# 4.1.2 Force & Acceleration

### Force & Acceleration

• As stated on the previous page, Newton’s Second Law of Motion tells us that objects will accelerate if there is a resultant force acting upon them
• This acceleration will be in the same direction as this resultant force

Newton’s Second law equation

#### Resultant Force

• Since force is a vector, every force on a body has a magnitude and direction
• The resultant force is therefore the vector sum of all the forces acting on the body. The direction is given by either the positive or negative direction as shown in the examples below

Resultant forces on a body

• The resultant force could also be at an angle, in which case addition of vectors is used to find the magnitude and direction of the resultant force.
• For more details on this, have a look at the page on “Scalars & Vectors”

#### Acceleration

• Given the mass, Newton’s Second Law means you can find the acceleration of an object
• Since acceleration is also a vector, it can be either positive or negative depending on the direction of the resultant force
• Negative acceleration is deceleration
• An object may continue in the same direction however with a resultant force in the opposite direction to its motion, it will slow down and eventually come to a stop

#### Worked example

Resultant force and acceleration on a rocket

#### Exam Tip

The direction you consider positive is your choice, as long the signs of the numbers (positive or negative) are consistent with this throughout the question

It is a general rule to consider the direction of motion the object is travelling in as positive. Therefore all vectors in the direction of motion will be positive and opposing vectors such as drag forces, are negative.

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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