# Physics

- Created by: Nelema Uddin
- Created on: 26-01-12 22:38

Newtons laws of motion

First law of motion - if the forces acting on an object an balanced then the object will either contiue at a constant speed or remain stationary

Second law of motion - An object will always accelerate in the direction of the larger force. Examples of acceleration:

- Stopping - Slowing down

- Starting - Changing direction

- Speeding up

Thrid law of motion: For every force acting on an object there is an equall and opposite force acting in the opposite direction

Obvious points

- The larger the force, the larger the acceleration

- The larger the mass the larger the force needed for the same acceleration

- The smaller the mass, the larger the acceleration

Mass, weight and gravity

Mass is the amount of stuff "matter" inside an object whereas weight is the force of gravity which acts on an object. Weight is measured in Newtons. Weight = mass(kg) x gravitational field strength (N/kg)

Resultant force is the sum of all the forces acting on an object. If the forces are un opposite directions then subtract them to get the resultant force. If the forces are in the same direction then add them up to get the resultant force. Eg. If I push a trolley with a force of 50N but super Nelema comes and pushes the trolley at 70N in the opposite direction then my resultant force would equal 50-70= -20 the minus just means in the opposite direction.

Drag and terminal velocity

Friction is a force which opposes the movement of objects. Friction always slows things down and always acts in the opposite direction of movement. Friction can also be called drag.

Car designers design cars to reduce the drag force on them by making cars a more streamlined shape so they are able to go faster. Whereas skydivers use the opposite of this concept and want to increase their drag force and so open a parachute when fallin, this increases their drag force.

It is easier to maintain a constant speed at 30mph then at 70mph as at 30mp the driving force is much lower and so it is easier for the frictional forces to balance the driving force and the car can travel a t a constant speed. But at 70mph the driving force is much larger and so the drag force has to increase to much larger to balance the driving force which is more difficult as the driving force is larger.

Cars and free-falling people

When a car sets off from rest initially its driving force is much larger than its drag force and so the car accelerates, speeding up. As the car accelerates it goes through more air and so the drag force acting on it increases. Eventually this balances out the driving force and there is zero resultant force. The car is said to have reached terminal velocity and will contiue to travel at a constant speed.

When a skydiver first jumps out of a plane, initially…

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