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# Newtons laws

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The presentation for chapter 2, newtons laws of the everything science series.

The presentation for chapter 2, newtons laws of the everything science series.

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• 1. 1 2. Newtons law Physics Grade 11Everything Science www.everythingscience.co.za
• 2. 2ForceA force is anything that can cause a change to objects. Forces can:● change the shape of an object● accelerate or stop an object● change the direction of a moving object.The unit of force is the Newton (N)Forces can be classified as either contact forces or a non-contact forces.A contact force must be A non-contact forcein touch or be in contact does not have towith an object to cause a touch an object tochange. Examples are: cause a change.push and pull forces and Examples arethe force of the wind to gravity, electricityturn a windmill. and magnetism. Photo by MountainAsh on Flickr Photo by Colleen Henning Everything Science www.everythingscience.co.za
• 3. 3Different types of forces in physicsThe normal force, N , is the force exerted by a surface on an object in contact with it. The normal force is always perpendicular (at a right angle) to the surface.Frictional force is the force that opposes the motion of an object in contact with a surfaceand it acts parallel to the surface the object is in contact with.Frictional forces always act parallel to surfaces.Tension is the magnitude of the force that exists in objects like ropes, chains and struts thatare providing support. Everything Science www.everythingscience.co.za
• 4. 4 More about frictional forces The magnitude of the frictional force depends on the surface and the magnitude of the normal force. Different surfaces will give rise to different frictional forces, even if the normal force is the same. Frictional forces are proportional to the magnitude of the normal force. F friction ∝ NEvery surface has a constant factor, the coefficient offriction. Since static and kinetic friction have differentmagnitudes we have different coefficients for the two typesof friction: for static friction and for  kinetic friction. s kStatic friction varies up to a maximum value while kineticfriction stays constant. We use the following two equationsto calculate frictional forces: f max = s N s f k = k N Everything Science www.everythingscience.co.za
• 5. 5Force diagramsForce diagrams are sketches of the physical situation you are dealing with, with arrows for allthe forces acting drawn on the system. When drawing force diagrams remember the following:● Make your drawing large and clear.● You must use arrows and the direction of the arrow will show the direction of the force.● The length of the arrow will indicate the size of the force. Arrows of the same length indicateforces of equal size.● Draw neat lines using a ruler. The arrows must touch the system or object.● All arrows must have labels. If necessary create a key on the side to show the forces.●The labels must indicate what is applying the force, on what the force is applied and in whichdirection● If the values of the forces are known, these values can be added to the diagram or key. Everything Science www.everythingscience.co.za
• 6. 6Free body diagramsIn a free-body diagram, the object of interest is drawn as a dot and all the forces acting on itare drawn as arrows pointing away from the dot. Everything Science www.everythingscience.co.za
• 7. 7Resolving forces into componentsWe have looked at resolving forces into components. There is one situation we will considerwhere this is particularly useful, problems involving an inclined plane. It is important becausethe normal force depends on the component of the gravitational force that is perpendicularto the slope.We can use the following two equations to find the components: F gx =F g sin  F gy =F g cos Everything Science www.everythingscience.co.za
• 8. 8Finding the resultant forceWe can find the resultant force quite easily be following a few simple guidelines.1. Draw a free body diagram.2. Resolve all forces into components parallel to the x- and y-directions.3. Calculate the resultant in each direction, and , using co-linear vectors.4. Use and x . to calculate the resultantR  Ry  Rx  Ry  R Everything Science www.everythingscience.co.za
• 9. 9Newtons lawsNewton’s First Law An object continues in a state of rest or uniform motion (motion with aconstant velocity) unless it is acted on by an unbalanced (net or resultant) force.This property of an object, to continue in its current state of motion unless acted upon by anet force, is called inertia.An ice skater pushes herself away from the side of the ice rink and skates across the ice.She will continue to move in a straight line across the ice unless something stops her.Objects are also like that. If we kick a soccer ball across a soccer field, according toNewton’s first law, the soccer ball should keep on moving forever! Photo by JosDieles on Flickr Photo by Elvert Barnes on Flickr Everything Science www.everythingscience.co.za
• 10. 10Newtons lawsNewton’s Second Law If a resultant force acts on a body, it will cause the body to acceleratein the direction of the resultant force. The acceleration of the body will be directly proportionalto the resultant force and inversely proportional to the mass of the body.Mathematically this is: F =m⋅ net a Everything Science www.everythingscience.co.za
• 11. 11Newtons laws – Newtons second lawForce is a vector quantity. Newton’s second law of motion should be applied to the y- andx-directions separately. You can use the resulting y- and x-direction resultants tocalculate the overall resultant as we saw in the chapter on vectors.We can use Newtons second law to solve problems involving:● Objects accelerating along a surface with or without frictional forces.● Two connected objects with a tension force. The tension force may be at an angle.● Objects being pulled along, with the pulling force applied at an angle.● Problems involving objects on an inclined plane.● Problems involving lifts and rockets. Everything Science www.everythingscience.co.za
• 12. 12Newtons lawsNewton’s Third Law If body A exerts a force on body B, then body B exerts a force of equalmagnitude on body A, but in the opposite direction.Using Newtons third law we can determine action-reaction pairs of forces. These have thefollowing properties: the same type of force acts on the objects; the forces have the samemagnitude but opposite direction; and the forces act on different objects. Everything Science www.everythingscience.co.za
• 13. 13Forces in equilibriumEquilibriumAn object in equilibrium has both the sum of the forces acting on it and the sum ofthe moments of the forces equal to zero.We mentioned that resultant forces cause objects to accelerate in a straight line. If anobject is stationary or moving at constant velocity then either,● no forces are acting on the object, or● the forces acting on that object are exactly balanced.In other words, for stationary objects or objects moving with constant velocity, theresultant force acting on the object is zero. Everything Science www.everythingscience.co.za
• 14. 14 Newtons lawsNewton’s Law of Universal Gravitation Every point mass attracts every other point mass bya force directed along the line connecting the two. This force is proportional to the product ofthe masses and inversely proportional to the square of the distance between them. m 1 m2 F =G 2 d Everything Science www.everythingscience.co.za
• 15. 15Newtons laws – Newtons law of universal gravitationAn important point to note about this law is that it is always attractive, and itdepends only on the masses involved and the distance between them.This law also involves the universal gravitational constant: G=6,67×10−11 N⋅m 2⋅kg−2We also note that for any large object we use the distance from the centre of theobject(s) to do the calculation.And finally we note that we can find the acceleration due to gravity for Earth (andfor any planet) by using: M Earth ao =G 2On Earth this value comes out to be d Earth g=9,8 m⋅s−2 Everything Science www.everythingscience.co.za
• 16. 16Weight and massMass is a scalar and weight is a vector.Mass is a measurement of how much matter is Measuring massin an object and is measured in kg.Weight is a measurement of how hard gravity ispulling on that object and is measured in N.Your mass is the same wherever you are. Yourweight depends on how strong a gravitationalforce is acting on you.We can use the following equation to calculateweight:  F g =m  g Photo by xJason.Rogersx on Flickr Everything Science www.everythingscience.co.za
• 17. 17Newtons law of universal gravitation – comparative problemsA common application of this law is to solve comparative problems. The following strategywill help you solve these equations:● Write out equations and calculate all quantities for the given situation● Write out all relationships between variable from first and second case● Write out second case● Substitute all first case variables into second case● Write second case in terms of first case Everything Science www.everythingscience.co.za
• 18. 18For more practice see:www.everythingscience.co.zaShortcode: ESBM3 Everything Science www.everythingscience.co.za