3.2  Forces in Action

3.2.1  Dynamics

Learners should be able to demonstrate and apply their knowledge and understanding of:

a) net force = mass × acceleration; F = ma

Remember that the F in this equation is the resultant force

(b) the newton as the unit of force

You should be able to deduce what the Newton is from the formula, F=ma.

(c) weight of an object; W = mg

The basic definitions have not changed and you still need to know them and what units are used for each.

(d) the terms tension, normal contact force, upthrust and friction

You may struggle with the normal contact force for a little while.  It comes back in the Newton's Laws section when it makes more sense.  It is also called the reaction force, which doesn't help.  Previous students have struggled with the concept of tension, although I have never understood why.  It just means a force in a wire or beam or whatever.  There is nothing complicated about it.  You don't need anything much about friction apart from the fact that it opposes motion.

(e) free-body diagrams

This just means drawing all the forces acting on an object using arrows, the length of which should reflect the size of the force acting.

(f) one- and two-dimensional motion under constant force.

This is basically the projectiles stuff again but stressing that it doesn't have to be a projectile.  This is also going to come back in the equilibrium section.

3.2.2  Motion with non-uniform acceleration

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) drag as the frictional force experienced by an object travelling through a fluid

Drag is a handy name for all sorts of forces that oppose motion i.e. friction, air resistance, water resistance etc.

(b) factors affecting drag for an object travelling through air

Basically these are speed and surface area!

(c) motion of objects falling in a uniform gravitational field in the presence of drag

You need to be able to describe how the acceleration changes as the drag increases thus the resultant forces decreases.  It is the same as at GCSE.

(d) (i) terminal velocity

Again there is nothing new here that wasn't around at GCSE. It is important to remember that it is not all about parachutes.

(ii) techniques and procedures used to determine terminal velocity in fluids.

We will do this is class as a PAG.

3.2.3  Equilibrium

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) moment of force

The basics are still the same.  The extra bits are that things may be at angles to each other.  You can either resolve the forces into perpendicular and horizontal components or work would what the perpendicular distances between force and pivot are.

(b) couple; torque of a couple

Weird words but a pretty straightforward concept really.  Just a special case of moments.

​(c) the principle of moments

(d) centre of mass; centre of gravity; experimental determination of centre of gravity

Again, this is the same as at GCSE.  The experiment is the same as well. The terms centre of mass and centre of gravity can be interchanged at this level.

(e) equilibrium of an object under the action of forces and torques

(f) condition for equilibrium of three coplanar forces; triangle of forces.

Basically if the triangle of forces joins up, there is no resultant force, thus the system is in equilibrium.

3.2.4  Density and pressure

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) density; p = m/V

(b) pressure; p = F/A for solids, liquids and gases

(c) p = hp g; upthrust on an object in a fluid; Archimedes’ principle.