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Physics Form 4 Chapter 2 Force and Motion I

Solution
EXAMPLE 2.16
Total momentum before coupling
A lorry with a mass of 8 000 kg moves with a = (2.5 × 10 × 10) + (2.5 × 10 × 0.8)
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velocity of 30 m s . The lorry then collides with = 4.5 × 10 kg m s –1
–1
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a car with a mass of 1500 kg and travelling with Total momentum after coupling
a velocity of 20 m s . After the collision, the two = 5.0 × 10 × v
–1
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vehicles stick and move together with a velocity v.
Determine the value of v. According to the principle of conservation of
momentum,
Solution Total momentum Total momentum

30 m s –1 20 m s –1 before coupling = after coupling
5.0 × 10 × v = 4.5 × 10 kg m s –1
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v = 0.9 m s –1 Chapter
Momentum of lorry before collision Assumption: The sum of the external forces acting
= 8 000 × 30 on the system consisting of the two boxcars is zero.
= 240 000 kg m s –1 2
Momentum of car before collision Note:
= 1 500 × 20 1. For experiments in this chapter, a friction
= 30 000 kg m s –1 compensated inclined plane is used.
Total momentum = Total momentum Friction
before collision after collision
Component of
240 000 + 30 000 = (8 000+1 500)v weight of the
trolley
270 000 kg m s –1 = 9 500v
270 000 Weight component cancels the effect of friction
Therefore, v = 9 500
= 28 m s –1 Figure 2.63
2. A friction compensated inclined plane has an
inclination in such a way that is the friction on
the surface of the inclined plane is balanced
EXAMPLE 2.17 out by the component of the weight along the
trolley.
Figure 2.64 shows two identical boxcars of a train 3. The two forces cancel out each other and hence
being coupled at a railway station. Each boxcar has
a mass of 2.5 × 10 kg. The velocity of the boxcar it is as if the inclined plane is frictionless.
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1 and boxcar 2 before the coupling are 1.0 m s 4. When this condition is achieved, the trolley
–1
and 0.8 m s respectively. will move with a uniform velocity when given
–1
a small push.
1.0 m s –1 0.8 m s –1
1 2 Explosion
Before 1. When a rocket is launched, the exhaust gas is
v forced out from the back of the rocket and the
1 2 rocket moves upwards. Initially the rocket is
After stationary at the launching pad. The launching
Figure 2.62 of a rocket is an example of explosion.
Find the velocity, v, of the boxcars after they are 2. In explosion, the total momentum is conserved,
coupled. State one assumption in your calculation. that is the total momentum before and after the
explosion is zero.

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