Force and motion

1
Force and motion
Objectives
When you have competed it, you should
understands Newtons first law of motion
know some different types of forces
know and be able to apply Newtons second law
to simple examples of objects moving in a
straight line
understands the idea of equilibrium.
2
Force and motion
Newtons first law
A particle will remain at rest or will continue
to move with constant velocity in a straight
line unless the forces act on it to change that
state.
Example
When you are riding a bicycle on a level path and
start to free-wheel, you can keep up an almost
constant velocity force some time.
But eventually you will slow down, partly because
of air resistance.
3
Force and motion
Newtons first law
Example
When a stone is sent sliding across the frozen
surface of a lake. However smooth the ice
appears, the stone eventually slows down because
of friction between the two surfaces
4
Force and motion
Newton second law
The force F applied to a article is proportional
to the mass m of the particle and the
acceleration produced.
Force mass ?? acceleration
F ma
SI units
Mass ?
kilogram (kg)
Acceleration ?
metres per second per second (m s-2)
Force ?
Newton (N)
1 N is the force needed to give 1 kg an
acceleration of 1 ms-2.
5
Types of forces
Pushing
or Thrust
Tension
Driving force
6
Force and motion
Example
An ice-yacht of mass 400 kg has an acceleration
of 1.2 ms-2. What force is needed to produce this
?
Solution
1.2 ms-2
F
F m a
F 400 ? 1.2
480 N
7
Force and motion
Example
Find the magnitude of the acceleration produced
when a particle of mass 5 kg is acted upon by a
force of magnitude 22 N.
Solution
a ms-2
F 22 N
F ma
22 5 ? a
a
4.4 ms-2
8
Force and motion
Example
A car of mass 1000 kg is pushed with a force of
200 N. Calculate the acceleration of the ca and
find how long it will take to reach a speed of 3
ms-1 form rest.
Solution
200 1000 ? a
a 0.2 ms-2
F m a
Data u 0 v 3 a 0.2 t?
Required equation v u at
9
Force and motion
Example
A curling stone of mass 18 kg is launched across
ice with a speed of 2 ms-1, and goes a distance
of 25 metres before coming to rest. Calculate the
deceleration, and find frictional ore between the
stone and the ice.
Solution
Data
u
2
v
0
s
25
02 22 2 ? a ? 25
Use the equation
v2 u2 2as
gives a
- 0.08 ms-2
F 18 ? ( - 0.06)
- 1.44 N
F ma