# Forces P2

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• Forces
• Gravity
• Gravity attracts all masses but you only notice when one of the masses is really big.
• Anything near a planet or star is attracted to it very strongly.
• On the surface of a planet, it makes all things accelerate towards the ground all with the same acceleration- 10m/s squared on Earth.
• It gives everything a weight.
• Weight and mass
• Mass is the amount of stuff in an object, it is the same wherever.
• Weight is caused by the pull pf the gravitational force, often the force pulling it towards the centre of the Earth.
• An object has the same mass whether its on the Earth or Moon but its weight will be different. A 1kg mass will weigh less on the moon.
• Weight is a force measured in Newtons,N. It's measured using a spring or newton meter.
• Mass is not a force, it's measured in Kilograms,kg, using a mass balance or scales.
• Weight = Mass * Gravitational field strength
• W = m*g
• Resultant forces
• In most real situations there are at least 2 forces acting on the object in any direction.
• The overall effect of these forces will decide the motion of the object. Stay stationary, accelerate or decelerate.
• If you have a number of forces acting on a single point, you can replace them with one single resultant force.
• if the forces all act along the same ling e.g. the'yre parallel or act in same or opposite direction, the overall effect is found by adding or subtracting them.
• A resultant force on an object means a change in velocity.
• If there is no resultant force on a moving object it will move at the same velocity.
• If there is a non-zero resultant force on a moving object it will accelerate or decelerate.
• Acceleration
• Reaction and Action forces
• Each action force exerts and opposite and equal reaction force.
• When two objects interact, the forces they exert on each other are equal and opposite.
• This means that if you push something like a shopping trolley, the trolley will push just as hard and if you stop pushing so does the trolley.
• Even though the 2 objects will feel the same force, they will accelerate at different velocity's due to their mass.
• Friction