Forces - Notes
pushes and pulls
Forces can
- change the speed of an object (Beschleunigung)
- change the direction of movement of an object (Richtung)
- change the size and shape of an object (Verformung)
Weight - Gewichtskraft
Weight is the force acting on an object because of the gravitational pull from the centre of the Earth.
It can vary depending where you are on Earth. In Germany it is around $9.81 \frac{m}{s^2}$
or $9.81 \frac{N}{kg}$
(the units are equivalent).
Often, for sake of simplicity, $10 \frac{N}{kg}$
is used.
$$F = m \cdot g$$
(the formula will be explained later)
Planets with different masses will have different gravitational constants.
Hooke’s Law
Key question - How can we measure forces?
After, experimenting with different springs, Hooke discovered that up to a certain point (…), the springs expand proportional to a weight pulling the spring down.
extension of a spring is directly proportional to the force stretching the spring.
$$F = D \cdot l$$
directly proportional
the elastic constant of a spring is
$$D = \frac{F}{l}$$
P is called the elastic limit where the spring is permanently deformed.
You may see the formula as $D = \frac{F}{s}$
in a “Formelsammlung”.
newtons - the unit of force
the unit of force is called newtons (N), after Sir Isaac Newton
spring balance
force-meter - Kraftmesser
experiment 1
- force-meters
vectors to represent forces
vectors are a mathematical tool that is an arrow with
- a direction
- a length (magnitude - from Latin magnus = great)
where is the forces acting upon the object - Angriffspunkt
direction - Richtung
size (magnitude) - Größe (magnus: Latin for “Great”)
addition of forces - Addition von Kräften
-
in a straight line (forwards / backwards) man pulling horse cart example
-
parallelogram
two small boats pulling a large vessel
subtraction of forces - Subtraktion von Kräften
Newton’s First Law of Motion
If the forces on a mass are balanced (no resultant force), then
- if at rest, it stays at rest
- if it is moving, it keeps on moving at a constant speed in a straight line
balance of forces - Kräftegleichgewicht
While the friction of the block on the surface is so great (because of it’s weight) the forces are in balance. That means the block remains motionless. As the same force the man is pulling with is equal to the force of the force acting on the block. This is because of friction. (On a very smooth surface or with wheels the situation is different.)
friction - Reibung
what causes friction? -> surface structure
- static friction - die Haftreibung
- dynamic friction - die Gleitreibung
- Rollreibung
inclined plane - geneigte (schiefe) Ebene
geogebra app
The force due to the weight of the ball (die Gewichtskraft) is split into two components, one downhill (die Hangabtriebskraft) and one perpendicular to the surface (die Normalkraft).
Drag the point H to change the weight. Drag C to change the angle (inclination) of the plane.
TASK
- observe how the forces
$F_H$
and$F_N$
change. - what happens to
$F_H$
when the angle is 0°? 90°?
-
Friction -Reibungskraft
-
Weight - Gewichtskraft
-
force normal (perpendicular to a surface) - Normalkraft
-
force (down)hill - Hangabtriebskraft
-
Haftreibungskraft
-
Haftreibungszahl
-
Gleitreibungskraft
-
Gleitreibungszahl
erwünschte und unerwünschte Reibung
air resistance - Luftwiderstand
force and acceleration - Kraft und Beschleunigung
Newton