Download Chapter 2 question 2 - leo physics website

Chapter 2 Question 2
a. State Newton’s 1st and 2nd Laws, giving a brief account on how they are
related.
4 marks
Newton's 1st Law:
A body continues in a state of rest or uniform motion in a straight line
unless it is acted upon by external force.
1
Newton's 2nd Law:
The rate of change of momentum of a body is proportional to and in the
same direction as the resultant force that acts on it.
1
The mathematical form of Newton's 2nd Law is:
or F = m a, when k is set to 1 by definition.
1
When an object is at rest or moving at a constant speed in a straight line,
its acceleration is zero (change of momentum is zero). According to 1st
Law, there is no external force. According to 2nd Law, the resultant
force is zero. Thus, the first law is a special case of the second law.
1
b.
Explain the meaning of inertia.
Inertia of an object is its reluctance to change in motion.
Therefore, a force is required to change its state of rest or state of
uniform motion.
It depends on the quantity of substance in the object.
Mass is a quantitative measure of inertia.
c. By comparing the force required to project a mass horizontally on the
surfaces of the Earth and the Moon, distinguish between mass and
weight.
1 mark
0.5
0.5
*
3 marks
The weight of a mass on Earth is about six times as that on the Moon. 0.5
However, if the mass is projected horizontally with the same speed, the
force required on Earth is the same as that on Moon.
0.5
This is because the changes in momentum in both cases are the same.
The mass is independent of the gravity. It is a measure of the amount of
substance.
1
The weight, which depends on the gravity g, is related to the mass:
1
d.
Discuss the uses of seat belt and head restraint in a car.
2 marks
The seat belt is useful when the car stops suddenly (e.g. crashing into a
tree).
It helps to pull the drivers/passengers backward while their inertia keeps
them moving forward.
The head restraint is useful when the car is pushed forward suddenly
(e.g. collide from the back by a truck).
It helps to support the head and neck which would otherwise be thrown
backward relative to the car.
e. Account for the momentum change of the objects involved in following
examples:
i)
a ball falls freely under gravity,
0.5
0.5
0.5
0.5
6 marks
A ball falling under gravity is accelerating. Its momentum is increasing.
This is explained by the Earth's gravity: The weight of the ball is the net
force that causes momentum change.
1
In order to apply the law of conservation of momentum, the system of
reference should consists both the ball and the Earth. There is no
external force. The weight on the ball and the gravitational attraction on
the Earth are internal forces.
1
1
ii)
a fast moving car is stopped by a strong rigid wall.
For each case, explain how you should choose the system of reference in
order to apply the law of conservation of momentum.
The momentum of the fast moving car drops to zero suddenly by the
rigid wall.
This is because the wall exerts a large force on the car, changing the
car's momentum to zero.
1
In order to apply the law of conservation of momentum, the system of
reference should consists both the car and the Earth. During the
collision, the car exerts a force on the Earth and the Earth exerts the
same force on the car in opposite direction. These forces of impact are
internal forces. There is no external force involved throughout.
1
1