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Physics Form 4 Chapter 2 Force and Motion I
SPM Tips
EXAMPLE 2.21
The definition of Newton’s second law of motion consists
of a few important facts: A box of mass of 5 kg moves with a uniform
• The object will only accelerate if there is a net external velocity when acted upon by a force of 10 N.
force acting on it.
• The acceleration is directly proportional to the net Calculate,
external force. (a) frictional force acting on the box, and
• The direction of the acceleration is the same as the (b) acceleration of the box if the force is 20 N.
direction of the external force.
• The magnitude of the acceleration of the object is Solution
inversely proportional to its mass.
(a)
SPM Tips
Friction F = 10 N
The meaning of F = ma is: Chapter
• The net force F, in the direction of the acceleration. Figure 2.78
• The direction of the force and the direction of the
change of momentum are the same. The force acting on the box in the horizontal
• The direction of the force F is the same as the direction direction is the force F = 10 N and frictional 2
of the acceleration, a. force. These forces are in equilibrium when the
box moves with uniform velocity. Hence, the
magnitude of the frictional force = 10 N, in the
EXAMPLE 2.20 opposite direction to F.
A 100 kg box is placed on a smooth floor and is (b)
pulled by a force F N. a
100 kg Friction = 10 N F = 20 N
F 1
Figure 2.79
Figure 2.77
(a) If F = 200 N, calculate the acceleration of the When F 1 = 20 N, the net force acting in the
direction of the acceleration, a
box. = F 1 – friction
(b) If the velocity of the box changes from rest to = (20 – 10) N
15.0 m s in 10 seconds when acted upon by = 10 N
–1
a force, what is the value of the force?
Solution Using the formula F = ma
(a) Given, m = 100 kg, F = 200 N 10 = 5 × a
F = ma a = 2 m s –2
F 200
Therefore a = = 100
m
= 2 m s –2
v – u Checkpoint 2.6
(b) Acceleration, a = t
15.0 – 0 Q1 A box with a mass of 5.0 kg is pulled along a
= 10 smooth floor. If the force is 12.0 N, what is the
acceleration of the box?
= 1.5 m s –2
Force, F = ma
= 100 × 1.5
= 150 N

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