Lesson: Force and Pressure
Topic 1: Force - Introduction and Effect
Describe two examples in which a force is applied to change the state of motion of objects.
A force can change the state of motion of a body. Force can either stop a moving object or make a stationary object start moving.
For example:
a) A goal keeper saving a goal. A goalkeeper can stop the moving ball by applying a force.
b) Hockey player flicking a ball. The ball that is already moving moves faster after the hockey player flicks the ball and the direction also changes.
What is a force?
A push or a pull or both on an object is called a force. Pushing a trolley and stretching a rubber band are some activities that involve using force.
Explain the phrase ‘state of motion’.
The speed and the direction of motion of a body is described as its state of motion. The state of rest is considered to be the state when the body is at rest or called state of zero speed. An object might be at rest or in motion; both are its states of motion.
What happens to an object moving with a constant speed when a force acts on it in a direction opposite to its motion?
The force will cause the object to come to a stop after sometime.
What happens to a moving object when a force acts on it in the same direction to its motion?
When the force acts on a moving object in the same direction as its motion, the object's speed increases as the force adds to the already existing force on the object.
If a force is acting on a body, will it always cause a change in its state of motion?
No. that’s not it. To change the state of motion of a body the force acting on the body has to be more than the resultant of the force acting previously. Additionally, force acting on a body has lots of other effects such as, change in shape, change in direction etc.
Give two examples of situations where the applied force causes a change in shape of the object.
Examples of change in shape of an object are as follows:
• Squeezing of a plastic bottle
• Deformation of clay when pressed between two hands
What are the different effects of forces?
Following are the different effects of forces on an object:
• Change in the state of motion
• Change in the direction of motion of an object
• Change in the shape of an object
• Turn or spin of an object
What is the effect of the force when a hockey player deflects a pass from his fellow team mate?
The application of force by the hockey stick on the ball results in a change in direction of motion of the ball.
Topic 2: Contact/Non-contact Forces
Peter is holding a bucket of water in his hand. What are the forces acting on the bucket? Why do the forces acting on the bucket not bring a change in its state of motion?
Peter uses muscular force to hold the bucket of water above the ground. The muscular force acts against the force of gravity that pulls the bucket towards the ground. Peter’s muscular force and the gravitational force are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence there is no change in its state of motion.
What is an electrostatic force?
The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. An example of an electrostatic force is the force resulting when two balloons are rubbed against one another.
What is muscular force?
The force resulting due to the action of muscles is known as muscular force. We apply muscular force to lift objects.
What is frictional force?
The force that acts on all moving objects in a direction opposite to the motion of body is called the force of friction. Birds and fish have streamlined bodies to reduce the respective air and water resistance that they feel.
Why do we call frictional force as a contact force?
Friction comes into play only when there is contact between two surfaces. That is why it is called a contact force.
How does force change a state of motion?
An object can either be in rest or in motion. A force can be applied on an object to stop its movement, if it is in motion or to make it move if is at rest.
Explain gravitational force.
Gravitational force is the natural force of mutual attraction between all physical bodies in the universe.
Topic 3: Pressure
What is atmospheric pressure?
The pressure exerted by the air around us is known as atmospheric pressure.
How does a rubber sucker stick to a smooth surface?
The sucker is designed in such a way that when pressed against a smooth surface, the air is forced out from beneath the sucker. Since the air pressure outside is greater than the air pressure beneath the sucker, it results in a tight contact that does not break easily.
Explain the phenomenon of a rubber sucker not sticking to a rough surface.
The rubber sucker is unable to make an air tight seal as rough surface, since the rough surface allows gaps through which air can enter and leave. Hence, the pressure outside is the same as inside and the sucker does not stick to the surface.
Why do porters place a thick rounded piece of cloth on their heads before carrying heavy luggage?
The thick rounded piece of cloth increases the area of the surface in contact with the luggage and hence the pressure on the porter’s head due to the luggage is considerable reduced.
Why are school bags provided with wide straps?
School bags carry lot of load and hence the bag is heavy. The straps are made wide so that the weight of the school bag acting downwards is effectively distributed over a large area and thus, the pressure on the hand or shoulder is reduced.
How do snow shoes help when walking on snow?
Snow shoes have large flat soles and hence the weight of the body is distributed over a large area. Thus, the pressure exerted on the snow is reduced and our feet do not sink too much in the snow.
Explain how our bodies are not crushed by the large atmospheric pressure.
The weight of air in a column of the height of the earth’s atmosphere and area 100 cm2 is as large as 1,000 kg. We are not crushed under this weight because the pressure in our own bodies due to fluids inside is equal to the atmospheric pressure and hence cancels the pressure from outside.