Free Force and Laws of Motion 03 Practice Test - 9th Grade
Question 1
A man is standing at an edge of a boat. He starts moving towards the centre of the boat. As no external force is acting on the system of boat and man, (neglect friction due to water) the boat will move in opposite direction.
SOLUTION
Solution : A
Initially, the total momentum was zero and after the man starts moving, the total momentum would still be zero as no external force is acting on the system of the man and the boat (law of conservation of momentum). So, the boat will move in the direction opposite to the direction of motion of the man.
Question 2
A bullet of mass 20 g moving with a velocity of 100 ms−1 strikes a wooden block of mass 80 g and gets embedded into it. Calculate velocity of the combined system.
100 ms−1
30 ms−1
20 ms−1
5 ms−1
SOLUTION
Solution : C
Let mass of bullet be m1 =20g
Let mass of the wooden block be m2 =80g
Let initial velocity of the bullet be u1 = 100 ms−1 and final velocity of the bullet be v1.
Let the initial velocity of wooden block be u2, since it was at rest, the initial velocity is zero, i.e u2=0 ms−1
Let final velocity of the wooden block be v2.
By applying law of conservation of momentum,
intial momentum of the system = final momentum of the system
i.e, m1u1+m2u2=m1v1+m2v2
After the bullet is fired, the bullet gets embedded on the block.
Therefore, it acts as a system
Hence , the final velocity of the bullet and wooden block after collision will be equal as it works as a system
i.e v1=v2=v where v is the velocity of the system after collision.
Substituting on the equation, we get
(0.020×100)+(0.080×0)=v×(0.020+0.080)
⇒ 2=0.1×v
⇒ v = 20 ms−1∴Velocity of system after collision = 20 ms−1
Question 3
The rate of change of momentum of a body is directly proportional to the force applied and change in momentum takes place in the same direction as the applied force.
SOLUTION
Solution : A
According to newton's second law of motion-"the rate of change of momentum is directly proportional to force applied and the change in momentum is in the direction of force applied." Newton's second law of motion gives mathematical expression of force.
Force ∝ dpdt
Question 4
A body is kept on a table. The object is pushed downwards with a force equal to its weight. But the table exerts an equal and opposite force on the object, hence keeping it at rest.
True
False
SOLUTION
Solution : A
The weight of the object exerts a force on the table. The table in return exerts a reaction force which is equal to the magnitude of the weight and opposite in direction (Newton's third law). Thus, the net force on the object is zero as two equal forces in opposite directions are acting on it and hence the object remains at rest.
Question 5
|
It is easier to stop a 60 g tennis ball as compared to a 150 g cricket ball, both having the same velocity. |
SOLUTION
Solution : A
As we know, Force=mass×acceleration. Hence, force is directly proportional to mass. Here, cricket ball has more mass than the tennis ball. So, even if the velocity is the same, the mass of the cricket ball and the associated momentum(which is the product of mass and velocity) is greater than that of the tennis ball and hence, it is easier to stop a tennis ball as compared to a cricket ball having the same velocity.
Question 6
Newton's second law of motion gives us the mathematical expression of force.
True
False
SOLUTION
Solution : A
Newton's second law of motion gives us the quantitative analysis of force. It gives us the mathematical expression of force as it states that the rate of change of momentum of a body is directly proportional to the force applied, and this change in momentum takes place in the direction of the applied force.
Question 7
According to Newton's second law of motion, force is directly proportional to the rate of change of momentum provided
velocity of body remains constant
mass of body remains constant
acceleration of body remains constant
SOLUTION
Solution : B
Newton's second law states that the rate of change of momentum of an object is directly proportional to the net force acting on it, provided that the mass of the body is kept constant. It gives the mathematical expression of force.
Question 8
An object of mass 2 kg is sliding with a constant velocity of 4 ms−1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is:
SOLUTION
Solution : B
Ideally, a frictionless horizontal table has no resistance to motion.
Therefore, the object will continue to slide at 4 ms−1 without any application of force.
∴ The force required to keep the object moving = 0
Question 9
A stone of 1 kg is thrown with a velocity of 20 ms−1 on the frozen surface of a lake. The stone comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?
5 N
3 N
4 N
SOLUTION
Solution : D
Initial velocity of the stone = 20 ms−1.
Final velocity of the stone, v = 0(finally the stone comes to rest).
Distance covered by the stone = 50 m.
According to third equation of motion:
v2=u2+2as
0 × 0 = 20 × 20 + 2 × a × 50
⇒a= −4ms−2.
The negative sign indicates that acceleration is in the opposite direction to the motion of the stone.
Mass of the stone, m= 1 kg.
From the newton's second law of motion:
Force = mass × acceleration. Therefore,
F=1×−4=−4N.
Hence, the force of friction between the stone and the ice is 4N. The negative sign indicates that friction oppposes the relative motion.
Question 10
The importance of Newton's first law of motion is that:
it mentions force
it defines inertia
it defines momentum
it defines acceleration
SOLUTION
Solution : A and B
Newton's first law states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force. This is also called the law of inertia because inertia is the property of any object by virtue of which it retains its state of motion. The importance of Newton's first law of motion also lies in the fact that it mentions force and gives us a qualitative idea about it.