4 min read•january 29, 2023

Daniella Garcia-Loos

Contact forces result from the interaction of one object touching another object, and they arise from interatomic electric forces. These forces include tension, friction, normal, spring (Physics 1), and buoyant (Physics 2).

Let's review some basic contact forces from Physics 1 and remind ourselves of what they are!

- Tension - Occurs when an object is pulled by a rope, string, or chain.

- Friction - Occurs when two surfaces are trying to slide (static) or sliding (kinetic)

- Normal - Occurs when an object is in contact with a surface

- Spring - Occurs when a spring or an elastic material is compressed or extended

Let's look at how to draw these on FBD!

- Tension - The direction the rope, string, or chain is attached

- Friction - opposes the direction of relative motion

- Normal - Perpendicular to the surface

- Spring - Opposes the direction of the extension or compression

Some other basic concepts not specifically in the AP Physics 2 curriculum but the writers may ask you to recall from Physics 1!

1) Hooke’s Law - the amount of stretching or elongation of a string when a mass is attached to it is directly proportional to the applied weight.

Where k is the spring constant in units of Newtons per Meter (N/m) and x is the stretching or elongation of the spring beyond its original length.

2) Friction: A key factor in understanding the setup of the equations surrounding Newton’s Laws is familiarizing oneself with friction. Friction acts as the force that opposes the motion or attempted motion of an object.

The equation for friction is given by

where μ (Greek letter mu) is the coefficient of either static or kinetic friction and N is the normal force.

Friction is present if a problem mentions a “rough” surface, or specifically states the coefficient of static or kinetic friction.

Contact forces can be divided into two categories: **non-conservative** and **conservative forces**. Non-conservative forces, such as friction and air resistance, convert some of the energy of motion into other forms of energy, such as heat. Conservative forces, such as tension and normal force, do not convert energy but instead store it in the form of potential energy, which can be recovered if the force is removed.

A box with a mass of 20 kg is sitting on a horizontal surface. A horizontal force of 60 N is applied to the box to move it. Calculate the force of friction acting on the box.

Since the force applied to the box is not enough to overcome the force of friction, the box will not move. Therefore, the force of friction acting on the box is equal to the force applied to it, which is 60 N.

A rope is hanging vertically from a pulley with a mass of 10 kg attached to the end of it. The rope has a tension of 200 N. Calculate the weight of the pulley

We know that tension = 200 N

g = 9.8 m/s^2

The weight of the pulley = 200 N / 9.8 m/s^2 = 20.4 kg

A wooden block with a density of 600 kg/m^3 and a volume of 0.02 m^3 is placed in a swimming pool filled with water (density = 1000 kg/m^3). Calculate the buoyant force acting on the block.

We know that

density of water = 1000 kg/m^3

volume of block = 0.02 m^3

g = 9.8 m/s^2

The weight of the fluid that is displaced = 1000 kg/m^3 x 0.02 m^3 x 9.8 m/s^2 = 19.6 N

The buoyant force acting on the block is equal to the weight of the fluid that is displaced, which is 19.6 N.

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