WJEC AS Physics Unit 1.3 - Dynamics

Force

A force on a body is a push or a pull acting on the body from some external body

Newton’s Third Law

If Body A exerts a force on Body B, then Body B exerts an equal and opposite force on Body A

F=ma

• The mass of a body x its acceleration is equal to the vector sum of the forces acting on the body

• This vector sum is called the resultant force

Momentum

• Momentum of an object is its mass multiplied by its velocity

• p=mv

• Vector

• Unit; Ns or kgms^-1

The principle of conservation of momentum

• The vector sum of the momenta of bodies in a system stays constant even if forces act between the bodies, provided there is no external resultant force

Elastic collision

A collision in which there is no change in total kinetic energy

Inelastic collision

A collision in which kinetic energy is lost

Momentum-time graphs

• gradient = Force

• Flat = constant momentum, no net force acting. Travelling at a constant speed. In equilibrium

• Negative gradient = negative force

• Curve; draw a tangent and calculate gradient for force

• Maximum force = gradient of the steepest part of the graph

F=ma and momentum

F=k Δp/Δt

F=Δp/Δt

=(p1-p2)/t

=(mv-mu)/t

=m(v-u)/t

=ma

F=ma

Mass must be constant during the period of acceleration

Newton’s 3rd Law Pairs

• All forces act in N3L pairs but their effects will be different

• The pair of forces must;

  • Act on different bodies

  • Be of equal magnitude

  • Be in opposite directions

  • Be of the same type

Free body diagrams

• can be used to represent forces on a particle or body

• Useful in identifying forces on interacting bodies

• From a fbd, we can;

  • Calculate the resultant force if all forces known

  • Calculate any unknown forces on an object in equilibrium if all other forces are known

  • Identify the N3L partner forces on any interacting objects

Newton’s First Law

• a body at motion remains at motion unless acted upon by an external resultant force

• When the resultant force is 0N, a body will remain stationary or continue moving with a constant velocity

• Unbalanced forces will cause a change in the motion of a body

Examples of forces

• Weight (W=mg); gravitational force acting through its centre of mass

• Friction; when 2 surfaces rub together. Force acts up the slope, counteracting the component of weight

• Drag; a force that opposes motion between the object and a fluid

• Tension; a force due to stretching or pulling

• Upthrust; upward buoyancy force in an object in a fluid

• Normal contact force, F_N; when an object rests on another object, the surface exerts a force on the object. This is experienced at right angles to the surface