KS4 Forces – Further Forces

7/50th of a second or 0.14 seconds Ticker timers A ticker timer is used as an accurate timing device for motion experiments.Ticker timers print dots on paper at a rate of 50 dots per second. This means the distance between two dots is equivalent to 1/50th of a second or 0.02 seconds. How much time is shown here? 7/50th of a second or 0.14 seconds

Describe the motion for the ticker tape shown: Accelerating/decelerating/constant velocity Accelerating/decelerating/constant velocity Accelerating/decelerating/constant velocity

Plastic Example? Plasticene If you put a force on an object making the object change shape and it stays the same shape after you remove the force we say the object is plastic.

Elastic Example? A spring If you put a force on an object making the object change shape and it returns to its original shape after you remove the force we say the object is elastic.

Write down four examples of plastic and elastic materials. Elastic and plastic Write down four examples of plastic and elastic materials. Elastic Plastic

Match the force with its definition: The force on a material when it is being squashed. Tension The force on a material when it is being stretched. Compression

Tension or compression? T – Tension C - Compression T Your triceps muscle when you bend your arm. A fishing line holding a fish. A football being kicked. A string holding a conker. Pushing in a syringe. An arrow in a bow. Your biceps muscle when you bend your arm. C

Standing on a plank: Where is the plank being compressed? Where is the plank being stretched? What force pulls down on the man? Tension Compression Gravity

Stretching springs What do you predict will happen as masses are attached to a spring? Attach a spring to a stand. Record the length of the spring using a ruler. Add a 50g mass to the spring. Record the new length of the spring. Continue to add masses to the spring up to 500g in total. Plot a graph of load against extension. What do your results tell you? Mass (g) Load (N) Length (cm) Extension 2.4 50 0.5

Load and extension for a metal spring Extension (cm) Plastic region Elastic limit Load  extension Load (N) What do we call the point after which the spring will not return to its original shape? The Elastic Limit Where the graph is a straight line. Where is load proportional to extension?

How are mass, force and acceleration related? Two masses of 1 kg and 4 kg are both subject to the same force, which one will accelerate the fastest? This is a FLASH animation. Please click on the red button to activate the animation. The lower the mass, the greater the acceleration for a specific force. This is why Grand Prix cars get faster as they use up their fuel. Less fuel, means less mass, less mass means greater acceleration.

F=ma We can express the relationship between force, mass and acceleration using the equation: Force = mass x acceleration F =ma Force measured in Newtons (N) Mass measured in kilograms (kg) Acceleration measured in metres per second per second (ms-2)

  x F m Formula triangles Formula triangles help you to rearrange formula. The triangle for the f=ma formula is shown below: Whatever quantity you are trying to find, cover it up and it will leave you with the calculation required. …and you are left with the sum… So if you were trying to find mass, m….. m = F a  F  …you would cover m up… m a x

Force calculations: Force = 20 N Acceleration = 25 ms-2 Mass = 800 kg What force will make a mass of 4 kg accelerate at 5 ms-2? A force of 50N acts on a 2 kg mass. What will be the acceleration of the mass? A force of 2 000N accelerates a car at 2.5 ms-2. What is the mass of the car? Force = 20 N Acceleration = 25 ms-2 Mass = 800 kg

    Force questions: Label the forces acting upon a moving bike. Fill in the table: B A - Thrust  C B - Reaction A D C – Air resistance E D - Friction E - Gravity A > (C+D) A =(C+D) A < (C+D) Bike accelerating Bike at steady speed Bike decelerating   

Turning forces Two masses on a see-saw. What force acts on the masses? Which way will the see-saw go? gravity pivot gravity The see-saw turns around the pivot. What factors effect the size of a turning force?

Moments The size of the turning force or moment depends upon: The distance of the force from the pivot. The size of the force. Moment = Force x perpendicular distance from pivot Moments measured in Newton metres (Nm) Force measured in Newtons (N) Distance measured in metres (m)

Anticlockwise moments = Clockwise moments Principle of moments Anticlockwise moments = Clockwise moments 2 m 1 m 25N 50N Where should a force of 50N be positioned to balance the see-saw? Anti-clockwise moments = 25 N x 2 m = 50 Nm Anti-clockwise moments = Clockwise moments 50 Nm = 50 N x ? M distance = 1 m Clockwise moments = 50 N x ? m

Principle of moments Drag and drop any of the masses onto the “see – saw” and try to get it to balance. The masses are in kilograms and the distance in metres. This is a FLASH animation. Please drag and drop the masses onto the ‘see-saw’.

Moments questions Where should a force of 60N be positioned to balance the ruler below? What size force should be positioned on the left to balance the ruler shown? 4 m 1 m 15N 60N 1 m 4 m Force = 60 N ? N 15N

Pressure Forces are pushes, pulls and twist. Pressure is a measure of how spread out a force is. As you in increase the size of the force you increase the size of the pressure. As you increase the size of the area over which a force acts, you decrease the pressure. More pressure Less pressure Less pressure More pressure

Pressure questions Why do tractors have such large tyres? Why would a lady in stiletto heels standing on your foot hurt you more than a elephant standing on your foot? The tyres have a large area so that the weight of the tractor is spread over a large area. This reduces the pressure the tractor puts on the ground, so that it doesn’t sink into the mud. The elephant has a greater weight than the lady and it would have a greater pressure if the area the weight acted upon was the same. However, the lady’s weight is concentrated into a smaller area than that of the elephant’s. This means that the stiletto heel exerts a greater pressure than the foot of the elephant, and might hurt more.

P=F/A We can express the pressure formula using the equation: Pressure = Force ÷ Area P =F/A Pressure measured in pascals (Pa) Force measured in newtons (N) Area measured in metres squared (m2)

Hydraulics We can use the fact that the pressure in a liquid is the same at any point to make a machine that magnifies force. Comment on the area of the plungers, the relative distance travelled and the energy input/output.

  x F P Formula triangles Formula triangles help you to rearrange formula. The triangle for the pressure formula is shown below: Whatever quantity you are trying to find, cover it up and it will leave you with the calculation required. …and you are left with the sum… So if you were trying to find pressure, P….. P = F A F   …you would cover P up… P A x

The pressure you put on the ground during a handstand: Put your hand on a piece of graph paper. Count the number of 1cm2 squares your hand covers. Multiply this number by two (you have two hands). Measure your weight in Newtons on some scales. Use the pressure formula to calculate the pressure you put on the ground. P=F/A

Pressure calculations: What pressure will a force of 20N acting over an area of 4 ms-2 put upon the ground? A force of 50N acts on an area and creates a pressure of 1.0Pa. Over what area is the force acting? A pressure of 6 000 Pa is created by a force acting on an area of at 5 m2. What is the size of the force? Pressure = 5 Pa Area = 50 m2 Force = 30 000 N

Pressure experiment A fluid is a liquid or a gas. Get a tin can and drill three holes in the side, then fill it with water. What do you observe? What does this tell you about the pressure in fluids? The water is forced out a greater distance as you move down the tin can. This is because the pressure is greatest at the bottom - pressure in fluids increases with depth.

Why are the walls of a dam wider at the bottom than at the top? It is cheaper to build Gravity increases with depth Pressure increases with depth Looks nice 

A pressure of 600 Pa acts over an area of 2m2 A pressure of 600 Pa acts over an area of 2m2. What force creates this pressure? 1 200 N 300 N 1 200 Pa 300 Pa 

Why are racing cars able to travel faster as a race progresses? The engine has cooled down The petrol has warmed up There is less petrol to accelerate There is more petrol to accelerate 