ADDITIONAL QUESTIONS:
NUMERICAL EXAMPLES:
1.A ball of mass 200 g moving with a velocity of 4 m/s is stopped by a player in 0.2 seconds. Calculate the force applied by the player to stop the ball.
Given the mass ‘m’ of the ball = 0.2 kg, initial velocity ‘u’ of the ball = 5m/s,
time’t = 0.2 second, final velocity ‘v’ of the ball = 0m/s.
Find the force.
Ans: F = m (v−u)/t = 0.2(0 − 4/0.2 )= − 4N
Negative sign indicates that the force is applied opposite to the direction of motion.
2.A car of mass 1000 kg moving with a velocity of 72 km/h collides against a tree and comes to rest in 4 seconds. Calculate the force exerted by the tree on the car.
Ans: Given the mass ‘m’ of the car = 1000 kg
The initial velocity u of the car = 72 km/h =72 x 5/8 = 20 m/s,
Final velocity of the car = 0 m/s time t = 4s,
So to find F ⇒ F= m (v−u)/t ⇒ 1000(0−20) /4 = −5000 N
Negative sign indicates that the force is applied opposite to the direction of motion.
3. The velocity of an object of mass 10 kg changes from 5 m s -1 to 20 ms -1 in 1minute, on the application of a constant force. Calculate the magnitude of the force.
Ans: F= m (v−u)/t = 10(20−5)/60 = 150/60 = 2.5 N.
4. In order to move a 5 kg object from rest, a constant force of 2N is applied for 4 minutes. Calculate the velocity of the object at the end of 4 minutes.
Ans: F = m (v−u)/t ∴ v =Ft/m + u = 2 x 240/5 + 0 = 96 ms-1
5. A roller of mass 100 kg moving with a velocity of 4 ms-1 strikes a wall and stops. If the impact with the wall lasts for 2 s, calculate the force exerted by the roller on the wall during the impact.
Ans: F = m (v−u)/t = 100(0−4)/2 = − 200 N
6. A ball of mass 200 g and moving with a velocity of 5 ms comes to rest after it collides with another stationary ball of mass 40 g. Calculate the velocity with which the second ball will travel.
Ans: m1 v1 = m2v2 ⇒ 200x 5 = 40 x V ⇒ 200 x 5/40 = 25 ms-1
7. A bat of mass 4 kg strikes a ball at rest. The mass of the ball is 250 g. After the bat strikes the ball it travels with a velocity of 20 m/s. find the velocity with which the bat strikes the ball.
Ans: By the law of conservation of momentum, Initial momentum = Final momentum
Initial momentum= (Mass of bat x velocity of bat) + (Mass of ball x velocity of ball
= (4 x velocity of bat) + (0.25 x 0) = (4 x velocity of bat)
Final momentum= (4 x 0) + (0.25 x 20) = (0.25 x 20)
⇒ (4 x velocity of bat) = (0.25 x 20)
Velocity of bat= (0.25 x 20)/4 = 1.25 ms -28.
8. A bullet of mass 10 g moving with a velocity 400 m/s strikes a freely suspended ball of mass 490 g and gets embedded in it. Calculate velocity of the ball with the bullet embedded in it.
Ans: Momentum of the bullet and ball before striking = (0.01 x 400) +0 = 4 kgms-1
Mass of ball with bullet embedded in it = 0.490+ 0.010.5 Kg
Momentum of ball and bullet after striking = 0.5 x V
By the Law of Conservation of momentum
Initial momentum = final momentum⇒ 4 = 0.5 x V ⇒ V = 4/0.5 = 8 ms-1
9. A car A of mass 1200 kg travelling at 20 m/s collides with another car B of mass 1800 kg travelling at 15 m/s. After the collision, the velocity of car B becomes 20m/s.
Calculate
A.The momentum of car A before collision,
B. The momentum of car B before collision
C. The momentum of car B after collision
D. Velocity of car A after collision
Ans:
A. Momentum of car A before collision = mA XVA= 1200 x 20 = 24000 kg m/s
B. Momentum of car B before collision = mB XVB = 1800 x 15 = 27000 kg m/s
C. The momentum of car B after collision= 1800x 20 = 36000 kg m/s
D. Velocity of car A after collision. Initial momentum= final momentum
24000+ 27000 = 36000 + 1200 x V ⇒ 51000− 36000 = 1200V
15000 = 12.5 m/s
V= 1200
Answer the following questions :
1. The factor on which the inertia of a body depends.
Ans: Mass.
2.The factors on which the momentum of a body depends.
Ans: Mass and velocity.
3. The two factors on which magnitude of force depends.
Ans: Mass and acceleration.
4. Explain the following in brief.
a) First Law of Motion.
Ans: An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.
b) The effects of force on the motion of an object.
Ans: Force changes the magnitude of velocity. Force changes the direction of motion.
c) Any two illustrations/examples of inertia.
Ans: A standing passenger falls backward when a bus begins to move
suddenly. When a fast moving striker strikes a pile of carrom coins, only the bottom coin is removed from the pile.
d) Second Law of Motion.
Ans: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.
e) The SI unit of momentum.
Ans: kg ms-1 (kilogram metre per second)
f).The unit of force.
Ans: kg ms-2 or Newton
g). Third Law of Motion.
Ans: When one object exerts a force on another object, the second object instantaneously exerts a force back on the first. OR for every action there is an equal and opposite reaction.
5. Give any two examples of the Third Law of Motion.
Ans: When a bullet is fired from a gun the bullet moves forward and the gun recoils back. When the water is driven backwards by the oars, the boat moves forward.
6. State the L.aw of Conservation of Momentum.
Ans:. During a collision the sum of the momentum of two objects before collision is equal to the sum of their momentum after collision.
7. State the effects of force on an object.
Ans: Force changes the magnitude of velocity of an object, changes the direction of its motion, changes the shape and size of the object.
8. State the characteristics of the force of friction.
Ans: The force of friction – arises between the surfaces in contact. It acts in a direction opposite to the direction of motion.
9. Define the following:
a).Inertia.
Ans: Inertia is the tendency of undisturbed objects to stay at rest or to keep moving with the same velocity. OR Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.
b).Momentum.
Ans: Momentum is the product of mass and velocity.
c).Unit force.
Ans:Unit force is that force which produces unit acceleration of 1 m s-2 in an object of unit mass of 1 kg.
d) Newton
Ans: A Newton is that force which produces an acceleration of 1 ms-2 in an object of mass 1 kg.
10.When a marble or a ball is rolled on the ground, it moves forward but its speed gradually decreases and finally it comes to rest. Why.
Ans: When a marble is rolled on the ground, the force of friction between the ground and the marble opposes the motion of the marble. Hence its speed gradually decreases and finally it comes to rest.
11. What is the effect of two balanced forces acting on a body.
Ans: (i) The shape of the body may be changed. (ii) The size of the body may be changed.
12. When are two forces said to be balanced forces.
Ans:. The two forces are said to be balanced when they are equal magnitude but acting in opposite directions.
13. What are the characteristics of the force of friction.
Ans: The force of friction arises between the two surfaces in contact. It is a self-adjusting force up to a certain limit. It acts in a direction opposite to the direction of motion i.e. it opposes
14. The momentum of a uniformly accelerated body constantly increases .Why.
Ans: Momentum is the product of mass and velocity. When a body is uniformly accelerated its velocity increases uniformly. Therefore its momentum constantly increases. uniformly accelerated its velocity increases uniformly. Therefore its
15. Action and reaction are equal and opposite forces. Still they are not balanced forces. Justify the motion of one body over the other.
Ans: Balanced forces are equal and opposite forces simultaneously acting on the same body. But action and reaction are equal and opposite forces which act on different objects. Hence they are not balanced forces.
16. Describe a simple experiment/ activity to verify the Third Law of Motion.
Ans: Fasten a spring balance to a table or a wall. Attach the hook of another spring balance to the hook of the first. Pull the second balance steadily. Observe the readings on both the balances. The readings are found to be the same. This shows that the action of pulling is equal to the reaction offered by the wall or the table.
17. Why is the first law of motion called law of inertia.
Ans: The first law of motion specifies that a body at rest shall continue to be at rest or a body in motion shall continue to be in motion unless an external unbalanced force acts on it. Thus a body resists the change of state. Hence it is called the law of inertia.
18. When a fast moving striker strikes a pile of carrom coins, only the bottom coin is removed from the pile. Give a reason.
Ans: When a fast moving striker strikes a pile of car Tom coins, the lowest coin, against which the striker strikes, starts moving while the upper coins are still at rest due to inertia.
19. An athlete running a race runs a certain distance after crossing the finish’ line. Why.
Ans:. An athlete running a race runs a certain distance after crossing the finish’ line due to inertia. When he crosses the finish’ line he is in motion and continues to be in that state for some distance till he slows down and finally comes to rest.
20. Why are cars provided with safety belts for the passengers.
Ans:When a moving car suddenly stops, the passengers continue to move due to inertia and collide against the front panel. Safety belts exert an opposite force on the passengers thereby slowing down the forward motion. This prevents or minimizes the damages caused due to collision with the front panels.
21. A person carelessly getting down from a moving bus falls forward. Why.
Ans:When a person gets down from a moving bus, his feet come to rest because they come in contact with the ground but the upper part of his body is still in motion due to inertia. Therefore he falls forward.
22. A greater force is required to produce motion in a car at rest than to produce motion in a bicycle at rest. Give a reason.
Ans: Greater the mass, greater is the inertia. A car is much heavier than a bicycle. Therefore more force is required to produce motion in a car than in a bicycle.
23. If a table tennis ball and cricket ball travelling at the same speed hit a person which of the two balls will cause more injury. Justify your
Ans: The cricket ball will cause more injury. Impact produced by an object depends upon momentum which is the product of mass and velocity. The tennis ball and cricket ball move at the same speed. But the mass of the cricket ball is more than that of the table tennis ball. Hence the cricket ball causes more injury.
24. In a cricket match, when the fielder catches a moving ball, he moves his hand backwards in the direction of the ball. Why.
Ans: When a fielder moves his hand backwards along with the ball he increases the time of bringing the ball to zero velocity. Force is equal to the product of mass and acceleration and acceleration is the rate of change of velocity. When the time required to change the velocity of a moving ball is increased, the acceleration is decreased and therefore less force is required to stop the ball.
25. Why do shock absorbers of scooters, motorcycles, and so on have springs.
Ans: When scooters, motorcycles, and so on run on uneven roads, they receive jerks. The springs of the shock absorbers increase the time of the jerk and in doing so they reduce the force that the rider may face due to the jerk.
26. The seat covers of scooters and motorcycles are made of sponge. Give a reason.
Ans:When scooters, and motorcycles, run on uneven roads, they receive jerks. The sponge of the seat increases the time of the jerk and in doing so they reduce the force that the rider may face due to the jerk.
27. When a person swims he pushes the water backwards. Why.
Ans: Pushing the water backwards during swimming is the action The reaction is, he gets a forward motion. Therefore water is pushed backwards during swimming.
28. Why does a gun recoil backward when a bullet is fired.
Ans:Before the bullet is fired from the gun, the sum of their momenta is zero. When the bullet is fired, the bullet gains momentum, by the law of conservation of momentum, the gun should also gain equal negative momentum so that the sum of the momenta after firing also remains zero. Therefore the gun recoils.
29. What are the characteristics of the force of friction.
Ans: The force of friction arises between two surfaces in contact. It is a self-adjusting force up to a certain limit. It acts in a direction opposite to the direction of motion.
30. Explain how the Second Law of Motion can be used to define unit of force.
Ans: Suppose an object of mass m moves along a straight line with an initial velocity ‘u’.
A constant force ‘F accelerates it for’t seconds as a result its velocity changes to v’.
Therefore the rate of change of momentum = m (v-u)/t = ma. By the second law, applied force ‘F is proportional to the rate of change of momentum.
Therefore F ∝ ma or F= kma where “k is the constant of proportionality
If we define unit force as that force which produces unit acceleration
(1 ms -2) in an object of unit mass (1 kg), then from the formula F= kma, we get
1 unit force = k x (1 kg) x (1ms -2)
Therefore, k becomes equal to 1
1 unit force = (1 kg)x (1 ms -2 )= 1 kg ms-2
Thus, the Second Law of motion can be used to define unit force.
31. What is force of friction.
Ans: Force of friction is a force that develops between the surfaces in contact which oppose the motion of one body over the other.