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Transcript
5.1-5.5 More forces The greatest invisible force of all Think very carefully about gravity: how do you know it exists? Newton described it, calculated, but didn’t tell us what it was… If you think about it, our concept of gravity is very abstract In order to explain why objects fall to the earth without an apparent force acting on it, gravity as a force was developed to explain the behaviour of objects that seemed to defy Newton’s first law Weight vs. mass Weight: The force of gravity acting on an object as measured by the base unit Newtons Mass: The amount of matter within a given object as measured with the base unit grams More of Newton’s observations By observation of the motion of planetary bodies, Newton formulated the behaviour of gravity between two objects Planets maintained circular orbits around the sun, suggesting that the sun generated gravity to keep them there But the discovery of moons around other planets also illustrated that planets had a gravitational pull I pull you, pull me? This suggested that objects affected each other The larger mass of the sun and its ability to override the gravitational pull of the earth suggested that the gravitational pull of an object was dependent on the mass of the object involved The famous apple It is suggested that the famous apple that started it all might have led Newton to consider the consequences of gravity at large distances If gravity could cause an apple to fall from a shorter branch, and taller ones above it – could it affect other objects much further away – like the moon? Diagrammatically… m2 m1 F m2 on m1 F m1 on m2 R = distance between centre of masses Force is dependent on: Both mass 1 and mass 2 Distance between masses Therefore: F = Gm1m2 r2 Where: G = 6.67 x 10-11 Nm2/kg2 Not so Normal force We described normal force sometime ago, but let’s see how much you remember What is the definition of NORMAL FORCE? Normal force Normal force is the force that a surface pushes back on an object that bears weight on it For example: your textbook sitting on your desk pushes down on your desk, and the desk pushes back on the book The force pushing back on the book is known as the normal force (FN) For example: which one of these situations does normal force exist? BANK Hanging sign Holding something against the ceiling Friction: joining the x and y axis When looking at an object that moves across a surface, we can draw a FBD that includes gravity and normal force in the picture But how does it really affect motion? If you think logically about it, gravity, and normal force don’t contribute directly to the horizontal motion of an object on a surface – since the motion is ultimately peripindicular to the axis of motion This follows Newton’s second law Gravitynormal force friction The connection between the y-axis forces in any object sitting on a surface is friction Since friction depends on a portion of normal force – which is determined by gravity – it is important to know how they relate to one in another in different questions Try to remember… What is friction? How do we know it exists? What factors affect friction? How does friction affect motion? What is friction? Friction is generated by the interaction of the irregular surfaces of different objects that are in contact with each other Imagine a surface with golf-ball sized bumps interacting with one that has tennis-ball sized bumps Irregular surfaces have raised and sunken surfaces Two irregular surfaces in contact that try to slide across each other will resist motion because the raised surfaces fall into the sunken surfaces Therefore, additional energy is needed to move the push the raised surfaces out of the sunken surfaces How do we know it exists? Think about the process of pushing an object across the floor Why does the box come to stop without any visible force acting in the opposite direction? Which one of Newton’s Laws does this violate? Friction comes into play Therefore, another force that is not inherently visible must be present to explain this – or else the phenomenon of sliding objects coming to a stop would violate Newton’s first law of inertia What factors affect friction? What do your car brakes do to your car? Why do car brakes work differently in the rain and snow? What does this tell you about the nature of friction? Friction stops cars At least, let’s hope so or else it means that you’re slamming into something else When you hit the brakes on your car, you are actually stopping your wheels from turning But you aren’t directly controlling the forces that cause you to come to a stop Stopping the wheels Means that you are essentially allowing the rubber on your wheels to come into contact with the pavement The rest is dependent on the friction between the asphalt and the rubber So why do rain and snow affect braking? Change in surfaces When it rains or snows, the nature of the road surface changes The bumpy surfaces on the asphalt are not as accessible anymore – because they are covered by another substance with a different surface Therefore, the contact surfaces change, changing friction Friction depends on surfaces Therefore, the amount of friction you can generate is dependent on the types of surfaces that are in contact How does friction affect motion? So how much frictional force is generated to oppose motion? Think about moving two identical cardboard boxes across the same floor One has a greater mass than the other Which one is easier push across the floor? Why? Remember – I didn’t ask you to lift them up Newton’s second law states that the heavier one would be harder to move Is that related directly to the mass if we are looking at horizontal motion? Why is a heavier object harder to slide across the floor? Normal force Remember – the equation for the calculation of friction includes normal force Therefore, frictional force is a fraction of normal force The amount of friction generated is affected by the normal force acting on the box Normal not gravitational Be aware: normal force is NOT always equal to gravitational force! You must be careful when calculating questions and be aware of this! How does friction affect motion? Think about the objects in these two situations – they are both at rest In which situation is the force of friction at work? Static friction vs. kinetic friction Static friction: Prevents the initiation of movement Kinetic friction: Prevents motion Which one always has a greater value? (Think about accelerating a car vs. keeping a car moving at a constant speed – which one requires more force?) Static friction is always greater The coefficient of friction What exactly is µ as a measurement? To summarize: Friction: Always opposes motion Is dependent on the surfaces in contact and on normal force Static friction is greater than kinetic friction Let’s see what you know A force is applied to an object that sits on a surface, but it doesn’t move. Is friction present? What is the value of the applied force compared to the static frictional force?