CBSE-IX-Science
03: 2-Forces and Laws of Motion
- #Science Forces and Laws of Motion part 2
- Qstn #1An 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.Ans : No, it is not possible for the object to be travelling with a non-zero velocity if an object experiences a net zero external unbalanced force since unbalanced forces cannot be equal to zero.
- Qstn #2When a carpet is beaten with a stick, dust comes out of it. Explain.Ans : When a carpet is beaten with a stick, dust comes out of it because carpet fibres vibrate in forward and backward direction as carpet is beaten but the loosely bound dust particles due to inertia remain at rest and so they come out.
- Qstn #3Why is it advised to tie any luggage kept on the roof of a bus with a rope?Ans : It is advised to tie any luggage kept on the roof of a bus with a rope because when bus moves the luggage also gets moving with the velocity same as that of the bus and in the same direction but when bus changes direction or deaccelerates, due to inertia of motion luggage moves in the same direction and may get thrown away from roof of buses.
- Qstn #4A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because
-(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion.
(d) there is no unbalanced force on the ball, so the ball would want to come to rest.
Ans : (c) there is a force on the ball opposing the motion.
- Qstn #5A 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.)Ans : According to the question,
initial velocity of truck (u) = 0
distance = s= 400 m and time = 20 s
mass of truck = 7metric tones = 7000kg
400 =0 + 200a
400 = 200a
a = 400/200
therefore,
= 14000 N
- Qstn #6A stone of 1 kg is thrown with a velocity of
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?Ans : since
(object comes to rest so v=0)
-100a = 400
a = 400/-100 =
therefore, the force of friction between the stone and the ice
= -4 N
- Qstn #7A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate:
- #7-athe net accelerating force;Ans : The net accelerating force = Force exerted by engine - frictional force of track = 40000 - 5000 = 35000 N
- #7-bthe acceleration of the train; andAns : the acceleration of the train = a = F/m =
= 35000/10000 = 
- #7-cthe force of wagon 1 on wagon 2.Ans : the force of wagon 1 on wagon 2
Wagon 1 will have to exert force on all 4 wagons next to it
so mass of other 4 wagons =
= 8000 kg
F = ma =
= 28000 N
- Qstn #8An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of
?Ans : since
= -2550 N (negative sign symbolises acceleration in opposite direction)
- Qstn #9What is the momentum of an object of mass m, moving with a velocity v?
-(a)
(b)
(c)
(d) mv
Ans : (d) mv
- Qstn #10Using 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?Ans : N
- Qstn #11Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is
before the collision during which they stick together. What will be the velocity of the combined object after collision?Ans : Momentum before collision took place =
= 0
Since the objects stick together after collision hence
momentum after collision
= 3v
momentum before collision = momentum after collision
0 = 3v, v= 0/3 = 0
the velocity of the combined object after collision (v)= 0