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Balanced forces are those forces which when acting on a body will not change the state of rest or uniform motion. It occurs when two or more forces acting on a body canceled out each other effect resulting in zero net force on the body.
- Balanced forces are responsible for a body to remain in an equilibrium position.
- The vector sum of all the forces acting on a body is zero when the body is subjected to balanced forces.
- A book on a table, a hanging object, or a car moving with uniform velocity are examples of balanced force.
Unbalanced forces are those which change the condition of equilibrium of the body. A body at rest will start to move and a body moving with uniform velocity start accelerating when subjected to unbalanced force and the vector sum of all the forces acting on a body will not be zero.
Very Short Answers Questions [1 Mark Questions]
Ques. What are balanced forces?
Ans. Balanced forces are those that when acting on a body do not change the state of rest or uniform motion of a body. When the sum of all the forces acting on a body becomes zero then these forces are known as balanced forces.
Ques. What happens to a body when balanced forces are acted on it?
Ans. When balanced forces are acted on a body then the body will remain in its state of rest or uniform motion.
Ques. A man at rest in the middle of an ice pond. If the ice is perfectly smooth, then he can get started himself to move the shore by making use of
- Newton’s first law of Motion
- Newton’s second law of Motion
- Newton’s third law of Motion
- First and third law of Motion
Ans. The correct option is c. Newton’s third law of motion.
Explanation: Using Newton’s third law of motion, when he throws something opposite to the direction of the shore then due to this action he will get the reaction opposite to it, which will help him to start moving towards the shore.
Ques. How will you determine whether a force is a balanced force or not?
Ans. When a number of forces are acting on an object, then if the vector sum of all the forces acting on that object is zero then the forces are said to be balanced and the object will not charge its state of rest or state of uniform motion. And if the vector sum of all the forces is not zero, then the forces are said to be unbalanced.
Ques. Give three examples of balanced forces.
Ans. Examples of balanced forces are
- A book resting on a table
- A car moving with a uniform speed
- A hanging object on the roof ceiling.
Short Answers Questions [2 Mark Questions]
Ques. What are the characteristics of balanced forces?
Ans. The characteristics of balanced forces are as follows
- When the forces acting on a body are balanced, then the vector sum of all the forces is zero.
- A body subjected to balanced forces has zero acceleration.
- There is no change in the state of rest or uniform motion on the body under the effect of balanced forces.
Ques. Give 3 examples of unbalanced forces.
Ans. Examples of unbalanced forces are
- An accelerating car
- Pushing a block of mass
- Launching a rocket
Ques. State Newton’s first law of motion.
Ans. According to Newton’s first law of motion, if a body is at rest it will remain at rest or if a body is moving with a constant velocity in a particular direction, it will keep moving with constant velocity in that direction, until an external force is applied to change its state of rest or uniform motion.
Ques. What are balanced forces equivalent to?
Ans. When two or more forces acting on a body are balanced, then the magnitudes and directions are canceled out by each, which gives a zero net force. Hence, balanced forces are equivalent to a state of equilibrium. In this state, if the body is initially at rest, it will remain at rest or if the body initially moving with uniform velocity, it will keep moving with this velocity.
Long Answers Questions [3 Mark Questions]
Ques. Two equal forces of magnitude 50 N each act on a body in opposite directions. Are the forces acting on the body balanced?
Ans. Let the two forces on the body be
- F1 = 50 N towards the +ve x-axis
- F2 = 50 N towards the -ve x-axis
Since the two forces are in opposite directions, the net force acting on the body is given by
F = 50 - 50 = 0
Hence the net force acting on the body is equal to zero, therefore the forces acting on the body are balanced.
Ques. The floating of objects is an example of balanced force, Explain.
Ans. When an object is placed in water or any other liquid two types of forces are acting on it i.e. buoyant force in a vertically upward direction and weight of the object in the vertically downward direction.
For the floating of an object, the magnitude of buoyant force must be equal to the weight of the object, which results in zero net force. Therefore floating of objects is due to the balanced forces acting on the object.
Ques. A cyclist traveling at a speed of 20m/s has a forward force of 300N but is acted on by air and friction resistance of 300N which pushes against her. What will happen to the cyclist’s motion?
Ans. The force of 300 N exerted by air and friction which opposes the motion of the cyclist is balanced by the forward force exerted by the cyclist of magnitude 300 N. Both the force are of equal magnitude but acted opposite to each other, hence they canceled out each other. Therefore the net force acting on the cyclist is zero and the cyclist will keep traveling at a constant speed of 20 m/s.
Ques. How does the acceleration of a body change when balanced forces are acting on it?
Ans. The sum of all the force of balanced forces is zero. According to Newton’s second law of motion
F = ma
Where F is the net force acting on the body, m is the mass, and a is the acceleration produced on the body.
If the net force acting on a body is balanced, then F = 0
⇒ a = 0
Therefore the acceleration of the body is zero if balanced forces are acting on the body.
Very Long Answers Questions [5 Mark Questions]
Ques. Two blocks A and B of masses m and 2m respectively are connected by a massless spring of stiffness K. At the instant shown, the force F acting on the B and the acceleration of A at this instant is ‘a’ towards left. Find the acceleration of B and the extension in the spring at this instant.
Ans. Since block A is having an acceleration towards the left, there must be compression in the spring at the instant.
This will exert a force on A towards the left. Producing the given acceleration ‘a’. Let the compression be x.
The free-body diagram of the two blocks is shown below
From the free-body diagram of body A, we have
Kx = ma ….(i)
Where
- a is the acceleration of body A
- K is the stiffness of the spring
From the free-body diagram of body B, we have
F + Kx = 2m aB
Where aB is the acceleration of body B
Using equation (i), we get
F + ma = 2m aB
⇒ aB = (F + ma) / 2m
Therefore, the acceleration of body B is (F + ma) / 2m.
If the spring is massless, then tension is equally transmitted at the two ends of the spring.
Ques. What are the differences between balanced and unbalanced forces?
Ans. Followings are the difference between balanced and unbalanced forces
| Balanced Forces | Unbalanced Forces |
|---|---|
| Two forces acting on a body are said to be balanced if they are equal in magnitude but opposite in direction. | Two forces acting on a body are said to be unbalanced forces if they are not equal in magnitude, giving a resultant net force. |
| The net force acting on a body is zero, if the body is under balanced forces. | The net force acting on a body is non-zero if the body is under unbalanced forces. |
| A body subjected to balanced forces does not change its state of rest or uniform motion. | Unbalanced forces subjected to a body change its state of rest or uniform motion. |
| A man standing on the floor, a floating object on water, and a car moving with constant velocity are examples of balanced forces. | Falling objects, an accelerating car, and kicking a football are examples of unbalanced forces. |
Ques. Two forces of magnitude 10 N and 8 N act on a body in opposite directions. What is the magnitude of net force? Are the forces acting on the body balanced?
Ans. Let the two forces on the body be
- F1 = 10 N towards the +ve x-axis
- F2 = 8 N towards the -ve x-axis
The magnitude of the net force acting on the body is given by
F = \(\sqrt{F_1^2+F_2^2+2F_1F_2cos \theta}\)
Where θ is the angle between F1 and F2.
Since the two forces are in opposite directions, therefore θ = 180o
F = \(\sqrt{F_1^2+F_2^2+2F_1F_2cos 180}\) = \(\sqrt{F_1^2+F_2^2+2F_1F_2}\) = F1 – F2
⇒ F = 10 - 8 = 2 N
Hence, the magnitude of the net force acting on the body is 2 N.
Since the net force acting on the body is not equal to zero, therefore the forces acting on the body are not balanced.
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