Forces

Force is a push or pull caused by an interaction between objects and is measured in newtons (N). Force is a vector quantity because it has both magnitude and direction. Contact forces include friction, air resistance, tension and normal contact force, while non-contact forces include gravitational, electrostatic and magnetic forces.

Scalar quantities have magnitude only, whereas vector quantities have both magnitude and direction. Distance and speed are scalars, while displacement, velocity, acceleration and force are vectors. Vector notation and arrows can be used to represent forces, with the arrow length showing magnitude and the arrow direction showing the direction of the force.

Weight is the gravitational force acting on an object and is different from mass. Weight is calculated using $W=mg$, where $W$ is weight in newtons, $m$ is mass in kilograms and $g$ is gravitational field strength in N/kg. Mass remains the same when location changes, but weight can change because gravitational field strength may be different.

The resultant force is the single force that has the same overall effect as all the individual forces acting on an object. Forces acting in the same direction are added, while forces acting in opposite directions are subtracted. If the resultant force is zero, the forces are balanced and there is no change in the object's velocity.

Work is done when a force causes an object to move through a distance in the direction of the force. Work done is calculated using $W=Fs$, where $W$ is work done in joules, $F$ is force in newtons and $s$ is distance moved in metres. One joule is equal to one newton-metre, and doing work transfers energy from one store to another.

Elastic objects such as springs can return to their original shape after the deforming force is removed, provided their elastic limit is not exceeded. Hooke's law states that the extension of a spring is directly proportional to the applied force within the limit of proportionality and is expressed as $F=ke$, where $k$ is the spring constant and $e$ is the extension.

When a spring is stretched or compressed, work is done and energy is stored in its elastic potential energy store. The elastic potential energy can be calculated using $E_e=\frac{1}{2}ke^2$, where $k$ is the spring constant in N/m and $e$ is the extension in metres. Beyond the elastic limit, the spring may become permanently deformed.

The turning effect of a force is called the moment of a force, sometimes described as torque. It is calculated using $M=Fd$, where $M$ is the moment in newton-metres, $F$ is the force in newtons and $d$ is the perpendicular distance from the pivot to the line of action of the force. For a balanced object, total clockwise moments equal total anticlockwise moments.

Pressure describes how much force acts on a given area and is calculated using $p=\frac{F}{A}$, where pressure is measured in pascals, force in newtons and area in square metres. The same force produces a greater pressure when it acts over a smaller area. In fluids, pressure acts normal, or at right angles, to a surface.

Pressure in a liquid increases with depth because a deeper point has a greater column of liquid above it. Liquid pressure can be calculated using $p=h\rho g$, where $h$ is depth, $\rho$ is the density of the liquid and $g$ is gravitational field strength. Pressure differences on submerged objects can create an upward resultant force called upthrust.

Atmospheric pressure is caused by air molecules colliding with surfaces. Atmospheric pressure decreases with increasing altitude because there is less air above the surface and therefore a smaller weight of air pressing down. The Earth's atmosphere becomes less dense as height above the Earth's surface increases.

Distance is the total length of the route travelled and is a scalar quantity, while displacement is the straight-line distance from the starting point to the finishing point together with its direction. Speed is calculated from $s=vt$, while velocity describes speed in a particular direction and is therefore a vector quantity.

Acceleration is the rate of change of velocity and is calculated using $a=\frac{\Delta v}{t}=\frac{v-u}{t}$. Acceleration is measured in m/s². For uniform acceleration, the equation $v^2-u^2=2as$ can also be used to connect initial velocity, final velocity, acceleration and displacement.

Newton's laws of motion explain how forces affect movement. Newton's First Law states that an object remains stationary or continues moving at constant velocity unless acted on by a resultant force. Newton's Second Law is represented by $F=ma$, showing that acceleration increases with resultant force and decreases when mass increases. Newton's Third Law states that interacting objects exert equal and opposite forces on each other.

Momentum is a property of moving objects and is calculated using $p=mv$, where momentum is measured in kg m/s. In a closed system, total momentum before an interaction equals total momentum afterwards. Force can also be described as the rate of change of momentum, $F=\frac{\Delta p}{\Delta t}$. Safety features such as airbags and seat belts increase the time over which momentum changes, reducing the force acting on a person.