Forces and Newton's laws

1. Forces and Newton's laws Teacher: Aurora Comis

3. Communication (What language do we need working with the content? What Physics language will learners communicate during the lesson?) • Simple Present (A Force is….) • Present Perfect (We have measured ….) • Imperative (Let us consider…..) • Content (Subject matter - What are my objectives? What are the learning outcomes?) • Definition of Force • Vectors and scalar quantities • Hooke’s Law • Gravity • Newton’s Laws • Mass and Weight • Friction The 4 C’s • Cognition (What thinking skills are demanded to the learners?) • Repeating procedures • Ordering steps • Checking results • Handling formulas • Defining concepts • Making hypotheses, interpreting, judging and evaluating to solve problems • Culture (What are the cultural implications of the topic?) • Some historical background • Practical and technological implications

4. What is a Force? We can’t actually see Forces, but we see their effect on objects. We see the Moon orbit the Earth, objects fall to the ground and birds fly – all of these are because of Forces. 1 Newton isn't a very big Force: it's about the weight of an apple. Force is a physical entity that can change the state of rest or motion of a body. Force can be defined as a push or a pull. (Technically, Force is something that can accelerate objects). For example, when you throw a ball, you apply a Force to the ball. The unit of Force is called the Newton, and it has the symbol N. So 100 N is a bigger Force than 5 N. A Force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton.

5. Vectors and Scalars We can broadly divide the measurements we make into two types: Vector quantities have both a magnitude and a direction. Examples include:Force, because it matters which way you're pushing, as well as how hard.Velocity, because velocity is speed in a particular direction.Weight, because weight is a force, and forces are vectors.Acceleration, because it matters which way you're accelerating. Scalar quantities don't have a direction, just a magnitude Examples include:Mass, which is simply a measure of how much material is there, in kilogrammes.Speed, which is simply how fast you're going.Volume, which is how much space an object takes up.Temperature: although we talk about temperature going "up" or "down", they aren't really directions.

6. Something about Forces Forces are vector quantities, because the direction is important. The resultant of the Forces applied to a body follows the properties of the composition of the vectors. The Forces are measured with Force meter. Force meters contain a spring connected to a metal hook. The spring stretches when a Force is applied to the hook. The bigger the Force applied, the longer the spring stretches and the bigger the reading. Force meter applied the Hooke’s law.

7. Springs: Hooke's Law & Elastic Limits "Hooke's Law" is about stretching springs and wires. When we apply a Force to a spring, it stretches. If we apply double the Force, it stretches twice as much, so long as we don't over-do it. F = kΔl The force is directly proportional to the variation of the length. Example: we measure the original length of the spring when we start. When it stretches, we measure the extension – that's how much longer it is than it was when we started. Extension = present length – original length • the extension is proportional to the Force • the spring will go back to its original length when the force is • removed so long as we don't exceed the elastic limit.

8. Hooke's Law states The elastic limit is where the graph departs from a straight line. If we go past it, the spring won't go back to its original length. When we remove the force, we're left with a permanent extension. Below the elastic limit, we say that the spring is showing "elastic behaviour": the extension is proportional to the force, and it'll go back to it's original length when we remove the force. Beyond the elastic limit, we say that it shows "plastic behaviour". This means that when a force is applied to deform the shape, it stays deformed when the force is removed.

9. About Forces • Forces can change: • the speed of an object • the direction that an object is moving in • the shape of an object Physicists devote a lot of time to the study of Forces that are found everywhere in the universe. The Forces could be big, such as the pull of a star on a planet. The Forces could also be very small, such as the pull of a nucleus on an electron. Forces are acting everywhere in the universe at all times.

10. Examples of Forces If you were a ball sitting on a field and someone kicked you, a Force would have acted on you. As a result, you would go bouncing down the field. There are often many forces at work. Physicists might not study them all at the same time, but even if you were standing in one place, you would have many Forces acting on you. Those Forces would include gravity, the Force of air particles hitting your body from all directions (as well as from wind), and the Force being exerted by the ground (called the Normal Force). If there is more than one force acting on an object, the forces can be added up if they act in the same direction, or subtracted if they act in opposition.

11. Gravity Gravity is a Force that acts towards the centre of the Earth. This means that, wherever you are in the world, "down" is always towards the ground - even though your "down" isn't the same direction as anybody else's. The gravitational pull of the Earth is what gives objects weight. Thus weight is a Force - it's how hard the Earth is pulling on an object. The Earth pulls on every kilogramme with a Force of ten Newtons.

12. We say that the Earth's gravitational field strength (at ground level) is 10 Newtons per kilogramme (10 N/kg). In other words, an object with a mass of 1kg has a weight of 10N. (Actually, it's more like 9.81, but for GCSE we usually call it 10.) Gravity is a very weak Force, you need a very large mass in order to get a noticeable gravitational pull. An odd thing about gravity: it always attracts objects and never repels them.

13. In the Universe every object that has mass exerts a gravitational pull, or force, on every other mass. The size of the pull depends on the masses of the objects. The gravitational force between the Earth and the molecules of gas in the atmosphere is strong enough to hold the atmosphere close to our surface. Smaller planets, that have less mass, may not be able to hold an atmosphere. Obviously, gravity is very important on Earth. The Sun's gravitational pull keeps our planet orbiting the Sun. The motion of the Moon is affected by the gravity of the Sun AND the Earth. The Moon's gravity pulls on the Earth and makes the tides rise and fall every day. As the Moon passes over the ocean, there is a swell in the sea level. As the Earth rotates, the Moon passes over new parts of the Earth, causing the swell to move also. The tides are independent of the phase of the moon. The moon has the same amount of pull whether there is a full or new moon. It would still be in the same basic place.

14. Isaac Newton December 25th 1642 - March 20th 1727 Newton was an English physicist and mathematician, and one of the most influential figures in science. He discovered that white light is composed of many colours, and laid the foundations for modern physical optics. He formulated his three Laws of Motion and the law of Universal Gravitation.

15. Newton's First Law of Motion You will have to learn a new terminology here: net Force. Net Force is the sum of all Forces acting on an object. When you slide your book on floor it will stop soon. When you slide it on icy surface, it will travel further and then stop. Galileo believed that when you slide a perfectly smooth object on a frictionless floor the object would travel forever. Isaac Newton developed the idea of Galileo further. He concluded that an object will remain at rest or move with constant velocity when there is no net Force acting on it. This is called Newton's First Law of Motion, or Law of Inertia.

16. Newton's First Law,says that if the Forces applied on an object are in balance, the object's speed and direction of motion won't change. In other words, if you leave it alone, it'll carry on doing whatever it was doing already. If the Forces applied on an object are in balance, then the object's velocity is constant. This means that if it's not moving, it'll stay still; or if it is moving, it'll continue in a straight line at a constant speed .

17. Newton's Second Law of Motion Newton's First Law deals with an object with no net Force. Newton's Second Law talks about an object that hasnet Force. It states that when the net Force acting on an object is not zero, the object will accelerate at the direction of the exerted Force. The acceleration is directly proportional to the net Force and inversely proportional to the mass. It can be expressed in formula: F = ma where: F is the net Force in N, m is the mass of an object in kg and a is its acceleration in m/s2.

18. Mass and Weight Mass and weight are different in physics. For example, your mass doesn't change when you go to the Moon, but your weight does. Mass shows the quantity, and weight shows the size of gravity. If you know your mass, you can easily find your weight because P = mg Where: P is weight in Newton (N), m is mass in kg, and g is the acceleration of gravity in m/s*2. Mass depends on the molecular structure of the matter Weight is a Force and so it depends by the gravity

19. Newton's Third Law of Motion When you kick the wall in your room, you will probably end up hurting your foot. Newton's Third Law of Motion can explain why: when one object applies a Force on a second object, the second object applies a Force on the first that has an equal magnitude but opposite direction. In other words, when you kick the wall, the wall kicks you back with equal Force. As a result you will get hurt. These Forces are called action-reaction Forces. Remember when you kick the wall, you exerts Force on the wall. When the wall kicks you back, it exerts Force on you. Therefore, the net Force on the wall is not zero and the net Force on your foot is not zero neither.

20. Normal Force You will have to learn another vocabulary before you proceed: the Normal Force. The Normal Force acts on any object that touches surface (either directly or indirectly). The Normal Force would be applied on a ball on a table, but not on a ball in the air, for instance. It always acts perpendicularly to the surface. The formula to calculate the Normal Force is: FN = - mg where: FN is the Normal Force in Newton (N), m is the mass in kg, and g is the gravitational Force in m/s*2.

21. Friction When you slide your book on floor, it will come to stop because of the Force of Friction. Friction is the force that acts between two object in contact because of action-reaction. Whenever anything moves, there's usually some form of friction trying to stop it. Friction is sometimes useful, at other times it's a problem. There are two main types of friction:

22. "Static" or "sliding" friction • This type of friction occurs when dry surfaces rub together. • The frictional force depends only on: • the type of surfaces • how hard the surfaces are pressed together. In this diagram, the weight of the block provides the force pressing the surfaces together. Watch the animation carefully: If we push the block harder and harder, the frictional force will increase, until it reaches a maximum (in this case, 2.5N). If we push harder still, (say, 2.6N), the block will start to move, because we're now pushing harder than the frictional force.

23. "Fluid" friction This type of friction is what happens with liquids and gases. (Remember: in Physics, liquids and gases are both called "fluids". They behave in similar ways.) Fluid friction is also known as "drag". On aircraft it's also called "air resistance". It depends on: 1. how thick the fluid is (its “viscosity) 2. the shape of the object 3. the speed of the object

25. Reducing friction We can reduce friction by oiling ("lubricating") the surfaces. This means that the surfaces no longer rub directly on each other, but slide past on a layer of oil. It's now much easier to move them. Hovercraft ride on a cushion of air, which reduces the drag dramatically compared to the drag on the hull of a ship. Thus hovercraft can easily achieve much higher speeds than ships.

26. In winter sports, we need friction to be as low as possible so that we can achieve high speeds. Ice skaters actually move on a layer of water, and don't skate on ice at all. When ice is subjected to high pressure it melts. The narrow blades of the skates create a very high pressure and thus the skaters glide along on a layer of water they've just melted. The water refreezes as soon as they've moved on. This is called "regelation" (sounds like something that happens to a football team, but it's spelt differently!)

27. Other methods of reducing friction include: •  using "ball bearings" or "roller bearings", where balls or rollers allow the surface to move easily without actually touching each other • using special materials, for example, Teflon, which have a very low coefficient of friction and thus slide easily (Teflon is used in "non-stick" frying pans for this reason) We use friction to help us grip. This means that our shoes grip the floor, so we don't fall over. Right now you're using a mouse, which works because of friction between the ball and the mouse mat.

28. If it wasn't for friction between the tyres and the road, driving a car would be like trying to drive on an ice rink. This would make cornering and stopping very difficult! Friction provides the force to accelerate, stop or change the direction of the car. Ice and water on the road reduce this friction, and make is easier to skid.

29. websites • http://library.thinkquest.org • www.physics.org • www.physicsworld.com • www.physics4Kids.com • www.darvill.clara.net/enforcemot/forces.htm