Laws of Motion 2 Flashcards
why does a cricketer lower his hand while catching a ball?
A cricketer lowers his hand while catching a circket ball.
F Δt= pi-pf
By lowering his hands, the cricketer increase the time interval in wich the change of momentum occurs. Since pf-pi is a constant, the force i inversely proportional to Δt, this reduces the force with which the ball hits the player and he’s less injured
what is newton’s third law of motionlaw of reaction
To every action, there is always an equal and
opposite reaction.
does action and reaction have a causeeffect relationship?
There is no cause-
effect relation implied in the third law. The
force on A by B and the force on B by A act
at the same instant. By the same reasoning,
any one of them may be called action and the
other reaction.
how does newton’s third law of motion help us to walk
when we walk, we push our foot in the backward direction by pressing against the ground. This push is the action force F. The reaction force is the force R The weight of the body acts in the downward direction. The vertical component of reaction force R balances the weight W, while the horizontal component of reaction helps us move forward.
state the law of conservation of momentum
The total momentum of an isolated system
of interacting particles is conserved.
explain recoil of a gun
before firing: p=0
after firing:
mbvb = mgvg
By Law of Conservation of Momentum,
mbvb+mgvg=0
vg=-mbvb/mg
therefore when a bullet is fired from a gun, the bullet moves with a velocity mbvb in order to balance this momentum ( as intial momentum is 0) the gun will also experience a momentum in opp direction.
define the unit of force
In SI unit force is one that causes an acceleration
of 1 m s-2 to a mass of 1 kg. This unit is known as
newton : 1 N = 1 kg m s-2.
explain rocket propulsion
Just befre laucnhing, the total momentum of the system is 0. When the gases are ejected they aquire a momentum downards. due to the law of conservation of momentum, this momentum must be balanced such that total momentum is 0. So, the rocket acquires a momentum which is equal in magnitude but opposite in direction ie upwards.
When we talk about force, we talk about it at that instant. it doesn not depend on the history of the particle explain.
Force on a body at a given time is determined by the situation at the location of
the body at that time. Force is not ‘carried’ by the body from its earlier history
of motion. The moment after a stone is released out of an accelerated train,
there is no horizontal force (or acceleration) on the stone, if the effects of the
surrounding air are neglected. The stone then has only the vertical force of
gravity.
Explain why
(a) a horse cannot pull a cart and run in empty space,
The hoorse-cart system has no external forc acting on it in an empty space. The mutual forces between the horse and the cart cancel each other( newton’s third law)
On the ground, the contact force between the system and the ground (ie friction) causes themto move.
