P8 Forces in balance

P8.1 Vectors and scalars

Distance without change of direction is displacement.

Physical quantities having direction are vectors - acceleration, force, weight - and physical quantities having size but no specific direction are scalars - speed, distance, time, mass, energy, power. The size of a quantity is magnitude. Vectors have magnitude and direction, but scalars only have magnitude. Vector quantities are represented by an arrow whose direction shows the direction of the vector quantity and length shows its magnitude.

When more than one force acts on an object, the forces on the object can be shown on a scale diagram.

P8.2 Forces between objects

Forces can change an object's shape, state of rest, or motion. Forces are a push or pull.Objects touching each other to interact are contact forces.

Newton's third law of motion states that when 2 objects interact with each other, they exert equal and opposite forces on each other. 

To pull a car in the mud the mud's force on the tractor must be greater than the mud's force on the car. The equal and opposite force to the mud's force on the tractor is the tractor's force on the mud, and the equal and opposite force to the mud's force on the car's force on the mud.

Driving force is the force making cars move. It pushes the car forward due to friction between the ground and tyre of each drive wheel, friction acting where the tyre is in contact with the ground. When the car moves forward, the road's friction on the tyre is in the forward direction and the tyre's friction on the road is in the reverse direction.

P8.3 Resultant forces

Resultant force is the single force having the same effect as all forces acting on the object.

Newton's first law of motion says if the forces acting on an object are balanced, the resultant force on the object is 0 and if the object is at rest, it stays stationary and if the object is moving, it keeps moving with the same speed and in the same direction. If only two forces are acting on an object with 0 resultant force, they must be equal to each other and act in opposite directions.

When the resultant force on an object isn't 0, the forces acting on it aren't balanced and its movement depends on the size and direction of the resultant force. The resultant force is equal to the difference between the two forces and in the direction of the larger force.

A free-body force diagra shows the forces acting on an object without any other objects or forces shown. Each force on the diagram is shown by an arrow…