# forces in balance

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• forces in balance
• vectors & scalars
• scalars have magnitude (size) but no direction
• eg: distance, speed, time, mass, energy, power
• vector quantities have magnitude AND direction
• eg: displacement, velocity, force, weight, momentum
• displacement - distance travelled in a given direction
• represented by a diagram: arrow = direction of vector, length = magnitude of vector
• forces between objects
• a force can change the shape of an object or change is motion or state of rest
• measured in newtons (N)
• contact forces: two objects must touch to interact
• eg: friction, air resistance
• newtons 3rd law of motion
• when two objects interact they exert equal and opposite forces on each other
• the forces are always the same type eg: both frictional forces
• eg: car driving forward - force from tyre on ground that pushes backwards (driving force) there is an equal and opposite force from the ground on the tyre that pushes the car forwards
• resultant forces
• resultant force: a single force that has the same effect as all the forces action on an object
• newtons 1st law of motion
• if the forces action on an object are balanced the resultant force is 0 and
• object stays at rest or at same speed and direction
• if forces greater than 0
• speed or direction of object will change
• movement of the object depends on the size & direction of the resultant force
• if 2 forces act on an object along the same line of motion the resultant force will be
• the difference between the forces (if act in different directions) and in the direction of the larger force
• the sum of the forces (if act in same direction) and in the same direction as both forces
• force diagrams
• forces = arrows, direction of arrow = direction of force, length of arrow drawn to scale = magnitude of force
• centre of mass
• centre of mass: the point where its mass is concentrated
• any object freely suspended will come to rest with centre of mass directly below point of suspension
• object is then in equilibrium
• flat object that's symmetrical = centre of mass lies along axis of symmetry
• wider the base of an object & the lower its centre of mass = the more stable it is
• parallelogram of forces
• to find the resultant of 2 forces that don't act along the same line you must take directions into account
• a scale diagram of 2 forces which can be uses to find their resultant
• 1) choose scale 2) draw arrow length = magnitude direction = direction of force 3) from same starting point draw arrow to represent 2nd force 4) angle between 2 arrows same as angle between 2 forces 5) draw lines to complete parallelogram 6) line from stating point to opposite corner = resultant force, length = magnitude, direction = direction
• direction of the resultant usually expressed as an angle relative to one of the other forces
• resolution of forces
• a single force can be split into 2 perpendicular components that together have the same effect as the single force (called resolving the force)
• 1) scale 2) arrow length = magnitude, direction = direction of force 3) from same point draw lines along two perpendicular directions you want to resolve the vector into 4) from head of arrow draw lines to other arrow heads forming a rectangle with force vector as it's diagonal 5) measure sides of rectangle to calculate magnitude
• an object at equilibrium has no resultant force acting upon it, so either at rest or moving in a straight line at a constant speed