# MASS, WEIGHT AND GRAVITY

#### branchMECHANICS (MOTION)

=Mass, Weight & Gravity= ===Mass=== Mass is a quantity which describes how an object changes its motion (accelerates), given an unbalanced force (m = F/a). Mass is measured in '''kilograms (kg)'''. A large mass will be harder to accelerate than a small mass given the same unbalanced force. ===Weight=== Weight is a force that results from the action of a gravitational field on a mass: it measures how strongly the force of gravity pulls on that mass. Because weight is a force it is measured in '''Newtons (N)''' ===Gravity=== Gravity is a natural phenomenon that causes any objects with mass to be attracted to each other. The greater the mass the greater the resulting force of attraction. The force due to gravity is greater on Earth than the moon, because the Earth has so much more mass. On the moon, gravity is about 1/6 that of earth so object will experience 1/6 the weight force. ==Calculating Weight Force== [image:http://i.imgur.com/fKafSyn.png?1] An astronaut weighs less on the moon because he is subjected to less gravitational force. However his mass does not change. To calculate the weight force: '''F (weight) = m (mass) x g (acceleration due to gravity)''' The value for the acceleration due to gravity here on earth is '''9.81 N/kg or (ms^-2^)''' but it can be approximated to '''10 N/kg or ms^-2^''' for lower level studies in Physics. Therefore on '''Earth''' we can use the following formula to calculate the weight force: '''F = m x 10''' ==Extra for Experts== [image:http://i.imgur.com/xQHnGfv.png?1] You'll notice that the formula for weight looks very similar to the '''F = ma''' formula. Gravity can in fact be expressed using the units for acceleration. The acceleration due to gravity on earth is approx 10 ms^-2^ If two different masses are dropped they will each experience a different weight force, but the resulting acceleration is always the same (10ms^-2^). This is why different masses will fall at the same rate (ignoring any air resistance). This was illustrated in a famous experiment where an astronaut on the moon dropped a feather and a hammer. They both fell at the same rate and hit the ground at the same time. ===A thought Experiment=== You may still feel like a heavier object would fall faster, but ponder this for a moment... Imagine you had two metal balls that were both the same size and shape, but one had a mass of 10kg and the other only was hollow and only had a mass of 1kg. If you dropped them off a bridge which one would fall the fastest? If you still feel like the heavier one would fall faster, then what would happen if I tied them together? Would the 1 kg mass slow down the other because it falls more slowly or would they both fall faster because their combined mass is 11kg? It just doesn't work! All object fall at the same rate unless one is subjected to greater air resistance (that's why a parachute slows you down).
Credit: Ben Himme, Tristan O'Hanlon