Newton’s First Law
Every object continues in its state of rest or uniform motion in a straight line, unless it is compelled to change that state by an external force acting on it.
A car will maintain a constant speed if the drive force and the drag are balanced. The total force is zero.
The graph below shows the acceleration of a car up to its maximum speed.
(a) Why is the graph a straight line at low speeds?
(b) Use Newton I to explain why
the car reaches a maximum speed which it cannot
Newton's Second Law
Rate of change of momentum is proportional to the
total force acting on a body, and occurs in the direction of the force.
An object of mass,
is acted on with a constant force,
that its velocity increases from an initial value,
to a final value,
v, in time,
Momentum is the product of mass and velocity. It is a vector and has units of kilogram metres per second (kg m s-1).
Change in momentum = momentum at end - momentum at start
Now we know that acceleration:
So we can write:
is a constant with no units.
(N) = Mass (kg) × acceleration (m s-2)
orce (N) = Mass (kg) × acceleration (m s-2)
Acceleration in m s-2.
Acceleration is always caused by a total force,
or the resultant force, the vector sum of all the forces. The
acceleration is always, without exception, in the same direction as the total force.
Strictly speaking, we should write the equation as:
The strange symbol S is Sigma, a Greek capital letter 'S'. It is physics code for "sum of". Consider two coplanar forces F1 and F2 acting at right angles. To get SF, we need to do a vector sum on the two forces:
If you have a vehicle providing a force to accelerate another vehicle, you must
add the mass of the towing vehicle to that being towed.
Third law states that:
body A exerts a force on body B, body B must exert an equal and opposite force
on body A.
In other words, forces always act in pairs. This is true whether the forces are in equilibrium, moving, stationary or accelerating.
Source not known
This boy is sitting on a simple hovercraft. The motor drives a fan which forces air downwards onto the floor. The force of the air going down produces a reaction force that lifts the machine off the ground. A simplified diagram of the machine is shown below:
This picture shows a small hovercraft that can skim on water or dry land. It is powered by a small (10 kW) petrol motor.
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Hover mowers are very simple small hovercraft. Some transport hovercraft are enormous machines of mass several hundred tonnes. They move easily across flat ground. The problem with hovercraft is that their performance on hills is hopeless. Can you think why?
Picture by Andrew Berridge, Wikimedia Commons
Newton I and III are often confused.