GCSE Forces Distance Speed and Time D Speed

GCSE Forces

Distance, Speed and Time D Speed = distance (in metres) time (in seconds) S T 1) Dave walks 200 metres in 40 seconds. What is his speed? 2) Laura covers 2 km in 1, 000 seconds. What is her speed? 3) How long would it take to run 100 metres if you run at 10 m/s? 4) Steve travels at 50 m/s for 20 s. How far does he go? 5) Susan drives her car at 85 mph (about 40 m/s). How long does it take her to drive 20 km?

Distance-time graphs 2) Horizontal line = 40 4) Diagonal line downwards = 30 Distance (metres) 20 10 0 Time/s 20 1) Diagonal line = 40 60 80 100 3) Steeper diagonal line =

40 Distance (metres) 30 20 10 0 Time/s 20 40 60 80 100 1) What is the speed during the first 20 seconds? 2) How far is the object from the start after 60 seconds? 3) What is the speed during the last 40 seconds? 4) When was the object travelling the fastest?

Speed vs. Velocity Speed is simply how fast you are travelling… This car is travelling at a speed of 20 m/s Velocity is “speed in a given direction”… This car is travelling at a velocity of 20 m/s east

Acceleration V-U Acceleration = change in velocity (in m/s) (in m/s 2) time taken (in s) A T 1) A cyclist accelerates from 0 to 10 m/s in 5 seconds. What is her acceleration? 2) A ball is dropped and accelerates downwards at a rate of 10 m/s 2 for 12 seconds. How much will the ball’s velocity increase by? 3) A car accelerates from 10 to 20 m/s with an acceleration of 2 m/s 2. How long did this take? 4) A rocket accelerates from 1, 000 m/s to 5, 000 m/s in 2 seconds. What is its acceleration?

Velocity-time graphs 1) Upwards line = 80 Velocity m/s 4) Downward line = 60 40 20 0 10 2) Horizontal line = 20 30 40 50 3) Upwards line = T/s

80 60 Velocity m/s 40 20 0 T/s 10 20 30 40 1) How fast was the object going after 10 seconds? 2) What is the acceleration from 20 to 30 seconds? 3) What was the deceleration from 30 to 50 s? 4) How far did the object travel altogether? 50

Balanced and unbalanced forces Consider a camel standing on a road. What forces are acting on it? Reaction These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere. Weight

Balanced and unbalanced forces Reaction What would happen if we took the road away? Weight

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards… Weight

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces 1) This animal is either ____ or moving with _____… 3) This animal is getting _______…. 2) This animal is getting _____… 4) This animal is…

Balanced and unbalanced forces

Force and acceleration If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers: Force (in N) = Mass (in kg) x Acceleration (in m/s 2) F M A

Force, mass and acceleration 1) A force of 1000 N is applied to push a mass of 500 kg. How quickly does it accelerate? F 2) A force of 3000 N acts on a car to make it accelerate by 1. 5 m/s 2. How heavy is the car? 3) A car accelerates at a rate of 5 m/s 2. If it weighs 500 kg how much driving force is the engine applying? 4) A force of 10 N is applied by a boy while lifting a 20 kg mass. How much does it accelerate by? M A

Terminal Velocity Consider a skydiver: 1) At the start of his jump the air resistance is _______ so he _______ downwards. 2) As his speed increases his air resistance will _______ 3) Eventually the air resistance will be big enough to _______ the skydiver’s weight. At this point the forces are balanced so his speed becomes ____ - this is called TERMINAL VELOCITY

Terminal Velocity Consider a skydiver: 4) When he opens his parachute the air resistance suddenly ____, causing him to start _____. 5) Because he is slowing down his air resistance will _______ again until it balances his _____. The skydiver has now reached a new, lower _______.

Velocity-time graph for terminal velocity… Parachute opens – diver slows down Velocity Speed increases… Terminal velocity reached… On oon M he t Time New, lower terminal velocity reached Diver hits the ground

Weight vs. Mass Earth’s Gravitational Field Strength is 10 N/kg. In other words, a 1 kg mass is pulled downwards by a force of 10 N. W Weight = Mass x Gravitational Field Strength (in N) (in kg) (in N/kg) M g 1) What is the weight on Earth of a book with mass 2 kg? 2) What is the weight on Earth of an apple with mass 100 g? 3) Dave weighs 700 N. What is his mass? 4) On the moon the gravitational field strength is 1. 6 N/kg. What will Dave weigh if he stands on the moon?

Friction 1) What is friction? 2) Give 3 examples where it is annoying: 3) Give 3 examples where it is useful: 4) What effect does friction have on the surfaces?

Stopping a car… Tiredness Too many drugs Thinking distance Poor visibility Wet roads Icy roads Tyres/brakes worn out Too much alcohol Braking distance Driving too fast

Work done When any object is moved around work will need to be done on it to get it to move (obviously). We can work out the amount of work done in moving an object using the formula: Work done = Force x distance moved in J in N W in m F D

Kinetic energy Any object that moves will have kinetic energy. The amount of kinetic energy an object has can be found using the formula: Kinetic energy = ½ x mass x velocity squared in J in kg KE = ½ in m/s mv 2

Some example questions… 1) A 70 kg boy is running at about 10 m/s. What is his kinetic energy? 2) A braking force of 1000 N is applied by a driver to stop his car. The car covered 50 m before it stopped. How much work did the brakes do? 3) What is the kinetic energy of a 100 g tennis ball being thrown at a speed of 5 m/s? 4) A crane is lifting a 50 kg load up into the air with a constant speed. If the load is raised by 200 m how much work has the crane done? (The answer isn’t 10, 000 J)

Pressure – some basic ideas Describe and draw experiments for each of the following ideas: 1) Pressure increases with depth 2) Pressure is the same in all directions at a certain depth

Pressure depends on two things: 1) How much force is applied, and 2) How big (or small) the area on which this force is applied is. 3) Pressure can be calculated using the equation: F Pressure (in N/m 2) = Force (in N) Area (in m 2) P A

Some example questions… 1) A circus elephant weighs 10, 000 N and can stand on one foot. This foot has an area of 50 cm 2. How much pressure does he exert on the floor (in N/cm 2)? 2) A 50 kg woman copies the elephant by standing on the heel of one of her high-heeled shoes. This heel has an area of 1 cm 2. How much pressure does she exert on the floor? Work through q 2 on p 58 Extension task: Atmospheric pressure is roughly equivalent to 1 kg pressing on every square centimetre on our body. What does this equate to in units called Pascals? (1 Pascal = 1 N/m 2)

Hydraulic systems

Hydraulic systems Pressure is constant throughout this liquid

Hydraulic systems Basically, a smaller force on piston A will produce a larger force on piston B because the pressure of the liquid is constant. Magic! 1) If the area of piston A is 5 cm 2 and piston B is 20 cm 2 how much will the force be multiplied by? 2) If the area of the slave piston is ten times bigger than the master piston what force will be needed to lift an object weighing 1000 N? 3) A force of 10 N is used to lift a weight of 50 N. What is the ratio of the master piston area to the slave piston area?

Pressure and Volume in gases Pressure Volume Pressure x volume

Pressure and volume in gases Conclusion When we multiplied the pressure of a gas by its volume we found that the answer was always __ _______. In other words, if you DECREASE the volume you _______ the pressure and so on. “One goes up, the other goes down”

Pressure and Volume in gases This can be expressed using the equation: Initial Pressure x Initial Volume = Final Press. x Final Vol. P IV I = P F V F 1) A gas has a volume of 3 m 3 at a pressure of 20 N/m 2. What will the pressure be if the volume is reduced to 1. 5 m 3? 2) A gas increases in volume from 10 m 3 to 50 m 3. If the initial pressure was 10, 000 N/m 2 what is the new pressure? 3) A gas decreases in pressure from 100, 000 Pascals to 50, 000 Pascals. The final volume was 3 m 3. What was the initial volume? 4) The pressure of a gas changes from 100 N/m 2 to 20 N/m 2. What is the ratio for volume change?