Transcript 1.1.1 newtons first law

Any body will remain in it’s
state of rest or uniform motion
in a straight line unless caused
by some external NET FORCE
to act otherwise.
It essentially means that a body will do one of two
things:
· accelerate if you apply a force to it
· not accelerate if you don’t
Explain the motion of the following, refer to all
the forces acting on the object:
•a car going “flat out” at 120 kmh-1
No acceleration as balanced forces
of drag and weight reach equilibrium
•a parachutist hitting the ground at 200 kmh-1 without
a parachute if he jumps from 2000m or 5000m
Has reached this equilibrium point - terminal
velocity - before he has fallen 2000m
•the rate of acceleration of car decreasing as it gets
faster.
Engine produces a constant force - accelerates
and so drag increases. This decreases the net
accelerating force
Using the air track and light gates make a
decision about how friction free the motion
of the glider actually is and whether it can
be used to demonstrate motion according to
Newton’s first law.
• Does it remain stationary when no forces
are acting?
• Does the velocity remain constant as it
passes from one light gate to the next?
• What happens to the velocity if the glider
is allowed to bounce off the ends?
Using your knowledge of balanced forces draw
on the size of forces and direction to
simplified diagrams of these people travelling
at constant velocity. Remember the size of the
arrow indicates the size of the force.
Objects in Equilibrium
Decide for yourself what would happen to
these objects!
In all cases there is no
resultant force so the
balls stay in place!
• In order to decide whether forces or velocity
vectors do cause a resultant in any given
direction we need to “add” them, taking into
account their direction.
• We have done this since Year 7 with Forces
acting along the same line of action, even in 2 or
3D.
• However the previous slide showed 3 forces at
totally different angles. How, apart from the
general feeling, can we prove they cancel each
other out?
• How can we calculate the resultant?
• In all cases the Resultant is the “vector sum”
of the components…………WHAT!!!!!
For example, if you were swimming in a moving
river, what direction would you end up moving in
and how fast?
Again you can get a feel for this from your
everyday experience – USE THIS “FEEL” IN
THE EXAM TO GAUGE IF YOU ARE RIGHT!
There are two methods to solve this
1) Using Trigonometry and Pythagoras
2) Drawing a scale diagram and
measuring the size and direction of the
resultant.
1) Trigonometry and Pythagoras
If a person can swim at 1.5 ms-1 in still water but
the current of the stream flows at 2 ms-1 at 90o
to the swimmer. What is their speed and
direction?
A person swims
The vectors should
at 1.5 ms-1 in
always be drawn
still water
“nose to tail” as
shown on the left.
The RESULTANT is
the vector that joins
the start of one vector
with the end of the
last one after joining
them as described!
The
current
flows at
2 ms-1
1.5ms-1

Tan = 2 / 1.5
 = 53.10
Using Pythagoras’ theorem
1.52 + 22 = 6.25
so R = 6.25 = 2.5ms-1
2 ms-1
So the
RESULTANT is the
person travelling at
2.5ms-1 in a
direction 53o from
the direction that
the person was
swimming in.
How do you add vectors if they are not
in the same line of action or at 90o ?
This is best achieved at A level by drawing
2) A scale diagram and measure the angle between
the vectors accurately. Make sure you specify the direction
of the angle eg from the horizontal, vertical or one of
the vectors
This method is known as the parallelogram law as the two
forces make up 2 sides of the parallelogram allowing us to
measure the size of the resultant!
RESULTANT
R=A+B
A
B
Example
A body of mass 0.6kg falls vertically. A wind
blows horizontally with a force of 8N. What is
the magnitude and direction of the resultant
force on the mass? (g=10 Nkg-1)
6N
Tan() = 6/8
R2 = 62 + 82
  = 37o
R2 = 36 + 64
R2 = 100
8N
R = 10N
Which angle on the diagram is measured though?!
These forces can be investigated using a force
board where the forces are in equilibrium and the
angles indicated by the string and magnitude by
the weights suspended at the 3 points.
Resolving Vectors
If we want to know what the effect of a force is in a
certain direction or if we are to add vectors, we
need to know what they are doing in specific
directions. The easiest to use are vertical and
horizontal directions ie.
Tension in
the rope
Vertical
component
Horizontal
component
Resolve them
vertically and
horizontally
Wind direction
Force produced by keel
Direction of boat
As you can see from the example above we can make
a triangle of forces from just about any situation.
If a force is not acting vertically or horizontally we
can consider it being made up of a vertical and
horizontal force, just as you can walk somewhere by
going forwards, backwards, left and right without
moving diagonally!
Sideways
pull of rope
on barge
Forward
pull of
horse
Actual Pull
of horse
Show your working and
annotate the actions
you are taking in
these questions!!
So how can we calculate
the components of a force
at 90o to each other?
If the Horse pulls with a
force of 750 N at an angle
of 45o What would be the
forwards force?
Diagonal
force
upwards eg
dragging a
sledge

If the force
needed to pull
this sled is
100 N and you
pull at an angle
of 25o what
are the
vertical and
horizontal
components of
this force?
If a husky pulls a sled at an angle of 10o to it’s line of
travel, what is the vertical component of the force
and the horizontal component if the dog pulls with a
force of 700N.
If the sled and load weighs 100Kg what acceleration
will the dog cause the sled to have?
Newton meter

Arrange the apparatus as
shown
Vary the masses on both
stacks recording the values
Record the value from the
Newton meter
Masses
Check to see if the
horizontal force and
vertical force should
theoretically combine to
produce the force on the
Newton meter.