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Physics 1
Dynamics
Newton’s second law of motion N II
If you push a vehicle with a force, the velocity of the vehicle is increasing: the vehicle
accelerates. If you double the net force, the acceleration will be double. If you double the
mass of the vehicle, the acceleration will be half of the first one if You use the same force.
The basic law of dynamics (Newton II). (Dynamiikan peruslaki)
The acceleration of a body is directly proportional to the
net force acting on it and inversely proportional to its
mass:
a=F/m
where
m=the mass of the body
F = the net force= sum of the forces acting the body
The force F can be counted from equation
F=ma
Units of force: [F]=[m]/[a] =
Physics 1
Examples
1. Net force
Count the net force affecting to the object. In what kind of state of motion is the object.
F1
F3
F1= 8N, F2=16N, F3=10N
F2
Physics 1
Newton’s second law of motion (cont)
Newton’s second law is giving a more precise definition to force as
an action capable of accelerating an object. Force is needed to
chance the direction of motion and to change the velocity of the
body. Force is also needed to chance the shape of a body. For
instance friction is slowing down the velocity. Friction is a braking
force to the motion.
Example1 Estimate the force capable of giving to a 1000kg car an
acceleration of ½g.
Example2 A car (m=1000kg) is accelerating from rest to the speed
of 100km/h in 10,5 s. Count the average force that’s making the
acceleration.
Physics 1
Weight
Weight is the force of the gravity acting on a body. The basic law of dynamics states that weight G of a
body is counted as
G=mg
(compare with F=ma)
where
m = the mass of the body
g= 9,81 m/s2
Ex2. If your mass is 57 kg so what is your weight a) on Earth b) on Moon?
Mass m
Mass is the quantity of matter. More precisely mass is a measure of the
inertia of a body. The more mass a body has, the harder is to change its
state of motion, for instance to accelerate or to slow down or to turn. A truck
has more inertia than a bicycle. It therefore has much more mass.
Notice
Be careful when speaking of mass and weight. Mass is the quantity of matter. Weight is force of gravity acting on a body
.
Physics 1
Newton’s First Law of Motion N I
=the law of inertia (in Finnish “hitauden laki”
“jatkavuuden laki”).
If the net force on an object is zero, an object originally
at rest remains at rest, and an object in motion remains
in motion in a straight line with constant velocity.
Net force= sum of all forces acting on the object
Examples
Pulling a papersheet, A puck on ice, a steel
ball on the floor, a passenger in a car, a car
turning in a curve
Physics 1
Forces resisting motion

Friction and resistance on medium

Static friction
Sliding friction
Rolling resistance

Resistance of medium: liquids, air


prevents moving
resists sliding
Physics 1
Forces resisting motion
Vortex
Rolling resistance
Lift
Air
resistance
Thrust
Force due to
the gravity
Physics 1
Forces resisting motion
Bearing
Flat tyre