IB Physics Sub-topic A2 Notes

Forces

In Topic A.1, the force of gravity and fluid resistance were discussed. In Topic A.2, you need to know more detail about forces and how they impact the motion of objects, including their momentum. Forces are actions that cause the deformation or change in velocity (acceleration) of an object, measured in Newtons (N). Forces act on one object at a time with magnitude and direction and may be exerted by other objects.

The main types of forces encountered are:

1. Gravitational (F_g) – the attractive force exerted by an object’s mass on another object, referred to as the affected object’s weight (W).
2. Electric (F_e) – the force between two electrically charged objects.
3. Magnetic (F_m) – the force between two magnetic objects.
4. Normal (F_N) – the force a surface perpendicularly exerts on an object due to its weight.
5. Friction (F_f) – the force a surface parallelly exerts to counteract another surface’s motion.
6. Elastic restoring force (F_H) - the force a spring exerts in both directions on another object when it is squished or stretched.
7. Tension (T) – the force a rope or cable exerts in both directions on another object when it is stretched.
8. Viscous drag force (F_d) - the force opposing the motion of a small sphere through a fluid.
9. Buoyancy (F_b) - the force on a body due to its displacement of fluid.

Free body diagrams

Forces are drawn using free body diagrams. These show an object and all the forces acting upon it. An example is shown below:

As stated before, weight (W) is the gravitational force on an object, measured in Newtons. This must be distinguished from mass (m), which is the amount of matter contained in an object, measured in kg. The formula for weight, where g = 9.8 ms^-2 is:

$W = mg$

Friction

As stated before, friction is a force exerted by a surface to counteract another surface’s motion. If a surface is said to be smooth, there is no friction between that surface and any surface in relative motion. In friction between solids, there are two types:

1. Static friction – the force exerted to stop another resting surface from moving. This acts up to a maximum F_f, keeping Σ F = 0. The formula for F_f is:

   $F_{f} = μ_{s}F_{N}$

   In this, μ_s is the coefficient of static friction, which relates the reaction force and the maximum static frictional force. If the applied force > F_s, the object begins to move and F_s decreases, becoming F_d.

2. Dynamic friction – the force exerted on a moving surface, with a similar formula:

   $F_{f} = μ_{d}F_{N}$

   Remember that coefficient of dynamic friction μ_d < μ_s. It relates the reaction force and the maximum dynamic frictional force, as described to the formula above.