Transcript Forces On Vehicles - science

Forces On Vehicles
Understand the terms motive force and
braking force.
Be able to explain how driving wheels
can generate a motive force.
Explain the importance of friction in
acceleration and braking.
Know that stopping distances of
vehicles relate to frictional forces and
speed.
Know how to interpret distance/time
graphs.
Basics – A Reminder
• Do you remember Newton’s Third law?
• It states “For every action there is an
equal and opposite reaction”
An Example
• This coach is
stationary on the
ground.
• There are two forces
acting on it….what
are they?
1. Its weight acting down
2. The floor pushing up
As the coach is not moving, these two must be equal and opposite.
What if they were not equal?
• If the floor did not push up on the coach, or it
did not push up enough, then the coach
would sink into the ground.
• If it pushed up too much, the coach would fly
up into the air.
• This doesn’t only happen in a vertical
arrangement – it happens if you lean on
something.
• In fact it happens everywhere, all of the time!
Motive Force
• “Motive” comes from the word “motion”.
• So this is a force which causes motion.
• Lets think of a car:
•Where does the motive force come from?
Motive Force
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•
The engine provides a torque to the driving wheels.
However, this is not really the motive force.
The motive force occurs at the wheels.
The wheels turning push backwards on the road.
By Newton’s third law, the road must be pushing
forwards on the wheel, like this:
Wheel pushing back
Road pushing
forwards
Motive Force
• This means that the purpose of the
engine is to provide a torque to the
wheels, so they push backwards on the
road.
• It is the road which then pushes the car
forwards.
Braking Force
• The car is moving – so the wheels are turning
– usually at high speed.
• The braking force is provided by the brake
pads gripping the wheel.
• The brakes provide a torque opposite to the
torque driving the wheels round.
• This reduces the force of the wheels on the
road, and hence the push of the road on the
car.
Brakes
Another Reminder - Friction
• Friction occurs when things rub
together.
• It is caused by tiny imperfections in the
surfaces locking and unlocking.
• The result is heat is generated, and
things wear down.
• Friction can be reduced by lubricating
surfaces: coating them with liquid,
which allows surfaces to glide over each
other.
Why is Friction so Important?
• Lets look at the motive force first:
If friction is absent, the road will not be able to push
back on the wheel.
The result will be that the wheel will spin – there will
be no grip.
This sometimes happens when people accelerate
too harshly-the motive force exceeds the force of
friction, and we get a wheelspin.
Movement and Friction
• If the car is moving and the friction force
reduces, you may not notice…
• Unless you are going round a bend!
• If friction disappears while you are
turning a corner, your car could go into
a spin.
• This is more likely to happen at high
speeds – when greater forces are
acting.
What About Braking?
• If friction is absent during braking, the
wheels will lock – they will stop turning.
• This is very dangerous – mainly
because the car will slide, but also
because the car cannot be steered.
Summary
• The Motive force is the force which
makes something accelerate.
• The braking force causes bodies to
decelerate.
• Friction is very important for both forces.
Road Safety
Know that stopping distances
of vehicles relate to frictional
forces and speed.
Know how to interpret
distance/time graphs.
Stopping Distance
• This is the distance moved by the vehicle
between the driver seeing the obstacle
and the vehicle stopping.
• The vehicle keeps moving at a steady
speed whilst the driver reacts
(thinking distance)
• The vehicle slows down whilst the brakes
are applied
(braking distance)
• Stopping = Thinking + Braking
Speed
(m/s)
Thinking distance
Braking distance
Reaction time
Time (s)

Other factors affecting stopping distance
• Apart from speed,
• What will increase the thinking distance
(make reaction time greater)?
• What will increase the braking distance
(reduce the frictional force)?