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Physics Form 4 Chapter 2 Force and Motion I

2.7 Impulse and Impulsive Force Hence, the = mv – mu
t
impulsive force
(1 000 × 0) – (1 000 × 25)
1. From the equation = 0.05
F = ma ............................ 
v – u
and a = ............................  = –500 000 N
t
Note: The negative sign shows the the force is
Substitute  into , acting on the car.
v – u 
F = m 
t

mv – mu
=
t Newton’s Third Law of Motion
Therefore, Ft = mv – mu
1. Newton’s Third Law of Motion states that
2. The product of force F and action time t is for every action, there is an equal but opposite
Chapter
2 known as the impulse of a force. reaction. This means when an object exerts a
Impulse = Ft force on a second object, the second object
exerts a force of equal magnitude but in the
3. Impulse is defined as change of momentum, opposite direction on the first object.
that is, 2. Examples of Newton’s Third Law of Motion.

Impulse, Ft = mv – mu (a) A book placed on a table exerts a force on
= Change of momentum the table. The table exerts a reaction force
of the same magnitude on the book in the
The SI unit for impulse is kg m s or N s. opposite direction, that is, upwards.
–1
Impulse is a vector quantity. (b) When a person is walking, his sole pushes
the floor backwards. The floor reacts by
4. Impulsive force is defined as the rate of pushing the sole forward with the same
change of momentum resultant from an action magnitude. Hence, the person can move
that occurs in a short period of time. An forward.
example is during a collision. (c) A boy pulls a tight drawer and he is
pulled by that drawer. This is because
Change of momentum
Impulsive force, F = Time the force exerted by the boy pulling the
mv – mu drawer causes the drawer to exert a force
= of the same magnitude but in the opposite
t direction to pull the boy towards it.

EXAMPLE 2.22

A car with a mass of 1000 kg moves with a
velocity of 25 m s . The car then hits a tree and
–1
is stopped in 0.05 second. Calculate the impulsive
force acted on the car.
Solution
Mass of car = 1 000 kg
Initial velocity of car, u = 25 m s –1
Final velocity of car, v = 0 m s –1
Time taken to stop, t = 0.05 s Figure 2.80

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