# Category Archives: 03 Dynamics

## Finding resultant force from 2 forces at 0 to 90 degree

**Answer: Option A**

Refer to the video tutorial for the explanation.

## resultant force-time graph link to speed-time graph

**Answer: Option A**

Refer to the video tutorial below for explanation.

## Impressive Bugatti Chiron

This impressive Bugatti Chiron can accelerate from **rest to 400 km/h and decelerate to a complete stop in merely 42 seconds**! Our normal cars on the expressway travel about 90 km/h and the F1 race yesterday night is about 300 km/h. This Bugatti Chiron is faster than most bullet trains and comparable to the speed of a magnetic levitation train!

https://www.bugatti.com/chiron

Before we look at the video, let’s do some calculations:

**Let’s find the acceleration of the car to reach 400 km/h in 32.6 sec:**

Converting 400 km/h to m/s: 400km/1h = 400 000m/3600s =111 m/s

acceleration, a = (v – u)/t = (111 – 0) / 32.6 = 3.4 m/s2

hmmm…. this acceleration doesn’t seem impressive… it is way below free fall acceleration!

But we are not being fair here. To achieve the max speed of 400 km/h is not easy due to the resistive force (air resistance and friction) as speed increases. We should compare fairly the acceleration to reach 100 km/h instead like how we typically compare sports car like Ferrari etc.

**Let’s find the acceleration of the car to reach 100 km/h (27.8 m/s) in 2.4 sec:**

acceleration, a = (v – u)/t = (27.8 – 0) / 2.4 = 11.6 m/s2

This is greater than acceleration due to gravity (free fall) and much faster than most sports cars in the market like Ferrari or Lamborghini!

**Now, let’s find the deceleration of the car when it slows down from 400 km/h to a complete stop in 41.9 – 32.6 = 9.36 s**

acceleration, a = (v – u)/t = (0 – 111) / 9.36 = -11.9 m/s2

Take note of the spoiler being activated when it decelerates. This increases the drag (air resistance) to slow down the car, in addition to using the normal brakes. It is the same principle as the aeroplane when it lands and slows down on the runway.

## Vector Diagram – Parallelogram Method for 4 different kinds of questions

For Combined Science Physics Olevel, when constructing vector diagram, a method called **Parallelogram Method** is commonly used.

**Parallelogram Method** is used when **2 forces are known**, and the** resultant (net) force R**, can be found.

Below show **5 different variations of questions** when can be solved by using Parallelogram Method:

(A) Body has **acceleration**, **forces are unbalanced**, there is a **resultant force R** *(Newton’s 2nd law)*

(B) Body **at rest**, **all forces are balanced**, there is **no resultant force R** *(Newton’s 1st law)*

(C) Body moving at **constant velocity**, **all forces are balanced**, there is n**o resultant force** *(Newton’s 1st law)*

(D) Finding **unknown force** (with **resultant force R + another known force are given**)

(F) **Vector diagram is applicable to all vector quantities **(with magnitude and direction). Not just for forces.

Click here to view video tutorials on how to construct parallelogram step-by-step

For Pure Physics, another method is the closed-loop triangle, where only 1 known force with 2 other unknown force. Click here to view the video tutorials.