart because the strong nuclear force of attraction between the protons and neutrons keeps them in place

Everyday Forces We experience several types of forces daily Ex. Two children playing outside with a wagon One child pulls forward on a rope tied to the front The other child pushes on the wagon from behind What forces act on the wagon?

Everyday Forces In order to analyse the forces acting in this situation we could look at two different force diagrams System diagram – sketch of all of the objects involved in a situation like the picture below: Free-body diagram (FBD) – object shown as a rectangle or a large dot with the forces drawn as arrows originating from the object and pointing away from the centre

Free-Body Diagram Labelling the FBD: Consider the applied force: results when one object is in contact with another object and either pushes or pulls on it The child behind the wagon exerts an applied force on the wagon by pushing on the back

FBD There is also a tension force: pulling force exerted on an object by a rope or a string The child at the front of the wagon pulls on the rope, causing tension in the rope. The rope exerts tension on the wagon, pulling it forward

FBD Whenever an object is in contact with a surface, the surface can exert two different forces on the object One, is the normal force : a perpendicular force exerted on an object by the surface with which it is in contact. In our example, the ground is pushing up on the wagon

The other force exerted by a surface on an object is friction, friction is a force that resists the motion or attempted motion of an object and always acts parallel to the surface If the wagon is moving right, then friction acts towards the left Even if the wagon was at rest with the children pushing and pulling on it, friction would still be present

FBD Some forces do not require contact….. A force such as gravity is called a non-contact force Gravity always acts towards Earth’s centre

Mini Investigation Pg 116 Materials: Spring scale Set of objects of known mass

Force of Gravity Is an example of a non-contact force (i.e. it is a force that acts at a distance) To calculate the force of gravity acting on an object, you can use the equation Fg = mg (m = mass; g = 9.8 m/s2

Everyday Forces copy Force of Gravity (Fg)  force of attraction between Earth and object Normal Force (FN)  force perpendicular to surface upon which object rests Friction Force (Ff)  force that opposes (attempted) motion of object Applied Force (Fa)  force exerted on object Tension Force (FT)  force exerted by string or rope on object Note: The force of gravity is a non-contact force (i.e. it does not require contact in order to be experienced) Fg = mg Free-Body Diagrams (FBD) are simple drawings representing the object and all external forces acting upon it

Free-Body Diagrams (FBD) copy A simplified picture showing all of the forces acting on the object Examples: Free-falling object (no air resistance) A book on a desk Pulling a wagon with a rope

External & Internal Forces In this course, we will usually be concerned with external forces External forces are caused by one object pushing or pulling on another Internal forces occur when an object exerts a force on itself For example, when skate 1 pushes on skater 2, the force on skater 2 is external If skater 1pulls forward on her own arms, then it is an internal force

Practice Draw a Free-Body Diagram for the following: A) a cup sitting a rest on a table B) a large trunk is pulled by a rope to the right side, the trunk does not move C) a baseball player is sliding to the left across the ground D) a desk is pushed to the left across the floor

A cup sitting at rest on a table

B) a large trunk is pulled by a rope tied to the right side, the trunk does not move

C) a baseball player is sliding to the left across the ground

D) a desk is pushed to the left across the floor

Textbook: Read over pages 114 – 121 Answer #1-5, 7

Net Forces Balanced Forces:- When two forces acting on an object cancel each other out because they are equal in magnitude, but opposite in direction. Net Force = 0 N When all of the forces acting on an object do not cancel each other out there is an Unbalanced Force or Net Force  Net Forces   Fnet = F1 + F2 + F3 + …

Adding Forces A Free Body Diagram can be used to construct a vector diagram which can then be used to calculate the net force acting on an object.   Example 1: An object has two forces being applied to it: F1 = 120 N [right] F2 = 50 N [left] What is the net force acting on the object?

Example 2:   An object has two forces being applied to it: F1 = 9 N [right] F2 = 3 N [down] What is the net force acting on the object?

Worksheet Pg. 122 #15