Forces between objects Resultant force Speeding upSlowing down Force & acceleration Terminalvelocity & falling Physics P2 Transport issues stopping & braking
What is friction? Why is it an important force?
Forces practical • Equal and opposite forces • Objects in equilibrium • How much force is required to drag an object over different surfaces? • Use the wooden blocks, shoes or other objects to make your evaluation • Make some measurements • How reliable are they? • Draw a force diagram
Forces • Make a hovercraft and test it • Design your tests and make measurements to be able to improve your initial design to make it travel further • Were you successful? How reliable are the results? Materials needed Old CD/cardboard or paper Plasticine/Blu Tack Sports Bottle Top/cotton reel Different sized balloons Using the blu Tack or plasticine stick the closed sports bottle top over the hole in the CD. Blow some air into the balloon and carefully stretch it over the bottle top trying to let as little air escape as possible. When the bottle top is opened the hovercraft will gently hover across your desk
Homework-07 June 2011 Complete the rest of the worksheets handed out today. Complete the following sections from the text: P2: 2.1, 2.2. and 2.6 Take a look at 2.3 and 2.4 next session. We will finish these chapters and try to look at 2.5 next session.
Force & acceleration-learning outcomes: • You must be able to state Newton’s 2nd Law of motion. • You should be able to rearrange and use the equation of acceleration to find the unknown. • You could be able to describe how objects of varying mass require different force to accelerate them.
To recap... Newton’s 3rd Law: • To every action there is an equal and opposite reaction! Newton’s 1st Law: • An object in motion will stay in motion and an object at rest will stay at rest unless acted on by an unbalanced force!
force = mass x acceleration x Force and acceleration If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers: • Resultant force is measured in newtons (N). • Mass is measured in kilograms (kg). • Acceleration is measured in metres per second per second (m/s2).
Newton’s 2nd Law of Motion A force F acting on a body gives it an acceleration which is in the direction of the force and has magnitude inversely proportional to the mass m of the body: F = m a • If the resultant force acting on a moving body is zero, the body will continue to move at the same speed and in the same direction. • If the resultant force acting on a moving body is not zero, the body will accelerate in the direction of the resultant force. http://zonalandeducation.com/mstm/physics/mechanics/forces/newton/newtonLaw2.html
How do we use Newton’s second law? A car has a mass of 1,000kg. What force must the car’s engine supply to cause an acceleration of 2m/s2? force = mass x acceleration = 1,000 x 2 = 2,000N
How do we use Newton’s second law? = 10,000 12,000 acceleration = force mass A lorry has a mass of 12,000kg. What acceleration is caused by a force of 10,000N? force = mass x acceleration = 0.83 m/s2
F M A Practice questions • A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? • A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car? • A car accelerates at a rate of 5m/s2. If it weighs 500kg how much driving force is the engine applying? • A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by?
The space ship is in zero gravity with its rocket working 1: Force due to rocket on space ship. It accelerates 2: F = m . a m = mass of space ship 3: Action of gases fired backward = Reaction on space ship
The fire extinguisher is pushing the man to the left 1: Force of extinguisher gases accelerates trolley 2: F = m . a m = mass of man, trolley and extinguisher 3: Action of moving gases causes reaction on trolley
The two dogs on the right are winning the tug-of-war 1: 2 dog force bigger than 1 dog so forces not equal 2: F = m . a m = mass of 3 dogs 3: Dogs making earth spin to the left
No one is winning the tug-of-war 1: Forces cancel They are equal and in opposite directions 2: a = 0 3: Force of left hand men = Force of right hand men
F = ma investigation • Connect the overshot pulley to the end of the bench. • Tie the string around the trolley and (the other end!) to the mass holder. • Hang the mass-holder off the end of the bench held by the string going over the pulley to the trolley. • When you release the trolley it the hanging mass should pull the trolley along the bench; making it accelerate. • The FORCE is therefore the weight of the hanging mass which pulls the trolley. This force is 1N for each 100 g mass (don’t forget to count the mass-holder). • The MASS being pulled is the mass of the trolley (usually between 0.7-1 kg) PLUS the masses hanging on the string. • You can calculate the ACCELERATION: • Either, using the speed light gate or by timing how long it takes the trolley to travel a given distance from a standing start. • The acceleration will be given by the sum: • acceleration = 2 x distance travelled ÷ (time)2
Set this up and records loads of data • Investigate masses on the hanger and trolley. Take a look at this animation and have a play if you understand it: http://www.walter-fendt.de/ph14e/n2law.htm
Analysis of results… acceleration = 2 x distance travelled ÷ (time)2 Your masses that hit the floor! Your datalogger measurement in milliseconds. Convert tto seconds. Calculate using your force and mass measurements 1 kg = 10 N Plot a graph of acceleration (y-axis) versus force Form a conclusion.
Force & acceleration-learning outcomes: • You must be able to state Newton’s 2nd Law of motion. • You should be able to rearrange and use the equation of acceleration to find the unknown • Check your partner’s answers and score • You could be able to describe how objects of varying mass require different force to accelerate them • The extension work in the practical by loading up the dynamics trolley did this?
Man on the moon?How can this happen? http://nssdc.gsfc.nasa.gov/planetary/image/featherdrop_sound.mov
Homework-21 June 2011 Complete the following sections from the text: P2: 2.1, 2.2 and 2.3 We will finish this chapter with 2.4 and 2.5 next session. Also take a look at the end of chapter questions and past exam papers. Bring with you the questions I gave you on Tuesday night as some of them are for stopping and on the road.