GCSE Physics Revision: Forces and Motion notes

Forces

A force is a push or pull on an object. It can cause a stationary object to start moving or a moving object to change speed or direction. Forces are measured in Newtons (N).

There are several types of forces, including:

  • Gravity - the force that attracts two objects with mass towards each other. On Earth, gravity causes objects to fall towards the ground with an acceleration of 9.8 m/s².
  • Friction - the force that opposes motion between two surfaces that are in contact with each other. Friction can be useful (e.g. to stop a car) or harmful (e.g. to slow down a machine).
  • Normal force - the force that is perpendicular to the surface that an object is in contact with. This force arises due to the repulsion between the atoms in the object and the surface.
  • Tension - the force that is transmitted through a string, rope, or cable when it is pulled tight.
  • Applied force - a force that is applied to an object by another object, such as a person pushing a shopping cart.

Forces can be added together using vector addition. When multiple forces act on an object, they can either reinforce each other (in the same direction) or cancel each other out (in opposite directions).

The motion of an object can be described using Newton's laws of motion:

  1. An object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity, unless acted upon by an external force.
  2. The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass. This is described by the equation F = ma, where F is the net force, m is the mass of the object, and a is the acceleration.
  3. For every action, there is an equal and opposite reaction. This means that if object A exerts a force on object B, then object B will exert an equal and opposite force on object A.

    4. Balanced Forces

    When two forces are equal in size and opposite in direction, they are said to be balanced. An object experiencing balanced forces will not accelerate (will stay still or continue to move at a constant speed).

    Unbalanced Forces

    When two forces are unequal in size or not opposite in direction, they are said to be unbalanced. An object experiencing unbalanced forces will accelerate (i.e., it will change speed or direction).

    Motion

    Motion is the movement of an object from one position to another. It can be described in terms of distance, displacement, speed, velocity, and acceleration.

    Distance and Displacement

    Distance (scalar quantity) is the total amount of ground covered by an object during its motion. Displacement (vector) is the distance between the starting and ending points of an object's motion, regardless of any changes in direction along the way, s= vt.

    Speed and Velocity

    Speed is the distance an object travels in a certain amount of time. It is measured in meters per second (m/s). Velocity is speed in a given direction. It is also measured in meters per second (m/s).

    Acceleration

    Acceleration is the rate of change of velocity over time: a = (v-u)/ t. It is measured in meters per second squared (m/s^2). An object can accelerate if it speeds up, slows down, or changes direction.

    Terminal Velocity

    SUVAT Equations

    The SUVAT equations are a set of equations used to solve problems involving motion with constant acceleration. Each letter in the acronym SUVAT represents a variable that is used in the equations:

      1. S = ut + 1/2at2
      2. V = u + at
      3. S = vt - 1/2at2
      4. V2 = u2 + 2as
      5. S = 1/2(u + v)t

      Where:

      • S is displacement (in meters)
      • U is initial velocity (in meters per second)
      • V is final velocity (in meters per second)
      • A is acceleration (in meters per second squared)
      • T is time (in seconds)

      Terminal velocity is the maximum speed that an object can reach as it falls through a fluid (such as air or water). At terminal velocity, the force of air resistance (or drag) on the object equals the force of gravity pulling it down, resulting in a constant speed.

      Moments

      A moment is a turning effect produced by a force. It depends on the size of the force and the distance from the pivot point (or fulcrum) where the force is applied. Moments are measured in Newton-meters (N·m).

      Calculating Moments

      The moment of a force can be calculated using the equation:

      Moment (Nm) = Force (N) × Distance from pivot (m)

      Momentum

      Momentum is a measure of the motion of an object. It is the product of an object's mass and velocity. Momentum is conserved in a closed system (i.e., no external forces acting on the system).

      Calculating Momentum

      The momentum of an object can be calculated using the equation:

      Momentum (kgm/s) = Mass (kg) × Velocity (m/s)