### What is Mass?

- Mass is the measure of the amount of matter in an object
- Consequently, this is the property of an object that resists change in motion
- The greater the mass of a body, the smaller the change produced by an applied force
- The SI unit for mass is the
**kilogram**(kg)

#### Exam Tip

- Since mass is measured in
**kilograms**in Physics, if it is given in grams make sure to convert to kg by dividing the value by 1000 - It is a common misconception that mass and weight are the same, but they are in fact
**very different** - Weight is the force of gravity acting upon an object
- Weight is a vector quantity

- Mass is the amount of matter contained in the object
- Mass is a scalar quantity

### Weight

- Weight is the effect of a gravitational field on a mass
- Since it is a force on an object due to the pull of gravity, it is measured in
**Newtons***(N*) and is a vector quantity - The weight of a body is equal to the product of its mass (
*m*) and the acceleration of free fall (*g*)

*Weight equation*

*g*is the acceleration due to gravity or the gravitational field strength- On Earth, this is
**9.81****m s**(or N kg^{−2}^{−1})

** **

#### Free fall

- An object in free fall is falling solely under the influence of gravity
- On Earth, all free-falling objects accelerate towards Earth at a rate of
**9.81 m s**^{−2} - In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass

#### Mass v Weight

- For example, the gravitational field strength on the Moon is
**1.63 N kg**, meaning an object’s weight will be about^{-1}**6 times**less than on Earth

- Although you only need to memorise
*g*on Earth, its value on other planets in our solar system is given in the diagram below. Notice how much this varies according to the size of the planet

*Gravitational field strength of the planets in our solar system*

#### Exam Tip

- You will be expected to remember 9.81 m s
^{−2 }as the value of*g*on Earth for your exam - It is a common misconception that mass and weight are the same, but they are, in fact,
**very different**- Weight is the force of gravity acting upon an object. It is a vector quantity
- Mass is the amount of matter contained in the object. It is a scalar quantity

### Air Resistance

- Air resistance is an example of a drag force which objects experience when moving through the air
- At a walking pace, a person rarely experiences the effects of air resistance
- However, a person swimming at the same pace uses up much more energy – this is because air is 800 times less dense than water
- Air resistance depends on the
**shape**of the body (object) and the**speed**it’s travelling - Since drag force increases with speed, air resistance becomes important when objects move faster

*A racing cyclist adopts a more streamline posture to reduce the effects of air resistance.*

*The cycle, clothing and helmet are designed to allow them to go as fast as possible.*

#### Exam Tip

If a question considers air resistance to be **‘negligible**’ this means in that question, air resistance is taken to be so small it will not make a difference to the motion of the body. You can take this to mean there are no drag forces acting on the body.