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Transcript
PSCI II – Chapter 3– Forces (pages 66 – 97) Section 1 – Newton’s Second Law I. Force, Mass, and Acceleration º º Remember Newton’s First Law. Force and motion are connected. Force and Acceleration º Difference between a toss and a throw? – more force º For any object the greater the force the greater the acceleration. Force and Mass º Difference between a softball and a baseball thrown? – mass º Acceleration depends on mass. º More mass means less acceleration – less mass means more acceleration. II. Newton’s Second Law º States: the net force acting on an object causes the object to accelerate in the direction of the net force. º According to this equation acceleration = net force/mass. º a = Fnet/m º SI units would end up being kgm/s2 so scientists created a new unit called the Newton (N) for force. º Example: you are pushing a friend on a sled. You push with a force of 40N. your friend and the sled together have a mass of 80kg. Ignoring friction, what is the acceleration. a Fnet Fnet m 40 N m 80kg a 40 N 0.5 m s 2 80kg PSCI II – Chapter 3 – Page 1 of 6 Using Newton’s Second Law º Can be used to calculate force by Fnet = ma. º Example: How much force would a tennis racket have to exert to give a ball an acceleration of 5500m/s2 if the mass is 0.006kg? Fnet m a m 0.006kg a 5500 m s 2 Fnet (0.006)(5500) 33 kgm III. s2 33N Friction º What happens when you push a skateboard? º Newton’s law says it should go forever. º Since it doesn’t there must be a force acting on the board. º Defn. – force that opposes motion between two surfaces. What Causes Friction º Even the smoothest surfaces have microscopic bumps. º These are called microwelds between the surfaces. Sticking Together º The stronger the forces pushing objects together the stronger the microwelds will be. º To break the microwelds force must be applied. Static Friction º Imagine you try to push a heavy box and it doesn’t move. º According to Newton’s second law the net force must be zero. º Another force is acting on the box – friction. º Static friction – friction between two surfaces that are not moving past each other. º Push is not enough to move the box. Sliding Friction º Same box but you and a friend push it and it moves but not easily and stops after you quit pushing. º Sliding friction – opposes the motion of two surfaces sliding past each other. º Caused by breaking and reforming of microwelds. PSCI II – Chapter 3 – Page 2 of 6 Rolling Friction º Think of a car stuck in the mud or on ice – what do you usually do to help them? – Gravel gets put down so friction is increased. º Rolling friction – friction between a rolling object and the surface its rolling on. º Microwelds break and reform as wheel rolls. º Usually less than sliding or static friction. IV. Air resistance º Acts on objects that fall through the air. º Think of crumples and flat paper. º Flat paper takes longer to fall so there’s a force acting on it. º Air resistance acting up opposing gravity acting down. º Acts opposite an objects motion. º Depends on speed, size, shape, of the object. º Everything falls at the same rate when no air is present (vacuum). Terminal Velocity º As objects fall they accelerate and the force of air resistance increases until it cancels out gravity. º At this point the forces cancel and are balanced and it falls at a constant rate/speed (terminal velocity). Section 2 – Gravity I. The Law of Gravitation º We’re very attractive people (gravity attracts us to everything). º Law of gravitation says that any two masses exert an attractive force on each other. º Depends on two things – the masses and the distance between them. º So as mass increases gravity increases. º As distances increases gravity decreases. º Why do we not get pulled off the Earth then and into the sun? – To far away Gravity – a Basic Force º Four basic forces (electromagnetic, strong nuclear, weak nuclear and gravity). The Range of Gravity º Even though something is very far away gravity never disappears. º Gravity is a long-range force. PSCI II – Chapter 3 – Page 3 of 6 II. Gravitational Acceleration º Near Earth’s surface all objects have an acceleration of 9.8 m/s2 (g). º Remember that F = ma so if a = 9.8 m/s2 then F = m x 9.8 m/s2 . º This is then the force of gravity and depends only on the mass of an object. º Force of gravity is always down. Inertia and Gravity º If you dropped a bowling ball and a tennis ball off a bridge at the same time, which would hit first? Neither – same time. º True gravity on the bowling ball is greater but they hit at the same time. º This is because more force is needed to change the velocity of the bowling ball and that cancels out gravity. III. Weight º No matter whether you’re standing, jumping or falling Earth exerts a force on you. º Weight – the gravitational force exerted on an object. º Gravitational force = mass x acceleration due to gravity or W = mg. º Example: a person has a mass of 50kg would have a weight of º W = (50kg)( 9.8 m/s2) = 490 N Losing Weight º What happens to weight far from Earth? º It gets weaker. Weight and Mass º Not the same. º Weight is a force and mass is the amount of matter. º More mass equals greater gravity. º Different places can have different weight – planets. IV. Weightlessness and Free Fall º Experienced in space very far from Earth’s pull. º About 400km above earth gravity is about 90% Floating in Space º Picture yourself on an elevator standing on a scale. º While you’re standing on the scale all forces are balanced. º Now picture that the elevator is going down – this is kind of free fall and the scale would read less than true value (maybe even zero). PSCI II – Chapter 3 – Page 4 of 6 V. Projectile Motion º Thrown objects tend to curve downward. º Anything thrown or shot through the air is a projectile. º Projectiles curve because of inertia and gravity. º They have a horizontal (the throw) and vertical (gravity) velocities. Horizontal and Vertical Motion º When you throw a ball you give it horizontal velocity. º When you let go of the ball gravity acts on the ball too. º Gravity is the unbalanced force that causes a change in motion. º If you throw a ball and drop a ball from the same height what would happen? – both would hit the ground at the same time. Centripetal Force º When a ball enters a curve remember it’s accelerating because direction is changing. º When a ball enters a curve the change in velocity is toward the center of the curve. º Centripetal acceleration – acceleration toward the center of a curved or circular path. º Centripetal means move toward the center. º Centripetal force is acting as the unbalanced force to cause motion change. º Defn. – a force directed toward the center of the circle for an object moving in circular motion. The Moon is Falling º Talks about why the moon stays in orbit. º Read the section. Section 3 – The Third Law of Motion I. Newton’s Third Law º States: when one object exerts a force on a second object the second objects exerts a force on the first that is equal in size and opposite in direction. Action and Reaction º How many action – reaction pairs can you come up with? PSCI II – Chapter 3 – Page 5 of 6 How You Move º If we have only equal action-reaction pairs how do we move? º The pairs are acting on different objects. º Example is swimming – a person pushes on the water and the water pushes the person forward. Rocket Propulsion º Main principle behind rockets. º Fuel is heated and produces gases. º Engine then exerts a force on the gases to push them out. º Newton’s third law gases exert a force as they’re pushed out forcing the rocket forward. II. Finding Planets With Newton’s º How we found Neptune. º Read the section. III. Momentum º Defn. – property that a moving object has because of its mass and velocity. º Momentum (p) = mass (m) x velocity (v) or p = mv º Units are kgm/s º Has direction because velocity has direction. Force and Changing Momentum º We can make some substitutions for velocity to calculate for changing momentum. F mv f mvi t mvi initialmom entum mv f finalmomentum Law of Conservation of Momentum º momentum doesn’t change unless mass or velocity change. º Can be transferred from one object to another. º States: if a group of objects exerts forces only on each other their total momentum doesn’t change. º No momentum is created or destroyed. When Objects Collide º Both moving in same direction – second puck will continue but more quickly. º Opposite directions – pucks will reverse direction and go with same speed PSCI II – Chapter 3 – Page 6 of 6