TODAY’S
FOCUS
01 Acceleration
02 Adding forces
03 Gravitational power
Understanding acceleration
An object accelerates if its speed
increases. Acceleration is the rate with
which speed increases. When it slows
down, it means it’s decelerating.
Understanding velocity
Velocity, on the other hand, is an object’s
speed in a particular direction. It is an
example of a vector quantity.
An aircraft may have the speed of
200 m/s but a velocity of 200 m/s due
north.
Vectors have both
MAGNITUDE AND DIRECTION.
Speed is a scalar quantity and
has only MAGNITUDE.
Adding forces
What happens if an object is acted on by two or
more forces?
In this picture, someone is pushing a car. Friction
opposes their pushing force. Because the forces
are acting in a straight line, it is simple to
calculate the resultant force, provided we take
into account the direction of the forces:
Resultant force = 500N - 350N
Resultant force = 150 N to the right
Adding forces
This is a more difficult situation. A firework
rocket is acted on by two forces
●
The thrust of its burning fuel pushes it
towards the right.
●
It’s weight acts vertically downwards.
Rules for vector addition
You can work out the resultant force in two ways.
● Draw a precise triangle, with correct proportions, and see what
the result is.
● Use Pythagoras’ theorem to find the length of the resultant
vector and trigonometry to find the angle.
Find the resultant force acting on the
rocket (page 60).
You are rowing across the Lembeh Strait at
1.5m/s. a Calculate your velocity if you row
against a current of 0.2 m/s.
If you drop an object, it falls to the ground. It is difficult to see how a
falling object moves. However, a multi-flash photograph can show the
pattern of movement when an object falls. Figure 3.10 shows a ball
falling. There are seven images of the ball, taken at equal intervals of
time. The ball falls further in each successive time interval. This shows
that its speed is increasing -it is accelerating.
When an object accelerates, there must be a force that is causing
it to do so. In this case, the force of gravity is pulling the ball
downwards. The name given to the gravitational force acting on
an object that has mass is its weight. Because weight is a force, it
is measured in newtons (N).
Calculating weight
We have seen that an object of mass 1 kg has a
weight of 9.8N; an object of mass 2 kg has a weight
of 19.6 N; and so on.
To calculate an object’s weight W from its mass
m,we multiply by 9.8, the value of the acceleration
of free fall g.
We can write this as an equation in words and in
symbols:
Weight = mass x acceleration of free fall
W = mg
List the differences between mass and
weight.
Weight changes on other planets
Mass is constant
Mass - kg
Weight - N
Mass is the amount of matter that an object has
Weight is the force that attracts the object to the
Earth
A bag of sugar has a mass of 1 kg so its
weight on Earth is 10 N. What can you say
about the bag’s mass and its weight if you
take it:
a) to the Moon, where gravity is weaker than on Earth?
b) to Jupiter, where gravity is stronger?
Air resistance
All objects fall with the same acceleration,
provided there is no other force acting to reduce
their acceleration. For many objects, the force of
air resistance can affect their acceleration.
Parachutists make use of air resistance.
Air resistance
A free-fall parachutist jumps out of an aircraft
and accelerates downwards. Figure B shows the
forces on a parachutist at different points in his
fall. Notice that his weight does not change (so the
length of the downward-pointing arrow does not
change).
Air resistance
At first, air resistance has little effect. However,
air resistance increases with the speed of motion.
As the parachutist falls faster, eventually air
resistance balances his weight.
Then the parachutist stops accelerating: he falls at
a steady rate known as the terminal velocity. The
resultant force on the free-fall parachutist is the
result of two forces acting along the same line and
acting in opposite directions.
Examples of motion along a
curved path. In each case,
there is a sideways force
holding the object in its
circular path.
The size of the resultant force needed to make an
object travel in a circle depends on the object’s mass,
its speed and the radius of the circle in which it is
moving.
A bigger force is needed if:
the object’s mass is bigger (and
speed and radius stay the same)
the object’s speed is bigger (and
mass and radius stay the same)
the radius of the circle is
smaller (and mass and speed
stay the same)
b
C
d
c