Physics 1
Acceleration
An object whose velocity is changing (increasing or decreasing) is said to be
accelerating.

A car whose velocity is changing from 0 km/h to 80 km/h is accelerating. If another car is
changing velocity in less time, this car has a greater acceleration.
Constant Acceleration
v
t
Physics 1
Acceleration
An object whose velocity is changing (increasing or decreasing) is said to be
accelerating.

A car whose velocity is changing from 0 km/h to 80 km/h is accelerating. If another car is
changing velocity in less time, this car has a greater acceleration.
1. Constant Acceleration



The velocity of the particle is changing the same amount at the same time
intervals
The graph of the motion in (t,v) – coordinate is a straight line
The angle coefficient of the line is called the acceleration
Ex1. A Graph of a constantly accelerating
v
motion in a co-ordinate (t, s) and (t,v).
10
m/s
Ex 2. Acceleration in (t,v) – coordinate.
Interprete the motion in different timeintervals?
What is the acceleration in different timeintervals?
5
2
4
6
8
10
s
t
Physics 1
2. Average acceleration
Average acceleration = Change in velocity : Time elapsed
a = Δv : Δt
where
Δv = v2-v1 (velocity at the end – v at the beginning)
Δt = t2-t1 (time at the end – t at the beginning)
Unit:
[a]= (1m/s):1s=1m/s2
If the object is slowing down, meaning that the final velocity is less than the initial velocity,
the acceleration is negative. When the object is increasing its velocity, the acceleration is
positive.
Ex1.
A car accelerates along a straight road from rest to 75 km/h in 5,0s. What is the
average acceleration?
Physics 1
3. Free Falling
One of the most common examples of uniformly accelerated motion if that of
an object allowed to fall freely near the Earth’s surface.
Galileo Galilei (1564-1642) found, after experiments in Pisa Tower, that all
objects fall with the same constant acceleration in the absence of air or other
resistance at a given location on the Earth.
This is called the acceleration due to the gravity and we use for it the
symbol g. Its magnitude on the Earth is approximately
g = 9,81 m/s2
The magnitude of g varies slightly according to latitude and elevation (the
height from the sea level). At the Poles its appx 9,83 and at the Equator its
appx. 9,78.
Example 1.
A ball is dropped from a tower. What is the velocity of the ball after 1,0s, 2,0s and 3,0s
of free fall?
Physics 1
Physics 1
Dynamics
Questions, do we have answers?
Why objects move like they do? What makes the object at rest
begin to move? What causes a body to accelerate of to slow down?
Dynamics is is the part of physics that investigate this kind of
problems.
Dymamics is the area of force and motion.
Force
symbol =
[ ] = 1N (newton)
Force can be detected by it’s effects.
Measuring a force

Devices: spring scale, force plate, …
Physics 1
Some observations about forces




Interaction between objects causes pair of
forces. Force generates the counter force,
which are affecting to different particles.
Contact forces (friction, push force, air
resistance, water resistance, …) and
Remote forces (gravitation, magnetic force,
electric force, …)
Force has always a direction marked by .
Physics 1
Force picture


Force in a picture is discribed by an arrow
Force picture describes how force is pulling,
pushing, supporting or burden the object.

forces are named in the picture

Ex1
Physics 1
Force Picture
Ex2 Draw and name forces affecting to the box
pulled on the ground
Physics 1
Examples of affecting forces
1. Hitting a ball
2. Kicking a football
Physics 1
Force

If an external force is affecting to a particle, so the
particle’s motion is changing.

Internal force cannot change the motion of the particle

Example
Pressing the pedal on a bike cannot move the bike if the bike is
not touching the groud.
The touching force due to the ground makes the bke move.
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.
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
Newton’s Third Law of Motion N III
= law of action and reaction forces (in Finnish “voiman ja
vastavoiman laki”)

A horse pulls a wagon and a hammer pushes a nail. But these things
are not one-sided. A hammer pushes a nail and its velocity is rapidly
reduced to zero in contact. Why? Where is the force that stops the
motion of hammer? The hammer exerts the nail, and the nail exerts a
force back to a hammer.
Whenever one body exerts a force on a second body, the
second body always exerts an equal and opposite force on
the first.

Examples:
This law means, that to every action force there is an equal but
opposite reaction force. But remember that the action force and the
reaction force are acting on different objects and on different
directions.
A book on a table, Pushing the Wall, Pulling a sledge,
Walking, Car braking/ accelerating