Types of Forces
Category : 8th Class
Types of Forces
There are different types of forces:
(i) Applied force (ii) Gravitational force
(iii) Normal force (iv) Frictional force
(v) Tension force (vi) Muscular force
(vii) Electrostatic force (viii) Magnetic force
Applied force
It is the force, which is applied to an object by a person or by another object. If a person is pushing a bed across the room, there is a applied force acting on the bed.
Gravitational Force
The force, which exists between any two planets, or any two objects in this universe, is called gravitational force. This force always attracts the object towards each other. All object on the surface of earth experience a force of gravity towards the centre of earth, which is equivalent to the weight of the object. Numerically it is given by,
\[F=m\times g\]
Here, m is the mass of the object and g is the acceleration due to the gravity.
The numerical value of g is 9.8 m/sec2.
Two objects of mass 10 kg and 35 kg are 50 m apart from each other. The force acting between two objects at any instant of time is _________.
(a) 2x 10–11 N
(b) 1.2x 10–11 N
(c) 2.2x 10–11 N
(d) 0.934 x 10–11 N
(e) None of these
Answer: (d)
Explanation
The net force between any two objects can be calculated by using the relation, \[F=\frac{G\times M\times m}{{{R}^{2}}}\]. On calculation it was founded that its value is 0.934 x 10–11N.
According to the universal law of gravitation, the value of gravitational constant remains same at all places in this universe. But, the acceleration due to the gravity changes from one place to another on the surface of the planet. The reason for this change in the value of acceleration due to gravity on the planet is________.
(a) Volume
(b) Density
(c) Mass
(d) Surface area
(e) None of these
Answer: (c)
Explanation
The acceleration due to the gravity depends on the mass of the planet.
Normal Force
The normal force is the force, which is exerted upon an object in contact with another stable object. For example, if a plate is resting on the table, the surface of the table exerts an upward force on the plate in order to support the weight of the plate. A normal force acts horizontally between two objects, that are in direct contact with each other. For instance, if a person leans against a wall, the wall will apply a horizontal force on the person.
.
A car moving on a slope is balanced by different types of forces. The force acting vertically downward is given by _______.
(a) Mg
(b) Mgcosq
(c) Mgsinq
(d) Zero
(e) None of these
Answer: (b)
Explanation
The force acting in vertically downward direction is given by Mgcosq.
An object is moving in the vertically upward direction, which is normal to the plane. The net work done by the object when it reaches a height of 35 m is__________.
(a) 0 J
(b) 35 J
(c) 20 J
(d) 15 J
(e) None of these
Answer: (a)
Explanation
When the object moves in vertically upward direction, normal to the plane, the net work done is 0 J.
Frictional Force
It is the force exerted by a surface when an object moves across the surface. It can also be defined as the force, which opposes the motion of an object. It is also known as the constant force. There are two types of frictional forces:
(i) Kinetic friction (ii) Static friction
Kinetic Friction
The frictional force between any two moving surface is called the kinetic friction. There are different types of kinetic friction, such as, sliding friction, rolling friction. For example, if a boy is sliding on a slider in the garden, the force exerted by the slider on the boy is a sliding friction. On the other hand, if a wheel is rolling down the slope, the friction between the wheel and slope is called the rolling friction. The frictional force also depends on the nature of the surface and degree to which they are pressed together. The maximum amount of frictional force is calculated by using the relation:
Force = Coefficient of friction x Normal force
Static Friction
It is the friction that acts between the surfaces of two motionless objects. It always acts in the direction opposite to the force, which tends to produce the motion in one of the two objects. It increases with the external force until motion is initiated in the object.
-----> Reaction force acting on the bicycle by the road, it is called friction too
A car of mass 1200 kg is moving on the road at the speed of 60 km/h. If the coefficient of friction is 38 and the normal force is 50 N, the frictional force acting on the car is given by _______.
(a) 300 N
(b) 3000 N
(c) 2500 N
(d) 30 N
(e) None of these
Answer: (b)
Explanation
The frictional force acting on the object is given by,
Force = Coefficient of friction x Normal force.
On calculation, it was found that the frictional force acting on the object is 3000 N.
A train at rest tends to move from the platform and it attains a velocity of 100 km/h within a few second. While trying to come in motion it has to overcome the frictional force; that frictional force is________.
(a) Static friction
(b) Rolling friction
(c) Sliding friction
(d) Kinetic friction
(e) None of these
Answer: (a)
Explanation
When an object tends to come in motion from the rest, the frictional force acting on the object is called static friction..
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