Download 11SD3 P2a revision notes Miss O`Neill file

RESULTANT FORCES
•
When 2 objects interact, the forces they exert are equal and opposite each other
•
Resultant force = when lots of forces acting on an object are replaced by a single force that has the
same effect as all the little forces acting together.
•
If a resultant force acting on a stationary object is:
• Zero – the object stays ?
• Not zero – the object will ?
•
If a resultant force acting on a moving object is:
• Zero – the object will ?
• Not zero – the object will ?
FORCE & MOTION
•
Acceleration – the change in velocity (speed) of an
object in a certain time
•
F=mxa
•
F = force (N)
•
m =mass (kg)
•
a = m/s2
•
The bigger the force put on an object the faster it
will move.
•
The heavier an object is the harder it is to move
and the higher the force needed.
•
a=?
F
m
x
a
FORCE & MOTION
The gradient (slope) of a distance – time graph
represents speed
•
Velocity = speed in a certain direction
•
Constant speed = straight line at an angle
•
Stationary = straight horizontal line – distance not
changing
Fastest
Distance
•
Stationary
Slowest
Time
CALCULATING ACCELERATION
•
Remember a = F/m
•
If you don’t know the force or mass of an object there is another formula:
•
a = v-u
t
a = acceleration (m/s2)
v = final velocity (m/s)
u = initial velocity (m/s)
t = time (s)
• For example:
• A car goes from 10m/s to 35m/s in just 5 seconds. What was its acceleration?
• a=?
• v = 35m/s
• u = 10m/s
• t=5s
• a = 35 – 10
= 5 m/s2
5
EXAM STYLE QUESTION
EXAM STYLE QUESTION
FORCES & BRAKING
•
A vehicle travelling at a steady speed – the forces acting on it are
balanced
•
The greater the speed of a vehicle to greater the braking force
needed to stop it in a certain distance
FORCES & BRAKING
•
What happens when brakes are applied?
•
What force is involved?
•
Friction increases between the brakes & the wheel.
•
The stopping distance: the distance the vehicle travels from when brakes are applied until it stops
•
•
It consists of the driver’s reaction time and the braking time added together
Stopping distance can be affected by the driver’s reaction times
• Reaction time affected by ??
• Drugs, tiredness, alcohol, road conditions, weather conditions & condition of the car itself.
WEIGHT & GRAVITY
•
What is weight?
• The force of gravity on an object (Newtons)
• The bigger an object, the higher the force of gravity, the higher the weight.
•
Mass = the amount of matter that an object is made of (Kg)
•
Weight can be measured with a Newton meter
•
1kg = 10N
•
10kg = 100N
•
100g = 1N
•
On Earth the force of gravity is about 10N/Kg.
•
Therefore we are pulled down by a force of 10N for every 1 Kg of mass
•
Weight = mass x gravitational force (N/Kg)
•
Weight, W = mg
w = weight (N)
m = mass (Kg)
g = gravitational pull/Kg
FORCES & TERMINAL VELOCITY
•
When an object is falling, if only gravity acts on it then if falls at:
• 10m/s2
•
Remember: a = f/m
• A 50Kg object falls. What is its acceleration?
• a = 500N/50Kg = 10m/s2
•
The faster an object moves through something the greater the
frictional force acting on it.
•
Think of a skydiver. The faster he falls through the air the
greater the friction (air resistance) acting on him.
•
When an object is dropped:
• It will accelerate due to the force of gravity
• It will then fall as fast as the force of friction around it =
terminal velocity
• Terminal velocity = steady speed.
• The acceleration of the falling object then __________ as it
falls. This is because the drag (air resistance) starts to
increase.
EXAM STYLE QUESTION
EXAM STYLE QUESTION
EXAM STYLE QUESTION
MARK SCHEME
MARK SCHEME
MARK SCHEME
FORCES & ELASTICITY
•
A force acting on an object may cause a change in shape of an object
•
A force applied to an elastic object such as a spring will result in the
object stretching & storing elastic potential energy
•
Extension = difference between the length of a spring & it’s original
length.
•
Extension is directly proportional to the strength of the force applied
to the spring
•
If the force is too strong the spring will be stretched beyond its limit of
proportionality
•
Hooke’s law can be calculated like this:
•
Force applied = spring constant x extension
(N)
(N/m)
(m)
• A spring has a spring constant of 25N/m. How much force is needed to
make the spring extend by 0.10m??
Hooke’s Law
EXAM QUESTION ON EXTENSION
EXAM QUESTION ON EXTENSION
EXTENSION QUESTION CONT.
MARK SCHEME