| Key Words Uniform motion, Force, Mass, Acceleration |
Newton I
Newton’s First Law
states:
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
states:
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, m,
is pushed with a constant force, F, so
that its velocity increases from an initial value, u,
to a final value, v, in time, t.
Time
Þ
mv - mu
µ
F
t
Þ
m(v - u )
µ
F
t
Now acceleration,
a = (v- u)
t
Þ
F
µ m × a
Þ
F = k × m × a
(
k
is a constant with no units.
k
= 1)
Force
in N,
Mass
in kg,
Acceleration in m/s2.
Acceleration is always caused by a total force,
the vector sum of all the forces. The
acceleration is always, without exception, in the same direction as the total force.
| A 70 kg athlete accelerates to his maximum speed of 9.5 m/s in a time of 2.5 s. What is the average force he applies to the track? |
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.
Newton’s
Third law states that:
If
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.
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.
Hover mowers are very simple small hovercraft. Some transport hovercraft are enormous machines of several hundred tonnes.
| Presentation | Newton's Laws | ||
| Now try Topic Test | Home | Unit 2 | Physics AS |
