9th Class Physics Chapter 3 Constructed response questions
9th Class Physics Chapter 3 Constructed response questions
3.1 Two ice skaters weighing 60 kg and 80 kg push off against each other on a frictionless ice track. The 60 kg skater gains a velocity of 4 metre per second. Considering all the relevant calculations involved, explain how Newton’s third law applies to this situation.
According to Newton’s third law of motion both the skaters will apply equal and opposite forces on each other.
Law of conservation of momentum
Initial momentum = Final momentum
0 = (60)(4) + (80)(v)
-80v = 240
v= -3 meter per second
The negative sign shows that the 80 kg skater will move in opposite direction with a speed of 3 meter per second.
Hence Newton’s third law tells us that the lighter skater accelerates more and will gain a greater velocity but the heavier skater accelerates less and will gain smaller velocity.
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3.2 Inflatable airbags are installed in the vehicles as safety equipment. In terms of momentum, what is the advantage of airbags over seatbelts?
When suddenly vehicle stops, the passenger’s momentum will change to zero. Airbags increase the time over which the momentum changes.
This increase in time will reduce the force because rate of change of momentum is equal to the applied force. Hence airbags reduce the force on the passenger and provide better protection.
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3.3 A horse refuses to pull a cart. The horse argues, ” according to Newton’s third law, whatever force I exert on the cart, the cart will exert an equal and opposite force on me. Since the net force will be zero, therefore, I have no chance of accelerating (pulling) the cart.” What is wrong with this reasoning?
The horse is confused. the force exerted by the horse on the cart and the force exerted by the cart on the horse act on different bodies. The horse pushes the ground backward and the ground exerts a forward force on the horse. Therefore the net force on the system will not be zero.
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3.4 When a cricket ball hits high, a fielder tries to catch it. While holding the ball he/she draws hands backward. Why?
When the ball is caught, momentum changes to zero. By drawing the hands backward, the fielder increases the time taken to stop the ball. This increase in time will reduce the force because rate of change of momentum is equal to the applied force. This prevents injury.
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3.5 When someone jumps from a small boat onto the river bank, why does the jumper often fall into the water? Explain.
Due to Newton’s third law of motion the boat exerts a backward force on the person. This makes the boat move and reduces stability, so the person may fall into the water.
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3.6 Imagine that if friction vanishes suddenly from everything, then what could be the scenario of daily life activities?
If friction vanishes suddenly from everything, daily life will be completely disturbed. Because holding, driving and walking will be impossible.
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