2.8.1 Safety Features of Vehicle

Posted on April 16, 2020 by user

Crumble Zone

The crumple zone increases the reaction time of collision during an accident.
This causes the impulsive force to be reduced and hence reduces the risk of injuries.

Seat Belt

Prevent the driver and passengers from being flung forward or thrown out of the car during an emergency break.

Airbag

The inflated airbag during an accident acts as a cushion to lessen the impact when the driver flings forward hitting the steering wheel or dashboard.

Head Rest

Reduce neck injury when driver and passengers are thrown backwards when the car is banged from backward.

Windscreen

Shatter-proof glass is used so that it will not break into small pieces when broken. This may reduce injuries caused by scattered glass.

Padded Dashboard

Cover with soft material. This may increases the reaction time and hence reduce the impulsive force when passenger knocking on it in accident.

Collapsible Steering Columns

The steering will swing away from driver’s chest during collision. This may reduce the impulsive force acting on the driver.

Anti-lock Braking System (ABS)

Prevent the wheels from locking when brake applied suddenly by adjusting the pressure of the brake fluid. This can prevents the car from skidding.

Bumper

Made of elastic material so that it can increases the reaction time and hence reduces the impulsive force caused by collision.

Passenger Safety Cell

The body of the car is made from strong, rigid stell cage.
This may prevent the car from collapsing on the passengers during a car crash.

2.7.2 Impulse

Posted on April 16, 2020 by user

Impulse is defined as the product of the force (F) acting on an object and the time of action (t).

Impulse is the product between the force, F with the time of impact, t.
Impulse is also defined as the change in momentum.
Impulse is a vector quantity.
An impulse will cause velocity change of an object.

Formula of impulse

Impulse is the product of force and time.

Impulse = F x t

Impulse = change of momentum

Impulve = mv - mu

Example:
A billiard ball weighing 0.25 kg is in a stationary state on a smooth billiard table. The ball is then given an impulse as much as 3.0Ns horizontally. What is the velocity of the ball after impact?

Answer:
Mass of the billiard ball, m = 0.25kg
Initial velocity of the billiard ball, u = 0
Impulse = 3.0Ns
Final velocity, v = ?

Impulse = momentum change = mv - mu
(3.0) = (0.25)v - (0.25)(0)
3 = 0.25v
v = 3/0.25 = 12 m/s

2.7.1 Impulsive Force

Posted on April 16, 2020 by user

Impulsive force is defined as the rate of change of momentum in a reaction. Mathematically, we write this as
It is a force which acts on an object for a very short interval during a collision or explosion.

Example:
A car of mass 1000kg is traveling with a velocity of 25 m/s. The car hits a street lamp and is stopped in0.05 seconds. What is the impulsive force acting on the car during the crash?

Answer:
m = 1000kg
u = 24 m/s
v = 0
t = 0.05s

Effects of impulse vs Force

A force determines the acceleration (rate of velocity change) of an object. A greater force produces a higher acceleration.
An impulse determines the velocity change of an object. A greater impulse yield a higher velocity change.

Examples Involving Impulsive Force

Playing football
Playing badminton
Playing tennis
Playing golf
Playing baseball

Long Jump

The long jump pit is filled with sand to increase the reaction time when atlete land on it.
This is to reduce the impulsive force acts on the leg of the atlete because impulsive force is inversely proportional to the reaction time.

High Jump

(This image is licenced under the GNU Free Document Licence. The original file is from the Wikipedia.org.)

During a high jump, a high jumper will land on a thick, soft mattress after the jump.
This is to increase the reaction time and hence reduces the impulsive force acting on the high jumper.

Jump off From a High Place

A jumper bends his/her leg during landing. This is to increase the reaction time and hence reduce the impact of impulsive force acting on the leg of the jumper.

2.6.2 Force, Mass and Acceleration

Posted on April 16, 2020 by user

Newton's Second Law

The rate of change of momentum of a body is directly proportional to the resultant force acting on the body and is in the same direction.

Implication:

When there is resultant force acting on an object, the object will accelerate (moving faster, moving slower or change direction).

Formula of Force

From Newton's Second Law, we can derived the equation
(IMPORTANT: F Must be the net force)

Summary of Newton's 1st Law and 2nd Law

Newton's First Law:
When there is no net force acting on an object, the object is either stationary or move with constant speed in a straight line.

Newton's Second Law:
When there is a net force acting on an object, the object will accelerate.

Example:
A box of mass 150kg is placed on a horizontal floor with a smooth surface; find the acceleration of the box when a 300N force is acting on the box horizontally.

Answer:
F = ma
(300) = (150)a
a = 2 ms^-2

Example:
A object of mass 50kg is placed on a horizontal floor with a smooth surface. If the velocity of the object changes from stationary to 25.0 m/s in 5 seconds when is acted by a force, find the magnitude of the force that is acting?

Answer:
We know that we can find the magnitude of a force by using the formula F = ma. The mass m is already given in the question, but the acceleration is not give directly.

We can determine the acceleration from the formula

From the formula
F = ma = (50)(5) = 250N

The force acting on the box is 250N.

Principle of Conservation of Momentum

The principle of conservation of momentum states that in a system make out of objects that react (collide or explode), the total momentum is constant if no external force is acted upon the system.

Sum of Momentum Before Reaction
= Sum of Momentum After Reaction

Formula

Example - Both Object are in the Same Direction before Collision
A Car A of mass 600 kg moving at 40 ms^-1 collides with a car B of mass 800 kg moving at 20 ms^-1 in the same direction. If car B moves forwards at 30 ms^-1 by the impact, what is the velocity, v, of the car A immediately after the crash?

Answer:
m₁ = 600kg
m₂ = 800kg
u₁ = 40 ms^-1
u₂ = 20 ms^-1
v₁ = ?
v₂ = 30 ms^-1

According to the principle of conservation of momentum,

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
(600)(40) + (800)(20) = (600)v₁ + (800)(30)
40000 = 600v₁ + 24000
600v₁ = 16000
v₁ = 26.67 ms^-1

Example - Both Object are in opposite direction Before Collision
A 0.50kg ball traveling at 6.0 ms^-1 collides head-on with a 1.0 kg ball moving in the opposite direction at a speed of 12.0 ms^-1. The 0.50kg ball moves backward at 14.0 ms^-1 after the collision. Find the velocity of the second ball after collision.

Answer:
m₁ = 0.5 kg
m₂ = 1.0 kg
u₁ = 6.0 ms^-1
u₂ = -12.0 ms^-1
v₁ = -14.0 ms^-1
v₂ = ?

(IMPORTANT: velocity is negative when the object move in opposite direction)

According to the principle of conservation of momentum,

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
(0.5)(6) + (1.0)(-12) = (0.5)(-14) + (1.0)v₂
-9 = - 7 + 1v₂
v₂ = -2 ms^-1

Elastic and Inelastic Collision

Elastic Collision

Elastic collision is the collision where the kinetic energy is conserved after the collision.

Total Kinetic Energy before Collision
= Total Kinetic Energy after Collision

Additional notes:

In an elastic collision, the 2 objects separated right after the collision, and
the momentum is conserved after the collision.
Total energy is conserved after the collision.

Inelastic Collision

Inelastic collision is the collision where the kinetic energy is not conserved after the collision.

Additional notes:

In a perfectly elastic collision, the 2 objects attach together after the collision, and
the momentum is also conserved after the collision.
Total energy is conserved after the collision.

Example - Perfectly Inelastic Collision:
A lorry of mass 8000kg is moving with a velocity of 30 ms^-1. The lorry is then accidentally collides with a car of mass 1500kg moving in the same direction with a velocity of 20 ms^-1. After the collision, both the vehicles attach together and move with a speed of velocity v. Find the value of v.

Answer:
(IMPORTANT: When 2 object attach together, they move with same speed.)

m₁ = 8000kg
m₂ = 1500kg
u₁ = 30 ms^-1
u₂ = 20 ms^-1
v₁ = v
v₂ = v

According to the principle of conservation of momentum,

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
(8,000)(30) + (1,500)(20) = (8,000)v+ (1,500)v
270,000 = 9500v
v = 28.42 ms^-1

Before explosion both object stick together and at rest.	After collision, both object move at opposite direction.
Total Momentum before collision Is zero	Total Momentum after collision : m₁v₁ + m₂v₂
From the law of conservation of momentum: Total Momentum Before collision = Total Momentum after collision 0 = m₁v₁ + m₂v₂ m₁v₁ = - m₂v₂ (-ve sign means opposite direction)

Crumble Zone

Seat Belt

Airbag

Head Rest

Windscreen

Padded Dashboard

Collapsible Steering Columns

Anti-lock Braking System (ABS)

Bumper

Passenger Safety Cell

Formula of impulse

Effects of impulse vs Force

Long Jump

High Jump

Jump off From a High Place

Newton's Second Law

Formula of Force

Unbalanced Force/ Resultant Force

Types of Forces

Friction

Tension

Weight (Gravitational Force)

Normal Reaction (Normal Contact Force)

Newton’s Third Law of Motion

Rocket

Jet Engine

Principle of Conservation of Momentum

Formula

Elastic and Inelastic Collision

Elastic Collision

Inelastic Collision

Momentum

Formula:

Ways to Reduce the Negative Effect of Inertia