Applications of Forces

Mass and Weight

In previous sections, the distinction between Speed and Velocity was discussed. Those two words can be used interchangeably in 'real life', but in Physics, they mean different things. The same applies to Mass and Weight.

The video below shows an introduction into the difference between Mass and Weight:-

In Physics, the following definitions apply:-

Mass - The amount of Matter contained within an object. The mass of an object does not change with location. Mass is measured in kg.

Weight - The Force on an object due to Gravity. Weight is measured in N.

As the Weight of an object is simply a Force, the formula used to explain Newton's 2nd Law can be used to calculate it. As this is a special case, however, the formula is altered to refer specifically to Weight. The changes made are as follows :-


1. Force is equivalent to Weight.

2. Acceleration in this case is the acceleration due to gravity.


Due to this, the Formula F = ma can be rewritten as:-


W = m x g


Where :-

W = Weight (N)

m = Mass (kg)

g = Acceleration due to gravity (on Earth = 9.8 ms-2)

Example 1 -

A Physics teacher uses a set of bathroom scales and measures his Mass as 70 kg. If he is standing on the Earth's surface, what is his Weight?

W = ?

m = 70 kg

g = 9.8 ms-2

W = m x g

W = 70 x 9.8

W = 686 N

As stated earlier, the value of g changes depending on the location within the Solar System. The table below shows values for g for other locations in the Solar System:-

The link below opens a page where you can calculate your Weight on other planets:-

The video below shows the effect of increasing the Force due to Gravity on Brian Cox:-

Gravity Case Study : Galileo's Leaning Tower of Pisa Experiment

In 1589, the scientist Galileo Galilei is said to have conducted an experiment from the top of the Leaning Tower of Pisa in Italy.

In the experiment, Galileo dropped two objects at the same time, a cannon ball and an apple (two objects of similar size). It was assumed at the time that the heavier object (the cannon ball) would fall faster.

It was seen during the experiment, however, that both hit the ground at the same time. This shows that all objects falling under Gravity experience the same acceleration, regardless of mass.

Note - The above experiment most likely never happened, it is a "Thought Experiment".

A modern variation of the above experiment was conducted by the astronauts of Apollo 15 on the surface of the Moon. The astronaut David Scott dropped a feather and a ammer in this recreation:-

Forces Case Study : Car Safety

A major part of all vehicle design is to ensure that the occupants are kept as safe as possible. There are three safety features that vehicles use that can be understood as an application of Newton's Laws:-

Seat Belts - Newton's 1st Law.

Crumple Zones - Newton's 2nd Law.

Airbags - Newton's 2nd Law.


Seat Belts

By law, all occupants of cars must wear a seat belt. The function of a seat belt is to restrain the occupant, preventing forward motion during a fast stop or crash. Seat belts do this by applying a Force in the opposite direction to motion, causing the occupant to experience a negative Acceleration, slowing them down.

The easiest method of explaining the Physics behind a seatbelt is to consider first the motion of an occupant in a crash without a seatbelt.

The video below shows what happens during a car crash, with and without a seat belt:-

As stated above, seatbelts can be explained as an application of Newton's 1st law. In a crash, the vehicle experiences a large negative acceleration and is brought to a stop in a short time.

If the occupant is not wearing a seatbelt, then following Newton's 1st law, the occupant will continue to move forward with a constant velocity until an external Force acts upon them. In this case, the external Force would then be provided by the steering wheel or the windscreen, causing severe injury.

If the occupant is wearing a seatbelt, then the occupant will have a Force applied to them by the belt, causing the occupant to slow down with the vehicle, reducing the risk of injury.


Airbags and Crumple Zones

Both of the above technologies function as an application of Newton's 2nd law. In a crash, the greater the Force an occupant experiences, the greater the risk of injury. Both airbags and crumple zones reduce the Force experienced by increasing the time of collision. This reduces the negative acceleration experienced, and therefore the Force experienced.

The video below shows the effect of a high speed collision on two different types of car:-

The video below shows a in-depth explanation of the Physics behind car safety technology:-