Forces and Movement

Understanding Forces

A force is a push or pull acting upon an object as a result of its interaction with another object.
Forces are vectors; this means they have magnitude and direction.
The SI unit for force is the newton (N), and its direction is given by a vector arrow.
The strength of a force can be represented by the size of the arrow in a force diagram.
Balanced forces cause no change in the velocity of an object.
When forces are unbalanced, they cause acceleration, which can mean an increase or decrease in speed, or a change in direction.

Newton’s First Law, also known as the law of inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton’s Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This can be written as F=ma.
Newton’s Third Law states that for every action, there’s an equal and opposite reaction.

Mass is a measure of the amount of matter in an object, measured in kilograms (kg).
Weight is a force acting on that mass due to gravity, measured in newtons (N).
Weight can be calculated by multiplying the mass of an object by the strength of gravitational field (g), which on Earth is about 9.8 m/s².

Speed is how fast an object is moving without regard to the direction.
Velocity is speed in a given direction.
Both speed and velocity are measured in metres per second (m/s).
Acceleration is the rate at which velocity changes with time; it is measured in metres per second squared (m/s²).

Momentum (p) is the product of an object’s mass (m) and its velocity (v), so p=mv, with its unit being kilogram metre per second (kg m/s).
Momentum is a vector quantity and is conserved in a closed system.
Impulse is the change in momentum of an object when a force is applied for an interval of time.

Work (W) is done when a force (F) moves an object over a distance (d), according to the equation W=Fd, measured in joules (J).
Energy is the capacity to do work, also measured in joules.
Power (P) is the amount of work done or energy transferred in a given time, computed with P=W/t, measured in watt (W). This is the rate of doing work or the rate of using energy.

Gravitational potential energy (GPE) is the energy stored in an object due to its height above the earth’s surface.
Kinetic energy (KE) is the energy an object possesses due to its motion.
The energy transfer between GPE and KE can be observed in pendulums and roller coasters.