G481: Mechanics

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

OCR Mechanics June 2013

Revision videos

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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

$\mbox{average} \mbox{speed} = \frac{\mbox{distance}}{\mbox{time}}$

$\mbox{acceleration} = \frac{\mbox{change in velocity}}{\mbox{time}}$

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:

$s = \frac{1}{2}(u+v)t$

$s = ut + \frac{1}{2}at^{2}$

$v^{2} = u^{2} + 2as$ ;

(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:

$\mbox{net force} = \mbox{mass} \times \mbox{acceleration}$ ( $F = ma$ )

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, $F = ma$ 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:

$\mbox{weight} = \mbox{mass} \times \mbox{acceleration of free fall}$ ( $W = mg$ );

(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:

$\rho=\frac{m}{V}$ ;

(j) select and use the equation for pressure

$p=\frac{F}{A}$ ,

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

$W = Fx$ and $W = Fx \cos \theta$ ;

(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 $E_{k} = \frac{1}{2}mv^{2}$ ;
(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 $E_{p} = mgh$ ;
(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

$\mbox{efficiency}=\frac{\mbox{useful output energy}}{\mbox{total input energy}} \times 100%$ ;

(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 $E = \frac{1}{2}Fx$ and $E = \frac{1}{2}kx^{2}$ ;
(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.

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