Question:
Two persons manage to push a motorcar of mass 1200 kg at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of 0.2m s-2. With what force does each person push the motorcar?
(Assume that all persons push the motorcar with the same muscular effort.)
Answer:
Let each person applies force = F N
Initially car is moving with uniform speed. When two persons apply force, there is no change in state, car moves with same uniform speed. It means the force of the two persons is cancelled by the kinetic friction of the car.
⇒ 2F + Fkinetic-friction = 0 ... (I)
When the third person applies there occurs change in state, due to which acceleration (a) occurs.
⇒ 3F + Fkinetic-friction = ma
where m is the mass of the car = 1200 kg and a is acceleration = 0.2 m s-2.
Using eq. I, we get
F = ma = 1200 × 0.2 = 240N
∴ Each person applies 240N force.
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.
Can balanced forces change the shape and size of an object?
Q 5.
Can balanced forces change the shape and size of a body ?
Q 6.
On what factor does inertia of a body depend ?
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.
Are action-reaction forces simultaneous ?
Q 10.
What is the unit of force in c.g.s system?
Q 11.
Using second law of motion, derive the relation between force and acceleration.
Q 12.
Can every force produce motion in every object?
Q 13.
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 14.
In the above question, which body is damaged more ?
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.
When a branch of a tree is shaken, some of the fruits may fall down.Why ?
Q 18.
It is difficult to balance our body when we accidentally slip on a peel of banana. Explain why?
Q 19.
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 20.
What acceleration is produced by a force of 5 N acting on a body of mass 20 kg ?
Q 21.
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 22.
State Law of conservation of momentum.
Q 23.
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 24.
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 25.
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 26.
Show that kg-
and N-s represent the same quantity.Which is that quantity ?
Q 27.
Explain why a player lowers his hands while catching a cricket ball.
Q 28.
Define 1N (Newton).
Q 29.
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 30.
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 31.
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 32.
What are balanced forces?
Q 33.
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 34.
A stone of 1 kg is thrown with a velocity if 20
across the frozen surface of a lake and comes to rest after travelling a distance of 50 m.What is the force of friction between the stone and the ice ?
Q 35.
Can balanced forces move a body at rest ?
Q 36.
The masses of two objects are 2 kg and 20 kg. Which has greater inertia ?
Q 38.
Explain what is meant by momentum of a body.Is it a scalar or a vector ? What are its units ?
Q 39.
Discuss briefly Galileo's ideas about motion.
Q 40.
Why does a gun recoil on firing ? Obtain an expression for recoil velocity of gun.
Q 41.
Give atleast four applications of Newton’s second law of motion.
Q 42.
What a force can do?
Q 43.
Fill in the blanks :
Newton's second law of motion states that the rate of change of ______ of a body is directly proportional to the force and takes place in the same direction as the ______.
Q 44.
Fill in the blanks :
Which has more inertia of Cricket ball or a rubber ball of the same size? __________
Q 45.
Fill in the blanks :
A cyclist on a levelled road does not come to rest immediately even he stops pedalling. It is due to ______________.
Q 46.
Fill in the blanks :
________ is a measure of intertia.
Q 47.
Why does a bicycle slow down when the cyclist stop pedalling?
Q 48.
What are unbalanced forces?
Q 49.
When a force acts on a body, what are the two essential effects it can produce?
Q 50.
Define force of friction.
