Basics of Physics Course II: Kinetics

Basics of Physics Course II: Kinetics | 1

This chapter gives you an overview of all the basic physics that are important in biomechanics. The aim is to give you an introduction to the physics of biomechanics and maybe to awaken your fascination for physics. The topics mass, momentum, force and torque are covered. In order to illustrate the physical laws in a practical way, there is one example from physics and one from biomechanics for each topic area. Have fun!

1. Mass 2. Momentum 3. Conservation of Momentum 4. Force 5. Torque

1.Mass

In physics, mass M is a property of a body. Its unit is kilogram [kg] .

Example Physics Example Biomechanics

2. Momentum

In physics, the momentum p describes the state of motion of a body. Its unit is kilogram times meter per second [\frac{kg * m}{s}] The momentum combines the mass with the velocity. The momentum is described by the following formula: p = M * v

Here p is the momentum, M the mass and v the velocity of the body.

Sometimes the formula sign for the impulse is also written like this: \vec{p} . This is because the momentum is a vectorial quantity and therefore has a size and a direction. For the sake of simplicity we write only p .

Example Physics

3. conservation of momentum

Let us assume that a billiard ball rolls over a billiard table at a constant speed and then hits a second ball that is lighter than the first ball. After the collision, the first ball will stop and the second ball will continue rolling, but the speed of the second ball is greater than that of the first. This happens because of the principle of conservation of momentum, which says that the momentum before the collision must be equal to the momentum after the collision. Let us assume that the velocity of the first ball is v_1 = 2 \frac{m}{s} and the ball has the mass M_1 = 0.2kg and the mass M_2 of the second ball is only half as large at 0.1kg. Then it follows from the conservation of momentum that the second (lighter) ball has a velocity v_2 twice as high as the first ball. The calculation path would be as follows: p_1 = p_2

M_1 * v_1 = M_2 * v_2 v_2 = M_1 ∗ \frac{v_1}{M_2} = \frac{P_1}{M_2} v_2 = \frac{0.4}{0.1} [\frac{kg∗\frac{m}{s}}{kg}] = 4 \frac{m}{s}

So, the velocity of the second ball is v_2 = 4 \frac{m}{s} .

Example Biomechanics

4. Force

In physics, the force F describes an action that causes a body to be accelerated or deformed. Its unit is Newton N . The force combines mass with acceleration. The formula for the force is

F = M * a

Here F is the force acting on a body, M is the mass of the body and a is the acceleration the body experiences due to the force F .

Also forces are vectorial quantities with a direction, which is why they are sometimes written like this: \vec{F} .

Example Physics Example Biomechanics

4. Torque

In physics, torque D describes the rotational eﬀect of a force on a body. Its unit is Newtonmeter [Nm] .

The formula for the torque is

D = F ∗ s = M ∗ a ∗ s

Here D is the torque, F the force, s the distance between the force and the center of rotation, M the mass and a the acceleration.

Example Physics Example Biomechanics