MOTION

The Four-stroke Petrol Engine

Vehicles without Engines

1. Many vehicles do not use engines to move about e.g. bicycles,

bullock carts and wheelbarrows.

2. The bicycle uses human energy to paddle and move it.

Vehicles with Engines

1. Many types of vehicles have engines which move them.

2. The four-cylinder engine is commonly used in motor vehicles.

3. A fuel is burnt in the engine to move the pistons. The up and

down motion of the pistons causes the shafts in the vehicle to

rotate and the wheels to turn. This makes the vehicle to move.

The Two-stroke Petrol Engine

Two-stroke Petrol Engine

1. It is used for small machines e.g. motor boats, motorcycles and

lawn mowers.

2. It is an inlet tube for the petrol-air mixture to go in and an outlet

tube for exhaust gases to up escape.

3. The inlet tube and the outlet tube are open or closed by the piston

moving up and down.

Upward stroke

1. The piston moves up the cylinder.

2. The inlet tube opens and the petrol-air

mixture enters the cylinder due to lower pressure in the cylinder.

3. Piston closes the outlet tube.

4. Petrol-air mixture above the piston is compressed by the rising

piston.

Downward stroke

1. When the piston reaches the top of the cylinder, a spark fired by the

spark plug ignites the petrol-air mixture in the upper part of the

cylinder.

2. The force of the expanding gases pushes the piston down, opening

the outlet tube and allowing the exhaust gases to escape.

3. The piston closes the inlet tube as it is moving down and forces

petrol-air mixture into the upper part of the cylinder through a transfer

passage.

Relationship between Operation of the engine and motion of the Vehicle

Transfer of Motion in a Car

1. When the engine of a car is started, the pistons move up and

down and rotate a crankshaft.

2. When the gear is engaged, the crankshaft is connected to the

transmission system (clutch and gearbox) and a drive shaft.

3. The rotating crankshaft rotates the drive shaft, the differential

gear, and the wheel axle, causing the wheels to turn.

The Gear System

Low gear (force multiplier)

1. When a low gear is used, a small gear (driving wheel) is used to

turn a large gear (driven wheel).

2. The large gear turns more slowly but exerts a greater force.

3. A low gear is used when the car is starting to move or going up a

deep slope.

High gear (speed multiplier)

1. When a high gear is engaged, a large gear (driving wheel) is used

to turn a small gear (driven wheel).

2. The small gear rotates faster (more speed) but has less force.

3. A high gear is used to increase speed.

Clutch, Foot Brake and Accelerator

1. The clutch is used when the driver wishes to change gear.

Modern cars have an automatic system for changing gears.

2. The foot brake is used for slowing down or stopping the car.

3. The accelerator is used to increase fuel to the engine and so

increases the speed of the car.

Speed, Velocity and Acceleration

Speed = kelajuan

Velocity = halaju

Acceleration = pecutan

Inertia

What is Inertia?

1. Inertia is the tendency of a body to maintain its state of

rest or uniform motion in a straight line unless it is

acted upon by a force.

2. Any body with mass has inertia.

Stationary inertia

Stationary inertia is the inertia possessed by a body at

rest.

(a)We need to use a force to push an object, because the

object’s stationary inertia resists motion.

(b) When the cardboard is hit out of its position quickly,

the coin falls into the glass. The coin tends to remain at

rest in its original position because of its stationary

inertia.

(c) When block R is hit out of the stack quickly,P and Q

move down vertically due to their stationary inertia.

Moving inertia

Moving inertia is possessed by a moving object. It resists any

force trying to slow it down, make it go faster or change its

direction of motion.

(a)When a moving car stops suddenly, the passengers in the car

are thrown forward because the moving inertia in the

passengers continue to move them forward.

(b) When an electric fan is switched off the blades continue to

rotate for some time due to the moving inertia of the rotating

blades.

(c) Trains, cars, aeroplanes and ships which are moving cannot

stop at once when the brakes are applied or the engines are

switched off. They continue to move for some distance before

stopping because of their moving inertia.

Relations between Mass and Inertia

1. The tin with the bigger mass (tin filled with sand) requires a larger

force to make it swing or to stop it from swinging.

2. This shows a body with a large mass has more inertia than a body

with a small mass.

Safety Measures Used in Vehicles to Reduce the Negative

Effects of Inertia

1. Seat belts prevent the driver and passengers from being

thrown forward in a collision.

2. Air bags prevent injury to the driver.

3. The collapsible steering column prevent injury to the

driver.

4. Headrests prevent the heads of passengers from being

jerked backwards.

5. Bumpers in the front and the back absorb the collision

force.

6. The strong body frame of the car protects the passengers.

Applications of Momentum

1. A falling pile driver has a large momentum because of

its huge mass and high velocity so that it can hit a

concrete pillar into the ground.

2. A bullet fired from a gun has high penetrating power

because of the high velocity of the bullet.

3. A steam roller has a large momentum for rolling a

surface because of its huge mass.

4. Gases escaping backward from a rockets (action)

creates a forward momentum (reaction) which pushes

the rocket forward.

5. A motor vehicle has safety features to protect the

driver and the passengers from the effect of the cars

high momentum in case of an accident

Principle of Conservations of Momentum

Conservation of Momentum

1. When two or more bodies collide with one another, the

total momentum before the collision is equal to the

total momentum after the collision.

Total momentum before collision = Total momentum

after collision

2. In an elastic collision the bodies separate after

collision.

3. In an inelastic collision, the bodies stick together after

collision.

Worked example

Trolley A collides with trolley B. After the collision the

two trolleys become attached and move together.

Calculate the velocity of the joined trolleys.

Experiment on Conservation of Momentum

1. When match sphere P is pulled aside and released, it

swings back, hits sphere Q and stops.

2. The momentum of P is transferred to Q. But Q cannot

move because it is sandwiched in the middle.

3. So the momentum of Q is transferred to sphere R.

4. Sphere R swings outwards at the same velocity as

sphere P.

5. This process is repeated with P and R swinging

alternately, until the energy of the swinging spheres is lost

due to resistance and friction with the air.

6. A Newton’s cradle usually has four or five metal

spheres.

Applications of Pressure

1. Some tools are designed to exert a large pressure by

having the force act on a small area (Table A).

2. Some gadgets and machines are designed to reduce

the pressure on a surface by having the force act on

a large area (Table B).

Principle of the Hydraulic System

Transmission of Pressure in a Liquid

1. Pascal’s Principle

The pressure exerted on a liquid in an enclosed container

is transmitted equally through the liquid in all directions.

2. This principle is used in the hydraulic system.

The Hydraulic Brake

1. When the driver’s foot pressses on the brake pedal, the

pressure exerted on the brake fluid is transmitted

unchanged to the four wheel cylinders.

2. This pressure acting on a large of the piston in the

cylinder produces a large force on the piston.

3. This force pushes the brake pads outwards to press on

the rotating drum or rotating disc and slow down or

stop the motor vehicle.

The Hydraulic Jack

1. A hydraulic jack uses a small force to lift a

compressor very large force such as a motor car.

2. When the compressor is switched on, the air pressure

on the small cylinder, causing the large piston to rise.

3. The pressure on the oil in the small piston is

transmitted unchanged to the large cylinder

4. This pressure acting on a large surface of the large

piston produces a big force which pushes the car up.

Motion of Vehicles in Water

Principle of Operation of Vehicles in Water

Vehicles without engines

1. Sampans and canoes are moved through water by

using human energy.

2. Sailing ships are moved by using the kinetic energy of

wind.

Ship

1. A ship is driven by an engine

which turns the propellers.

2. The turning propellers push the

water behind (action) and

causes a forward momentum

(reaction) which drives the ship

forward.

3. The rudder of the ship controls

the direction of motion of the

ship.

Hovercraft

1. A hovercraft moves on a cushion of air on the surface of

the sea.

2. The engine turns the fans which produce the cushion of air

to lift the hovercraft from the sea.

3. The large fans on top of the hovercraft produce a strong

backward wind (action) which causes an equally strong

forward momentum in (reaction) that pushes the boat

forward.

Hydrofoil

1. The lower surface of a hydrofoil has wing-shaped

structures called hydrofoils.

2. At a certain speed, the hydrofoils get lifted above the

water surface (aerofoil principle) and the boat moves

faster because of reduced friction with the water.

Archimede’s Principle

1. When a body is immersed in a fluid (gas or liquid), it

experiences a loss in weight (up-thrust) equal to the

weight of the fluid displaced by the body.

2. When the ballast tanks of a submarine are filled with

sea water, the submarine becomes dense and

submerges in the sea.

3. When the ballast tanks are emptied, the submarine

becomes less dense and rises to the surface of the sea.

This is because the weight of the submarine is now

equal to the upthrust

Motion of Vehicles in Air

The Jet Engine

1. Air is sucked from the front of the engine into the

compressor and compressed so that it contains more

oxygen for its volume.

2. The hot compressed air is directed into the combustion

chamber where a fuel such as kerosene is sprayed into

it.

3. The mixture of hot air and fuel burns and releases hot

exhaust gases, which escape from the back of the engine

and produce a great backward momentum (action).

4. This action causes as equally large forward momentum

(reaction) which pushes the jet plane forward.

The Rocket Engine

1.Liquid hydrogen and liquid oxygen are carried in the

rocket engine.

2. The hydrogen fuel burns fiercely in the oxygen in the

combustion chamber producing exhaust gases.

3.The exhaust gases escape from the back of the engine

with great backward momentum (action).

4. This action causes an equally powerful forward

momentum (reaction) which pushes the rocket upwards.

5. Unlike a jet plane, a rocket can move outside the

Earth’s atmosphere because it carries its own hydrogen

and oxygen.

Bernoulli’s Principle and Its Application in

Aircrafts Bernoulli’s Principle