Chapter 5: Motion

5.1 Motion of Vehicles on Land

1. Vehicles on land either powered with or without engine.

2. Without engine (bicycle) – pedal-driven two wheeled land vehicle.

3. With engine – driven by internal combustion, piston moved up and down when fossils fuels are burnt. (Exp: cars, lorries, vans).

5.1.1 The Four Stroke Petrol Engine

1. Contains 4 cylinders.

2. Internal combustion engine.

3. Uses petrol as fuel.

4. Each stroke works in a cycle.

5. In a complete cycle, crankshaft will make 2 revolutions

Chemical energy -> Heat energy -> Kinetic energy

Induction Stroke(intake)

Intake valve opens, exhaust valve closes. Piston moves down, low pressure in cylinder. Petrol-air mixture enters the cylinder through intake valve from carburetor. Intake valve will close when piston reach lowest position.

Compression Stroke

Intake and exhaust valves close. Piston moves up & compresses petrol-air mixture (1/5 of original volume). Spark plug produces sparks when piston reaches highest position. Crankshaft makes a complete revolution.

Power Stroke (combustion)

Intake and exhaust valves close. Spark plug ignites fuel. Explosion occurs. Hot gases expand & push piston down. Moving piston turns crankshaft and drives car ahead.

Exhaust Stroke

Intake valve close, exhaust valve open. Piston moves up, burned gas pushed out through exhaust valve. Exhaust valve close when piston reached highest position. Crankshaft makes another complete revolution (2nd revolution).

5.1.2 The Four Stroke Diesel Engine

1. Contains four cylinders, have fuel infector instead of spark plug.

2. Use diesel as fuel.

3. There is no carburetor.

4. In a complete cycle, crankshaft will make 2 revolutions

Chemical energy -> Heat energy -> Kinetic energy

Induction Stroke (intake)

Intake valve opens, exhaust valve closes.

Piston moves down, low pressure in combustion space.

Intake valve closed when piston reaches lowest position.

Atmospheric pressure causes the air sucked into the combustion space through

intake valve.

Compression Stroke

Intake and exhaust valves close.

Piston moves up to compress the air inside the combustion space (1/15 of

original space, temperature raised to 5500C.)

Diesel sprayed into cylinder through fuel injector when piston reaches highest

position.

Power Stroke

Intake and exhaust valves close.

Diesel-air mixture burns and explosion occurs.

Hot gases expand and push piston down.

Exhaust valve open when the piston reaches lowest position.

Exhaust Stroke

Intake valve closes, Exhaust valve opens.

Piston moves up, burned gas pushed out through exhaust valve.

Exhaust valve will close when the piston reaches highest position.

5.1.3 The Two Stroke Petrol Engine

1. Only have 2 stroke (upward & downward stroke).

2. Used in motorcycle & lawnmowers.

3. Piston act as valve to open and close the intake tube, exhaust tube and transfer tube.

4. Crankshaft makes one revolution in a complete cycle.

5. Less powerful than 4-stroke engine.

Upward Stroke

Piston move upwards to open intake tube, close exhaust tube & transfer port.

Petrol-air mixture drawn into cylinder and compressed by upward piston.

Ignition occurred.

At the same time, petrol-air mixture drawn into crankcase for next cycle.

Downward Stroke

Piston moves downward due to hot hair expansion. Exhaust tube & transfer port

open, intake valve closes.

Burned gas pushed out through exhaust tube.

Petrol-air mixture drawn into cylinder through transfer port for next cycle.

5.1.4 Comparison between Petrol Engine and Diesel Engine

Similarities

1. Both are internal combustion four-stroke engine.

2. Crankshaft makes 2 revolutions in a complete cycle.

3. Combustion of fuels causes the expansion of exhaust gases, energy obtained is

used to move the components in engine.

Differences

Petrol FUEL Dieselsmall & light SIZE OF ENGINE big & heavy

Mixture of air and petrolSUBSTANCES IN THE

CYLINDER Air only

Fire sparks from spark plug IGNITIONHigh temperature from air

compression

Less powerful & less efficientENGINE POWER &

EFFICIENCY More powerful & more efficientLight vehicles USAGE Heavy vehicles

5.2 Speed, Velocity and Acceleration

1. Speed is the rate of change of distance with time.

Speed = Distance travelled (m) / Time taken (s)

- Unit : m s-1 or km h-1

- a scalar quantity

2. Velocity is the change of displacement with time.

Velocity = Displacement (m) / Time taken (s)

- Unit : m s-1 or km h-1

- a vector quantity , has both magnitude and direction.

3. Acceleration is the rate of change of velocity with time.

Acceleration = Change in velocity (m s-1) / Time interval (s)

- Unit: m s-2

- a vector quantity

4. When the velocity of an object decreases, its rate of change is known as deceleration.

5.3 Inertia

1. the resistance of an object to a change in its original state of conditions.

2. the larger mass of an object, the greater its inertia value.

3. Examples : - aeroplane need long way to take off and landing to overcome its inertia.

- a ship need to off its engine a few kilometers away before arrive at jetty.

- passengers jerked forward when the bus stopped suddenly.

4. Seat belts, headrests and airbags are installed in vehicles as safety features based of concept

of inertia:

(a) Seat belts exert pulling forces on passengers to prevent them being thrown towards

the interior part of vehicle.

(b) Headrests help protect the neck of the driver and the passengers of the vehicle.

(c) Air bags prevent drivers and passengers from hitting the steering and dashboard.

5.4 Momentum

1. the product of its mass and velocity.

Momentum = Mass x Velocity

-The greater mass of an object, the greater its momentum

-The faster of an object moves, the greater its momentum.

2. measured in kg m s-1 / N s, it is a vector quantity.

3. Momentum is applied in the following:

(a) The speed and weight limits for heavy vehicles.

(b) The bumpers installed in the front and rear parts of vehicles.

(c) The use of the piledriver, the machine used to push heavy posts into the ground.

4. The Principles of Conservation of Momentum

Total momentum before collision = Total momentum after collision

- Conditions: ( m=mass, u=initial velocity, v=final velocity)

(a) Elastic collision

- mAuA + mBuB = mAvA + mBvB

- Kinetic energy conserved

- Objects separated after collision.

(b) Inelastic collision

- mAuA + mBuB = (mA + mB)v

- Kinetic energy not conserved

- Objects stick and move together after collision

(c) Explosion

- mBvB = - (mAvA)

- momentum is zero because objects are stationary.

- objects move in opposite direction.