Force & Laws of Motion (Physics) motion in a straight line.

Force & Laws of Motion (Physics)
Q1. What is a force?
Ans. Force is a push or pull which tries to bring the change in the state of rest or of uniform
motion in a straight line.
Q2. What are the effects of force?
Ans.1. Force can be used to change the magnitude of velocity of an object (that is, to make
the object move faster or slower)
2. Force can change its direction of motion.
3. Force can change the shape and size of objects
Q3. Differentiate between balanced and unbalanced forces?
Ans. When the resultant of a set of forces acting on a body is zero, the set is said to be a set
of balanced forces. Balanced forces do not change the state of rest or of motion of an
object They can change the shape of the object
When the resultant of a set of forces acting on a body is not zero, the
set is said to be a set of unbalanced forces. An unbalanced force acting on an object can
bring change in its state of motion, direction or magnitude of velocity.
Q4. State Newton laws of motion.
Ans. The first law of motion is stated as: An object remains in a state of rest or of
uniform motion in a straight line unless compelled to change that state by an applied force.
The second law of motion states that the rate of change of momentum of an object is
proportional to the applied unbalanced force in the direction of force
The third law of motion states that when one object exerts a force on another object, the
second object instantaneously exerts a force back on the first. These two forces are always
equal in magnitude but opposite in direction
Q5. Why does a gun recoil?
Ans. When a gun is fired, it exerts a forward force on the bullet. The bullet exerts an equal
and opposite reaction force on the gun. This results in the recoil of the gun Since the gun has
a much greater mass than the bullet, the acceleration of the gun is much less than the
acceleration of the bullet
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Q6. State and prove the law of conservation of momentum.
Ans. Suppose two objects (two balls A and B, say) of masses mA and mB are travelling in the
same direction along a straight line at different velocities uA and uB, respectively and there
are no other external unbalanced forces acting on them.
Let uA>uB and the two balls collide with each other
During collision which lasts for a time t, the ball A exerts a force FAB on ball B and the ball B
exerts a force FBA on ball A.
Suppose vA and vB are the velocities of the two balls A and B after the collision, respectively
the rate of change of momentum of ball A during the collision will be= the rate of change of
momentum of ball B during the collision will be Since (mAuA + mBuB)is the total
momentum of the two balls A and B before the collision and(mAvA+ mBvB) is their total
momentum after the collision the total momentum of the two balls remains unchanged or
conserved provided no other external force acts.
The sum of momenta of the two objects before collision is equal to the sum of
momenta after the collision provided there is no external unbalanced force acting on
them. This is known as the law of conservation of momentum.
Q. 7. Define One Newton.
Ans. one unit of force is defined as the amount that produces an acceleration of 1 m s-2in
an object of 1 kg mass.
Q.8. While catching a fast moving cricket ball, a fielder in the ground gradually pulls
his hands backwards with the moving ball?
Ans. In doing so, the fielder increases the time during which the high velocity of the moving
ball decreases to zero. Thus, the rate of change of momentum of the ball is decreased and
therefore the impact of catching the fast moving ball is also reduced. If the ball is stopped
suddenly then its high velocity decreases to zero in a very short interval of time Thus, the
rate of change of momentum of the ball will be large. Therefore, a large force would have to
be applied for holding the catch that may hurt the palm of the fielder.
Q.9. Define Inertia.
Ans. Inertia is the natural tendency of an object to resist a change in its state of motion or of
rest
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Q10 When a bus suddenly starts moving forward the passengers in the bus fall
backward. Explain why?
Ans This is because the lower part of the bodies of the passengers being in contact with the
floor of the bus come in motion along with the bus. On the other hand the upper part of
their bodies remains at rest due to inertia of rest. Hence the passengers fall backward.
Q.11 Do all bodies have the same inertia?
Ans. No, mass of an object is a measure of its inertia. Heavier or more massive objects offer
larger inertia.
Q12. Define Momentum.
Ans. It is the product of mass and velocity of the body. Its units are kgm/s. It is a vector
quantity.
Q13 Show that Newton’s first law of motion is a special case of Newton’s second law of
motion.
Ans. The first law of motion can be mathematically stated from the mathematical
expression for the second law of motion.
F = ma
Or
F =m (v- u)/t
That is, when F = 0, v = u for whatever time, t is taken. This means that the object will
continue moving with uniform velocity, u throughout the time, t. If u is zero then v will also
be zero. That is, the object will remain at rest.
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