Question:
According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the road side, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
Answer:
Two cases arise here:
Case I: No external force is applied. In this case, the truck weight (gravitational force) acting downwards, is cancelled by the contact force applied on the truck by the ground.
i.e.
FWeight = - Fground
Case II: If a person or group of persons pushes truck, the static friction acting on the truck horizontally, cancels this push-force. Static friction is a self-adjusting force. More push is applied, more static friction will oppose it (upto a limit) and forces are balanced. In order move the truck, the push-force must overcome static friction.
i.e. FWeight = - Fground
and FPush = - FFriction
Force and Laws of Motion
Q 1.
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 2.
A bullet of mass 10 g travelling horizontally with a velocity of 150 m/s strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.
Q 3.
A force of 5 N gives a mass
an acceleration of 10
,and a mass
,an acceleration of 20
.What acceleration would it give if both the masses were tied together ?
Q 4.
What would be the force required to produce an acceleration of 2
in a body of mass 12 kg ? What would be the acceleration if the force were doubled ?
Q 5.
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 6.
A body of mass 1 kg is kept at rest.A constant force of 6-0 N starts acting on it.Find the time taken by the body to move through a distance of 12 m.
Q 7.
A bullet of mass 50 g is fired from a gun of mass 6 kg with a velocity of 400 m/s.Calculate the recoil velocity of the gun.
Q 8.
A hockey ball of mass 200 g travelling at 10 m/s is struck by a hockey stick so as to return it along its original path with a velocity of 5 m/s.Calculate the change in momentum of the hockey ball by the force applied by the hockey stick.
Q 9.
Can balanced forces move a body at rest ?
Q 10.
Do action-reaction forces act on the same body ?
Q 12.
Define momentum of a body.
Q 13.
Derive the relation F = ma,where the symbols have their usual meanings.
Q 14.
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 15.
Fill in the blanks :
An umbrella protects us from rain it is based on the property of _____________.
Q 16.
Fill in the blanks :
It is dangerous to jump out of a moving bus because due to _________ one falls forward.
Q 17.
Fill in the blanks :
An athlete often jumps before taking a long jump. It is due to ________________.
Q 18.
Which principle is involved in the working of a jet plane?
Q 19.
Which would require a greater force : accelerating a 2 kg mass at 5
or a 4 kg mass at 2
?
Q 20.
Derive the unit of force using the second law of motion. A force of 5 N produces an acceleration of
on a mass
and an acceleration of 24
on a mass
.What acceleration would the same force provide if both the masses are tied together ?
Q 21.
What is the SI unit of force ?
Q 22.
Name any two applications of the law of conservation of linear momentum.
Q 23.
State Newton's First Law of motion.
Q 24.
What are balanced and unbalanced forces?
Q 25.
State Newtons second law of motion.
Q 26.
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 27.
Force can change the shape and size of an object. Give an example.
Q 28.
Why does a bicycle slow down when the cyclist stop pedalling?
Q 29.
Which Newton's law is also known as the Law of Inertia?
Q 30.
Define 1 Newton force.
Q 31.
What do you mean by a resultant force?
Q 32.
Two objects of masses 100 g and 200 g are moving along the same line in the same direction with velocities of 2 m/s and 1 m/s,respectively.They collide and after the collision, the first object moves at a velocity of 1.67 m/s in the same direction.Determine the velocity of the second object
Q 33.
What are balanced forces ?
Q 34.
What is the other name for Newton's first law of motion ?
Q 35.
What is the SI unit of momentum ?
Q 36.
What is the total momentum of the bullet and the gun on firing ?
Q 37.
In the above question,which body undergoes greater change in momentum ?
Q 38.
A person in a bus tends to fall backwards, when it starts suddenly ?
Q 39.
Show that kg-
and N-s represent the same quantity.Which is that quantity ?
Q 40.
What is force ? What is inertia ? How is inertia measured ?
Q 41.
Why does a gun recoil on firing ? Obtain an expression for recoil velocity of gun.
Q 42.
Explain briefly any three applications of Newton’s third law of motion.
Q 44.
What are non-contact forces? Give examples.
Q 45.
A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 metric tonnes. (Hint: 1 metric tonne = 1000 kg.)
Q 46.
How Newton's Second law of motion is different from First law?
Q 47.
When two objects act on each other, does action-reaction force pair cancel out each other?
Q 48.
Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?
Q 49.
What is collision? What are the types of collision?
Q 50.
Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 m/s before the collision during which they stick together. What will be the velocity of the combined object after collision?
