Question:
While catching a fast moving ball, fielder gradually pulls his hand backwards. Give reasons.
Answer:
By doing so, the fielder increases the time during which the high velocity of the moving ball
decreases to zero. The acceleration of the ball is decreased and therefore the impact of catching the fast moving ball is also reduced.
We can prove it mathematically,
Let u be the initial velocity of ball before it reaches fielder hands. Let m be the mass of the ball.
Initial momentum of the ball = mu.
When the ball reaches field hands, it stops. ∴ final velocity (v) = 0.
And the final momentum = mv = 0
The ball will exert force (F) on fielder hands = Change in momentum / time(t1)
In this case F1 = (0 - mu)/ t1 = -mu/t1
If fielder increases the time (say t2) i.e. t2 > t1 , the force will reduce
i.e. F2 = - mu/ t2 and F2 < F1
Force and Laws of Motion
Q 1.
While driving on a highway, an insect strikes on the car windshield and splatters. Which experiences greater impact (force): an insect or the windshield?
Q 2.
What is the total momentum of a bullet and a gun before firing?
Q 3.
A car weighs 1500 kg.Its speed time graph is shown in Fig. Calculate the force required at the end of 5 seconds to bring the car to a stop within one second.
Q 4.
On what factor does inertia of a body depend ?
Q 5.
Can balanced forces change the shape and size of an object?
Q 6.
Can balanced forces change the shape and size of a body ?
Q 7.
Do action-reaction forces produce the same magnitude of acceleration ?
Q 8.
Fill in the blanks :
The study of motion of a body under the action of a force is called ______.
Q 9.
Using second law of motion, derive the relation between force and acceleration.
Q 10.
Are action-reaction forces simultaneous ?
Q 11.
What is the unit of force in c.g.s system?
Q 12.
Can every force produce motion in every object?
Q 13.
In the above question, which body is damaged more ?
Q 14.
Fill in the blanks :
The momentum of an object is the product of its _____ and _______ and has the same direction as that of the ________.
Q 15.
Honey and Sunny are two friends.Both are financially well off.Honey is fond of driving an SUV while Sunny enjoys driving Tata Nano.Honey explains to Sunny the risks of driving a light vehicle and Sunny explains to Honey the cost factor.
Read the above passage and answer the following questions :
(i) With whom do you agree,with Honey or with Sunny?
(ii) Explain, why?
(iii) Which values of life do you learn from this discussion?
Q 16.
A TV set is lying on a table. The TV set experiences a gravitational force (pull) downwards (FW) by the Earth. It also experiences a force on it due to table in contact i.e. (FTable-TV) in upward direction. Do these forces form an action-reaction pair? Do these forces follow Newton's third Law of motion?
Q 17.
It is difficult to balance our body when we accidentally slip on a peel of banana. Explain why?
Q 18.
When a branch of a tree is shaken, some of the fruits may fall down.Why ?
Q 19.
A hammer of mass 500 g, moving at 50 m/s, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer ?
Q 20.
State Law of conservation of momentum.
Q 21.
From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m/s. Calculate the initial recoil velocity of the rifle.
Q 22.
A man pushes a box of mass 50 kg with a force of 80 N.What will be the acceleration of the box ? What would be the acceleration if the mass were halved ?
Q 23.
A feather of mass 10 g is dropped from a height.It is found to fall down with a constant velocity.What is the net force acting on it ?
Q 24.
An object experiences a net zero external unbalanced force.Is it possible for the object to be travelling with a non-zero velocity ? If yes,state the conditions that must be placed on the magnitude and direction of the velocity.If no,provide a reason.
Q 25.
What acceleration is produced by a force of 5 N acting on a body of mass 20 kg ?
Q 26.
Fill in the blanks :
When the playing card is flicked with the finger the coin placed over it falls in the tumbler. It is due to ____________.
Q 27.
Fill in the blanks :
When a hanging carpet is beaten with stick, the dust particles start coming out of it. It is due to ______________.
Q 28.
For how long should a force of 100 N act on a body of mass 20 kg so that it acquires a velocity of 100 m/s ?
Q 29.
In the above question,which body undergoes greater change in momentum ?
Q 30.
Show that kg-
and N-s represent the same quantity.Which is that quantity ?
Q 31.
Explain why a player lowers his hands while catching a cricket ball.
Q 32.
Give atleast four applications of Newton’s second law of motion.
Q 33.
A body of mass 25 kg has momentum of 125 kg m/s. What is the velocity of the body?
Q 34.
Define 1N (Newton).
Q 35.
A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km/h. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.
Q 36.
Fill in the blanks :
The natural tendency of objects to resist a change in their state of rest or of uniform motion is called ______.
Q 37.
Fill in the blanks :
When we shake a branch of a mango tree the mangoes fell down, this happens because of ___________.
Q 38.
Fill in the blanks :
When a car enters a curved path, the person sitting in it is thrown outwards. This is because of the property of ______________.
Q 39.
Fill in the blanks :
On what factor does the inertia of a body depends? ____________
Q 40.
Fill in the blanks :
When a bus starts suddenly, the bus passengers standing in the bus tend to fall backwards. It is due to _______________.
Q 41.
Fill in the blanks :
________ is the tendency of an object to resist a change in motion.
Q 42.
What are balanced forces?
Q 43.
Define force of friction.
Q 44.
Why do we tend to fall outwards when a car takes a steep turn?
Q 45.
When a body moves on flat friction-less surface, will its speed change?
Q 46.
Is force a scalar or vector quantity?
Q 47.
Name the principle on which a rocket works.
Q 48.
What is the momentum of a man of mass 75 kg when he walks with a velocity of 2 m/s ?
Q 49.
A certain force exerted for 1 -2 s raises the speed of an object from 1.8 m/s to 4.2 m/s. Later,the same force is applied for 2 second. How much does the speed change in 2 s ?
Q 50.
Two hockey players of opposite teams,while trying to.hit a hockey ball on the ground collide and immediately become entangled.One has a mass of 60 kg,and was moving with a velocity 5.0 m/s,while the other has a mass of 55 kg and was moving faster with a velocity of 6.0 m/s towards the first player,In which direction and with what velocity will they move after they become entangled ? Assume that the frictional force acting between the feet of the two players and ground is negligible.
