Download Forces

Key Areas covered
• Balanced and unbalanced forces. The
effects of friction. Terminal velocity.
• Forces acting in one plane only.
• Analysis of motion using Newton’s
first and second laws. Frictional force
as a negative vector quantity.
• Tension as a pulling force exerted by
a string or cable on another object.
What we will do today:


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•
Revise Newton’s first two laws.
Use free body diagrams to analyse the
forces on an object.
Carry out examples on the above
Carry out calculations using Fun = ma on
problems involving lifts.
Newton’s First and Second Laws
of Motion
In your class jotter write down
Newton’s first two laws
An accelerating object has an
unbalanced (resultant) force, F, acting
on it in the same direction as the
acceleration.
If an object is not accelerating then
the unbalanced force F = 0.
Newton’s First Law


An object will remain stationary or
continue at constant velocity unless
acted on by an unbalanced force.
i.e. if forces are balanced an object
will remain stationary or move at
constant velocity.
Newton’s Second Law
• Motion can exist without forces, but for
change of motion to occur, forces must
be involved.
• i.e. Unbalanced forces cause
acceleration.
Fun = ma
Fun = Unbalanced Force in N
m = mass in kg
a = acceleration in ms-2
One Newton is the (unbalanced) force that
gives a 1 kg mass an acceleration of 1 ms-2.
Force is a vector.
Solving Force Problems
1) Draw a free body diagram showing the
forces acting on the object and the
direction of its acceleration.
2) Deal with one object at a time.
However, when objects are tied
together, they behave like a single
larger object.
3) Use F = ma, but remember F is the
unbalanced force!!
Example 1 – Single Force,
Single Mass
A man pushes a trolley
of mass 20 kg along a
flat surface of 40 N.
If the effects of
friction can be ignored,
what is the
acceleration of the
shopping trolley?
a
40 N
20 kg
a =F/m
= 40 / 20
= 2 ms-2
Example 2 – Multiple Force,
Single Mass
A guided missile of
mass 1000 kg is fired
vertically into the air.
The missile’s rocket
motors provide a
thrust of 20 000 N,
and there is a drag
force of 2 000 N.
What is the
acceleration of the
missile?
Thrust = 20000 N
Fun = 20000
- 9800
- 2000
= 8200 N
Drag
= 2000 N
a =F/m
= 8200 / 1000
= 8.2 ms-2
N
A
S
A
a
W = mg = 9800N
Example 3 – Single (External)
Force, Multiple Mass
A ski-tow pulls two
skiers, who are
connected by a thin
nylon rope, along a
frictionless surface.
The tow uses a force
of 70 N and the
skiers have masses of
60 kg and 80 kg.
a) What is the
acceleration of
the system?
b) What is the
tension in the rope?
80 kg
a)
T
60 kg
70 N
a =F/m
= 70 / (80 + 60)
= 70 / 140
= 0.5 ms-2
b) Tension T is a Force.
It is caused by 80 kg
person.
T = ma
= 80 x 0.5
= 40 N
2003 Qu: 3
2005 Qu: 3
2008 Qu: 3
2009 Qu: 4
2010 Qu: 21
Lift Problems
In all Lift problems, there will always be a
tension in the cable supporting it, and a
weight (W = mg).
Tension
or
Fun = ma
Lift
of
mass ‘m’
Weight = mg
There could also be an unbalanced force, Fun.
There are 6 possible lift situations:
1) Lift at rest
Tension = Weight (no Fun)
2) Lift travelling at constant speed
Tension = Weight (no Fun)
3) Lift accelerating up
Tension = Weight + Fun = mg + ma (Fun up)
4) Lift decelerating up
Tension = Weight – Fun = mg – ma
(Fun down)
5) Lift accelerating down
Tension = Weight – Fun = mg – ma
(Fun down)
6) Lift decelerating down
Tension = Weight + Fun = mg + ma (Fun up)
Example
A package of mass 4 kg is hung from a spring balance
attached to the ceiling of a lift which is accelerating
upwards at 3 ms-2.
What is the reading on the spring balance?
a = 3 ms-2
Fsb
4 kg
W = mg
Solution
Fsb = force exerted by spring balance
[to acc up it has to overcome the
weight (mg) and then have Fun (ma)]
Fsb = W + Fun
Fsb = mg + ma
Fsb = (4 x 9.8) + (4 x 3)
Fsb = 51.2 N
2006 Qu: 4
2007 Qu: 3
Example 2 (…cont)
•
As the rocket rises its acceleration is found
to increase, state a possible reason why.
1. As it rises fuel is burnt up so mass
decreases.
2. Thinner air results in a reduction in friction.
3. The value of ‘g’ decreases so weight is less.
• (Learn two of these for the exam).
Questions
• Activity sheets:
• Page 22 – 30
• Forces Questions Revisited & Higher
Forces
• You should now be able to answer all
questions in class jotter