### Mr Cowen’s definitive definitions for Mechanics

### Past paper written walkthroughs

Forces & Motion January 2008

Forces & Motion June 2008

Forces & Motion January 2009

Forces & Motion June 2009

Mechanics June 2009

Mechanics January 2010

### Past paper video walkthroughs

### Revision videos

Click links to view videos related to that spec point. New videos are added regularly: please use the contact form if you’d like to request a video for a particular topic.

#### Module 1: Motion

**1.1.1 Physical quantities and units**

(a) explain that some physical quantities consist of a numerical magnitude and a unit;

(b) use correctly the named units listed in this specification as appropriate;

(c) use correctly the following prefixes and their symbols to indicate decimal sub-multiples or multiples of units: pico (p), nano (n), micro (µ), milli (m), centi (c), kilo (k), mega (M), giga (G), tera (T);

(d) Make suitable estimates of physical quantities included within this specification.

**1.1.2 Scalars and vectors**

(a) define *scalar* and *vector* quantities and give examples;

(b) draw and use a vector triangle to determine the resultant of two coplanar vectors such as displacement, velocity and force;

(c) calculate the resultant of two perpendicular vectors such as displacement, velocity and force;

(d) resolve a vector such as displacement, velocity and force into two perpendicular components.

(Forces on an object on a slope)

**1.1.3 Kinematics**

(a) define *displacement*, *instantaneous speed*, *average speed*, *velocity* and *acceleration*;

(b) select and use the relationships

to solve problems;

(c) apply graphical methods to represent displacement, speed, velocity and acceleration;

(d) determine velocity from the gradient of a displacement against time graph;

(e) determine displacement from the area under a velocity against time graph;

(f) determine acceleration from the gradient of a velocity against time graph.

**1.1.4 Linear motion**

(a) derive the equations of motion for constant acceleration in a straight line from a velocity against time graph;

(b) Select and use the equations of motion for constant acceleration in a straight line:

(c) apply the equations for constant acceleration in a straight line, including the motion of bodies falling in the Earth’s uniform

gravitational field without air resistance;

(d) explain how experiments carried out by Galileo overturned Aristotle’s ideas of motion;

(e) describe an experiment to determine the acceleration of free fall g using a falling body;

(f) apply the equations of constant acceleration to describe and explain the motion of an object due to a uniform velocity in one

direction and a constant acceleration in a perpendicular direction.

#### Module 2: Forces in action

**1.2.1 Force**

(a) Solve problems using the relationship:

appreciating that acceleration and the net force are always in the same direction;

(b) define the *newton*;

(c) apply the equations for constant acceleration and to analyse the motion of objects;

(d) recall that according to the special theory of relativity, cannot be used for a particle travelling at very high speeds because its mass increases.

**1.2.2 Nonlinear motion**

(a) explain that an object travelling in a fluid experiences a resistive or a frictional force known as drag;

(b) state the factors that affect the magnitude of the drag force;

(c) determine the acceleration of an object in the presence of drag;

(d) state that the weight of an object is the gravitational force acting on the object;

(e) select and use the relationship:

();

(f) describe the motion of bodies falling in a uniform gravitational field with drag;

(g) use and explain the term *terminal velocity*.

**1.2.3 Equilibrium**

(a) draw and use a triangle of forces to represent the equilibrium of three forces acting at a point in an object;

(b) state that the *centre of gravity* of an object is a point where the entire weight of an object appears to act;

(c) describe a simple experiment to determine the centre of gravity of an object;

(d) explain that a couple is a pair of forces that tends to produce rotation only;

(e) define and apply the *torque of a couple*;

(f) define and apply the *moment of force*;

(g) explain that both the net force and net moment on an extended object in equilibrium is zero;

(h) apply the principle of moments to solve problems, including the human forearm;

(i) select and use the equation for density:

;

(j) select and use the equation for pressure

,

where *F* is the force normal to the area *A*.

**1.2.4 Car safety**

(a) define *thinking distance*, *braking distance* and *stopping distance*;

(b) analyse and solve problems using the terms thinking distance, braking distance and stopping distance;

(c) describe the factors that affect thinking distance and braking distance;

(d) describe and explain how air bags, seat belts and crumple zones in cars reduce impact forces in accidents;

(e) describe how air bags work, including the triggering mechanism;

(f) describe how the trilateration technique is used in GPS (global positioning system) for cars.

#### Module 3: Work and energy

**1.3.1 Work and conservation of energy**

(a) define *work done* by a force;

(b) define the *joule*;

(c) calculate the work done by a force using

and ;

(d) state the principle of conservation of energy;

(e) describe examples of energy in different forms, its conversion and conservation, and apply the principle of energy conservation to

simple examples;

(f) apply the idea that work done is equal to the transfer of energy to solve problems.

**1.3.2 Kinetic and potential energies**

(a) select and apply the equation for kinetic energy ;

(b) apply the definition of work done to derive the equation for the change in gravitational potential energy;

(c) select and apply the equation for the change in gravitational potential energy near the Earth’s surface ;

(d) analyse problems where there is an exchange between gravitational potential energy and kinetic energy;

(e) apply the principle of conservation of energy to determine the speed of an object falling in the Earth’s gravitational field.

**1.3.3 Power**

(a) define *power* as the rate of work done;

(b) define the *watt*;

(c) calculate power when solving problems;

(d) state that the efficiency of a device is always less than 100% because of heat losses;

(e) select and apply the relationship for efficiency

;

(f) interpret and construct Sankey diagrams.

**1.3.4 Behaviour of springs and materials**

(a) describe how deformation is caused by a force in one dimension and can be tensile or compressive;

(b) describe the behaviour of springs and wires in terms of force, extension, elastic limit, Hooke’s law and the force constant (ie force per unit extension or compression);

(c) select and apply the equation , where *k* is the force constant of the spring or the wire;

(d) determine the area under a force against extension (or compression) graph to find the work done by the force;

(e) select and use the equations for elastic potential energy and ;

(f) define and use the terms *stress*, *strain*, *Young modulus* and *ultimate tensile strength* (*breaking stress*);

(g) describe an experiment to determine the Young modulus of a metal in the form of a wire;

(h) define the terms *elastic deformation* and *plastic deformation* of a material;

(i) describe the shapes of the stress against strain graphs for typical ductile, brittle and polymeric materials.