PHYSICS SPM

1. Analyzing
Physics Form 4

3. Forces in Equilibrium
. remains stationary (if • The principle of the
the object is stationary) forces in equilibrium
• (ii) moves at a constant states,
velocity ( if the object is • “When forces act upon
moving) an object , the object is
said to be in a state of
equilibrium when the
resulting force acting on
the object is zero (no
net force acting upon it)
”

6. Examples Forces in Equilibrium
Weight = Normal reaction Buoyant force = Weight
Weight = Normal reaction
Weight = Tension

7. Examples Forces in Equilibrium
Weight = Lifting force
Weight = Normal reaction
Pulling force = Frictional force Driving force = Dragging force

8. Resultant force
Force is a vector quantity and
hence it has magnitude and
direction.
Two or more forces which act on
an object can be combined into a
single force called the resultant
force.
If two forces are in same line, we
simply add the forces if both pull or
push together; subtract them if
one is in the opposite direction

10. Two Forces in Equilibrium
P + Q=0
P =-Q

11. Three Forces in Equilibrium
P
Q
R

12. 3 forces in balance

14. Triangle method

16. Parallelogram method
Two forces of 40N and 60 N act at 600 to each other at a point as represented
below
1. Choose a scale
Suppose we let 2cm represent 10N. Then an 8cm line represents the
40N force and a 12cm line represents the 60N force. Draw these lines
with a 600 angle between them:

17. 2. Complete the parallelogram

18. 3. Draw the diagonal from the point of
application of the forces
The diagonal labelled R represents the resultant force - measure this and
convert its length to newtons:Diagonal R = 17.4cm, so the resultant force R
= (17.4/2)*10 = 87N
4. Measure a suitable angle:
The angle A = 230.
Thus, the resultant of the two original forces is a force of size 87N acting at
230 to the 60N force

20. Resolution of forces
• is an angle between the force F to the horizontal line
•the sign of the force depend on the quadrant where the force , F is placed
Fx = F cos 
Fy = F sin 
A force can be resolved into two components, that is,
(i) the horizontal component, Fx and
(ii) the vertical component , Fy

21. Inclined plane
A = W sin 
B = W cos 

22. example
Find the values of Px and Py for the following figures.

23. Figure shows a stationary wooden block of
mass 50 g which is placed on a inclined plane
that is at an angle of 40o to the horizontal.
What is the magnitude of the weight parallel
to the inclined plane