Informative line

Various Forces

Learn direction of gravitational force, tension, spring and normal contact force. Practice examples of tension in a pulley system and normal force on an incline.

Direction of Gravitational Force

• Direction of gravitational force is given towards the center of the earth.
• The magnitude of gravitational force is equal to product mg, where m is the mass of the body, g is the acceleration due to gravity.

Case I :- Stationary body of any shape placed on horizontal surface

• Consider a block of mass M placed on a surface.

• Direction of gravitational force will be as shown in figure.
• Mg always acts vertically downwards, towards the center of earth.

Case II :-  Moving body of any shape

• Consider a body of mass M, falls free under gravity.

Important Note

• Mg always acts vertically downwards, irrespective of path of motion.

Case III :- Stationary or moving body on any surface (other than horizontal surface)

• Consider a body of mass M placed upon an inclined plane at $$\theta$$.
• Mg always acts vertically downwards, irrespective of orientation of surface.

A block of mass M is kept on a smooth horizontal floor, which of the following figure shows the correct direction of Mg?

A

B

C

D

×

Mg always acts vertically downward

Option (A) is correct.

A block of mass M is kept on a smooth horizontal floor, which of the following figure shows the correct direction of Mg?

A
B
C
D

Option A is Correct

Normal Contact Force

Push or Pull

• There are two types of forces, i.e., push and pull.
• Normal force comes into play when two bodies are in contact and either one or both are trying to push each other by penetrating the other's surface.

• Block A tends to penetrate into ground and ground does not allow to do so, thus pushing it back.
• So, block is being pushed by the ground away from itself hence normal force on the block is in upward direction.

Steps to draw the normal force

1. Mark point of contact.
2. Draw a line perpendicular to the surface.
3. Its direction is towards the object experiencing the normal contact force.

• Normal force on block A is towards itself hence, in upward direction.
• Normal contact force on a block placed on inclined.

• Normal contact force on curved surface.
• Two bodies are in contact such that one having curved surface and other having flat surface, as shown in figure.

• The direction of normal contact force (N) is perpendicular to the flat surface.

NOTE: When two bodies are curved, then the normal contact force is perpendicular to the common tangent.

A ball is kept on a smooth floor. Which of the following figure shows the correct direction of normal force on the ball?

A

B

C

D

×

The direction of normal contact force (N) is perpendicular to the flat surface towards the ball.

Option (A) is correct.

A ball is kept on a smooth floor. Which of the following figure shows the correct direction of normal force on the ball?

A
B
C
D

Option A is Correct

Normal Contact Force on a Ladder

• In this case the normal is perpendicular to the flat surface, as shown in figure.

A ladder is kept, as shown in figure. Which of the following option correctly shows the direction of normal contact force at A?

A

B

C

D

×

The normal will be perpendicular to the flat surface and away to it.

Option (D) is correct.

A ladder is kept, as shown in figure. Which of the following option correctly shows the direction of normal contact force at A?

A
B
C
D

Option D is Correct

Spring Force

• Spring applies force along its length.
• It can be both pushing or pulling in nature depending upon whether the spring is compressed or extended.

Natural length

• Length of spring when it is not under any force is called its natural length.
• It is denoted by $$\ell_0$$

Direction of spring force

• The direction of spring force is always directed towards its natural length.

A spring is extended towards right, as shown in figure. What is the direction of spring force?

A Towards right

B Towards left

C Upward

D Downward

×

The direction of spring force is always towards the natural length.

Option (B) is correct.

A spring is extended towards right, as shown in figure. What is the direction of spring force?

A

Towards right

.

B

Towards left

C

Upward

D

Downward

Option B is Correct

Tension

• Tension acts on a body when a string attached to the body is taut. As you must have experienced that by rope or a string you can only pull things towards you.
• Using rope or string you can not push objects away.
• It means tension is a pulling force.
• Tension force acts away from the body, along the string/rope.
• To draw tension force acting on block A due to string BC.
• Mark point of contact of string on the block as point B.
• Now draw a vector along string pointing away from the body.

A block is hanged with a string, as shown in figure. Choose the direction of tension force.

A

B

C

D

×

Tension force acts away from the body, along the string/rope.

Option (B) is correct.

A block is hanged with a string, as shown in figure. Choose the direction of tension force.

A
B
C
D

Option B is Correct

Tension Force on Pulley

• A pulley can change the direction of the force exerted by a cord.
• An ideal pulley has no mass and no friction.
• As a result the tension of an ideal cord that runs through an ideal pulley is the same on the both sides of the pulley.
• An ideal pulley can change the direction of the force exerted by a cord by change in the magnitude.
• Tension force acts away from the body, along the string/rope.

Consider an ideal pulley system. Find the direction and magnitude of F.

A

B

C

D

×

Over an ideal pulley the magnitude of tension force remains the same.

Tension force acts away from the body, along the string/rope.

Option (A) is correct.

Consider an ideal pulley system. Find the direction and magnitude of F.

A
B
C
D

Option A is Correct