# Forces Types of forces Contact and non-contact forces Friction

## Presentation on theme: "Forces Types of forces Contact and non-contact forces Friction"— Presentation transcript:

Forces Types of forces Contact and non-contact forces Friction
Balanced and unbalanced Speed Air resistance Terminal speed

Types of Force I A PULL is when you move something TOWARDS YOU.
I give up! Can you think of any other examples?

Types of Force II A PUSH is when you move something AWAY FROM YOU.
Can you think of any other examples?

Measuring Forces Sir Isaac Newton was an English scientist who had many good ideas about science and maths. A story is told that he was sat under a tree one day and an apple fell on his head. This gave him ideas about the force of gravity. The size of a force is measured in Newtons (N). A Newtonmeter is a device used to measure small forces. OUCH!

Exercise 1: Insert the Key Words
______ are pushes, pulls or twists. Forces are measured in _______ (N). Sir Isaac Newton was a famous English _______ who had many good ideas about the science. If you grab hold of something and move it towards you, we call it a ____. If you grab hold of something and move it away from you, we call it a ____. A ____ is when you pull and push at the same time to turn something around. Forces Newtons scientist pull push twist twist pull push scientist Newtons forces

Used to measure small forces Newtons Had many ideas about gravity Newtonmeter What forces are measured in Sir Isaac Newton

Forces Friction Air resistance Gravity Nuclear Electrostatic Tension
Forces are PUSHES, PULLS and TWISTS. What different types of forces are there? Friction Small forces can be measured using a device called a NEWTONMETER. Air resistance Gravity The units of force are NEWTONS (N). Nuclear When drawing force diagrams ALWAYS include ARROWS to show the DIRECTION a force is acting in. Electrostatic Tension Remember, FRICTION always acts in the OPPOSITE DIRECTION to the way an object is moving. Compression Upthrust Magnetism Remember, GRAVITY always PULLS DOWN on an object. Thrust

Exercise 3: Link the Labels and the Diagrams
Push Pull Twist

Exercise 4: Types of Forces
1. Which force keeps the planets in orbit around the Sun? 2. Which force causes compass needles to point north? 3. Which force keeps electrons in orbit around atoms? 4. Which force is caused by materials rubbing together? 5. Which force is the upwards push of water on a floating object? 6. Which force is the opposite of compression? Gravity Magnetism Electrostatic Friction Upthrust Tension

Force Diagrams Force diagrams show the directions forces are acting in using arrows. The bigger the arrow, the bigger the force. Which directions are the forces below acting in?

Gravity The force of GRAVITY is what keeps us on the SURFACE of the EARTH. You do not have to be TOUCHING the Earth to feel the force of gravity. GRAVITY ON EARTH PULLS YOU DOWN. GRAVITY ALWAYS PULLS; IT NEVER PUSHES. Draw an arrow on the shark and car to show the direction of gravity:

Upwards Forces I If there is a downwards force of gravity pulling on a car then why does it not get pulled into the ground? Reaction There must be a force PUSHING UP on the car that stops it getting pulled into the ground. Do you know what we call this force? Gravity The REACTION FORCE.

Upwards Forces II If there is the downwards force of gravity pulling down on a floating shark then why does it not sink deeper into water? Upthrust There must be a force PUSHING UP on the shark that stops it getting pulled further down. Do you know what we call this force? Gravity The UPTHRUST.

Exercise 5: Force Diagrams 1
Tension Which force pulls down on the boy? Which force along the ropes balances the downwards pull on the boy. Draw arrows on the diagram and label these two forces. Gravity Tension Gravity

Exercise 6: Force Diagrams 2
Label the forces acting on a rocket moving through the Earth’s atmosphere. Upthrust Thrust Gravity Air resistance

Exercise 7: Force Diagrams 3
Label the forces acting on the shark. Upthrust Thrust Friction Gravity

Investigating Forces 1. What happens when you let go of a pencil?
2. What force causes this to happen? 3. Bring a steel paperclip near a magnet, what happens? 4. What force causes this to happen? 5. Push a toy car across a table. What happens eventually? 6. What force causes this to happen? Some forces do not require contact with an object to affect the object. These forces are called NON-CONTACT FORCES. Examples of this type of force are MAGNETISM and GRAVITY. Some forces have to be in contact with an object to affect it. These forces are called CONTACT FORCES. Examples of this type of force are FRICTION and AIR RESISTANCE.

Oil acts as a lubricant on bicycle chains
Friction Whenever two materials move past each other they RESIST the MOTION. This force is called FRICTION. Friction ALWAYS acts in the OPPOSITE DIRECTION to the way a material is moving. If an object is NOT MOVING then it will have ZERO FRICTION. Whenever friction occurs you always get the effects of HEATING and WEARING. The effects of friction can be reduced by using a LUBRICANT so two materials are not actually in contact. Oil is used in car engines for such a purpose. Animals in nature are often STREAMLINED to reduce friction effects. Sharks are streamlined to help them swim faster. Oil acts as a lubricant on bicycle chains

Exercise 8: Friction Questions
1. In which direction does friction act? 2. What two effects do you get when friction occurs? 3. Name two ways of reducing friction effects. 4. If a car is not moving what do you know about the size of the friction force acting on it? 5. Give an example where friction is useful. 6. Give an example where friction is a problem. The opposite direction to the way an object is moving. Heating and wearing. Lubrication and streamlining. The friction force is zero. Car-braking systems. Wearing in car engines.

Balanced Forces If the forces acting on an object are EQUAL in ALL DIRECTIONS we say the forces are BALANCED. For an object with balanced forces… If it is moving it stays moving with CONSTANT SPEED If it is moving it stays moving in the SAME DIRECTION Its SHAPE does NOT CHANGE If it is STATIONERY it STAYS STATIONERY Examples of balanced forces A computer resting on a table A car driving at 70 mph A helicopter hovering

Unbalanced Forces If the forces acting on an object are NOT EQUAL in ALL DIRECTIONS we say the forces are UNBALANCED. For an object with unbalanced forces: If it is moving its SPEED could INCREASE or DECREASE If it is moving its DIRECTION COULD CHANGE Its SHAPE COULD CHANGE If it is STATIONERY it will START TO MOVE Examples of unbalanced forces A car turning a corner Stretching an elastic band A bike slowing down

Exercise 9: Which of the Following Are Examples of Balanced Forces
Exercise 9: Which of the Following Are Examples of Balanced Forces? Which Are Examples of Unbalanced Forces? A. An aeroplane taking off. B. A book resting on a table. C. Kicking a football. D. A picture hanging on a wall. E. A parachutist falling at constant speed. F. A car slowing down. G. A bike speeding up. H. A boy standing still. I. Stretching a spring. unbalanced balanced unbalanced balanced balanced unbalanced unbalanced balanced unbalanced

Speed Formula The speed formula allows you to calculate the speed, distance or time for a problem given two of the factors. Speed = Distance Time Speed measured in metres per second (m/s). Distance measured in metres (m). Time measured in seconds (s). Example: A car covers a distance of 200 m in a time of 25 s. What is the car’s speed? Speed = Distance ÷ Time Speed = 200 ÷ 25 m/s Speed = 8 m/s

Exercise 10: Speed Calculations 1
An rhino runs a distance of 360 metres in a time of 30 seconds. What is the average speed of the rhino? Speed = Distance ÷ Time Speed = 360 m ÷ 30 s Speed = 12 m/s

Exercise 11: Speed Calculations 2
An elephant walks a distance of metres in a time of 8 minutes. What is the average speed of the elephant in metres per second? Speed = Distance ÷ Time Speed = m ÷ 480 s Speed = 5 m/s

Exercise 12: Speed Calculations 3
1. A aeroplane covers a distance of m in 20 seconds. What is the speed of the aeroplane? 2. A car moving at 20 m/s will take how long to cover a distance of 1000 m? 3. A bike moving at 15 m/s travels in a straight line for 40 seconds. What distance will the bike cover in this time? 4. A dog running at 11 m/s will take how long to run 100 m? 200 m/s. 50 seconds. 600 m. 9.1 seconds (to 1 d.p.).

Moving Through Air Air resistance
AIR RESISTANCE is the special case of FRICTION between MOVING OBJECTS and AIR. Like friction, air resistance always acts in the OPPOSITE DIRECTION to the way an object is moving. Gravity For a FALLING OBJECT, air resistance acts in an UPWARDS DIRECTION. Falling sycamore seed Air resistance Direction of movement For a SPEEDING CAR, air resistance acts in the DIRECTION OPPOSITE TO THE WAY THE CAR IS MOVING. Speeding car

Air Resistance AIR RESISTANCE is the FRICTION between MOVING OBJECTS and AIR. Like friction, air resistance always acts in the OPPOSITE DIRECTION to the way an object is moving. Air resistance is caused by AIR PARTICLES striking a moving object. What factors will affect the amount of the air resistance for an object moving through air? The SPEED of an object. The FASTER an object is moving, the MORE AIR PARTICLES that can strike it, the MORE AIR RESISTANCE there is. The SURFACE AREA of an object. The LARGER the SURFACE AREA of an object, the MORE AIR PARTICLES that can strike it, the MORE AIR RESISTANCE there is. PARACHUTES work by INCREASING the SURFACE AREA of a FALLING OBJECT which INCREASES its AIR RESISTANCE which SLOWS ITS DESCENT.

Terminal Speed Why do cars have a top speed? Why if you keep your foot on the accelerator does a car not just keep getting faster and faster? When a car accelerates it gets faster because the engine is giving the car more THRUST. Air resistance (and friction) Thrust The FASTER a car is moving, the MORE AIR RESISTANCE there is. Forwards and backwards forces are balanced when a car reaches its terminal speed. The air resistance acts in the OPPOSITE DIRECTION to the thrust of the engine. This REDUCES the acceleration of the car. Eventually as the car gets faster and faster the force of air resistance EQUALS the force of thrust from the engine. The forces are BALANCED. When forces are balanced the speed of a moving object remains CONSTANT. The car has reached its MAXIMUM or TERMINAL SPEED.

Exercise 13: Air Resistance
1. In which direction does air resistance always act? 2. For the reversing car below, mark on the direction of the air resistance. 3. If you were falling through the air how could you make yourself fall slower? The opposite direction to the way an object is moving. Direction of movement Air resistance You could open a parachute. A parachute has a large area and this would make you fall slower.