P5 Weight Mass and Gravity (page 202) Flashcards
What is Gravitational Force?
it is the Force of Attraction between Masses.
Gravity attracts all masses, true or false?
True, but you only notice it when one of the masses is really really big (e.g. a planet - anything near a planet or a star is attracted to it very strongly).
What two affects do gravity have on the surface of a planet?
1) on the surface of a planet, it makes all things fall towards the ground.
2) It gives everything a weight.
Is Weight and Mass the same thing?
No.
Explain what Mass is?
its the amount of ‘stuff’ in an object.
(for any given object this will have the same value anywhere in the universe).
Explain what Weight it?
it is the force acting on an object due to gravity (the pull of the gravitational force on the object)
close to Earth, this force is caused by the gravitational field around the Earth.
Explain what Gravatational Field Strength is?
Gravatational field strength varies with location. It’s stronger the closer you are to the mass causing the field, and stronger for larger masses.
The Weight of an object depends on what?
the strength of the gravitational field at the location of the object. This means that the weight of an object changes with its location.
An object has the same mass whether it’s on Earth or on the Moon, but its weight will be different.
A 1kg mass will weigh less on the Moon (about 1.6 N) than it does on Earth (about 9.8 N) why?
simply because the gravitational field strength on the surface of the moon is less.
Weight is a force measured in what?
Newtons.
You can think of a force as acting from a single point on the object, called?
its centre of mass
(a point at which you assume the whole mass is concentrated)
(For a uniform Object (one that’s the same density, see page 192, throughout and is a regular shape), this will be at the centre of the object.
How is Weight measured?
using a calibrated spring balance (or newtonmeter).
How is Mass measured?
its measured in kilograms with a mass balance (an old-fashioned pair of balancing scales)
Remember Mass is not a force.
How can you calculate the weight of an object if you know its mass (m) and the strength of the gravitational field that it is in (g)?
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
(see picture of triangle on page 202).
For Earth g= 9.8 N/kg and for the Moon its around 1.6 N/kg.
(dont worry youll always be given a value of the g to use in the exam).
Increasing the Mass of an objet increases its weight, why?
if you double the mass, the weight doubles too, so you can say that weight and mass are directly proportional.
What is the direct proportionality symbol?
W ∝ m
Remember, weight is a force due to gravity that acts from an objects centre of mass. It changes depending on what?
on the strength of the gravitational field of the object is in (and is directly proportional to the objects mass)
(mass and weight are directly proportional).
Calculate the weight in newtons of a 5 kg mass?
a) on Earth (g - 9.8 N/kg)
b) on the Moon (g = 1.6 N/kg) (4 marks)
a) W = mg = 9.8 (1 mark) = 49 N (1 mark)
b) W = 5 x .6 (1 mark) = 8 N (1 mark)
