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−2 (or N kg−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
- An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field on different planets
- For example, the gravitational field strength on the Moon is 1.63 N kg-1, meaning an object’s weight will be about 6 times less than on Earth
On the moon, your mass will stay the same but your weight will be much lower
- 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
Worked example
Calculating mass of a space probe
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.
