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Physics Form 4 Chapter 2 Force and Motion I

Table 2.7
Before
explosion After explosion
Total left Total right
Total –d 1 d 2 Total
momentum v 1 =  t  v 2 = t m 1 m 2 momentum, momentum, momentum
m 1 v 1 m 2 v 2

Note: The value of t need not be measured. Chapter
Conclusion:
The total momentum of a system before and after explosion are the same. The hypothesis is accepted. 2

SPM Tips
EXAMPLE 2.18
To answer questions related to the principle of conservation
An astronaut with a mass of 80 kg throws a box of of momentum:
instrument with a mass of 40 kg in order to return to • Sketch a diagram to show the motion of the objects
the space capsule. If the box moves with a velocity involved
of 6 m s , what is the velocity of the astronaut after • Label the mass and velocity (magnitude and direction)
–1
of each object in the diagram.
throwing the box? • Use the formula (Total initial momentum = Total final
momentum).
Solution
Let v be the velocity of the astronaut.
The total momentum before the box is thrown is
zero because both the astronaut and the box are EXAMPLE 2.19
stationary.
Figure 2.65 shows 2 trolleys A and B with a mass
Total momentum after the box is thrown of 1.5 kg and 1.0 kg respectively. A compression
= momentum of the box + momentum of the spring is placed between the 2 trolleys and is
astronaut compressed by forcing the 2 trolleys close to
= (40 × 6) + (80 × v) each other. When the 2 trolleys are released, each
= 240 + 80v moves away from each other. What is the distance

According to the principle of conservation of travelled by trolley B when trolley A has moved
momentum, 40 cm?
total momentum before the box is thrown
= total momentum after the box is thrown
Therefore, A B
240 + 80v = 0
v = –3.0 m s Figure 2.65
–1
(The negative sign shows that the astronaut is
moving in the opposite direction to the box.)

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