Force and Pressure Science Notes
Newton’s first law of motion:
A stationary object on which no force is acting, remains stationary, An object in motion continues to move with the same speed and direction when no force is acting on the object.
→ A force acts on two bodies through an interaction between them. [Note: a body = an object.]
→ Contact force: A force that acts through a direct contact of two objects or via one more object, is called a contact force.
→ Non-contact force: A force that acts between two objects even if the two objects are not in contact, is called a non contact force.
→ Balanced forces: If a body is acted upon by two forces, equal in magnitude, opposite in direction and having the same line of action, the forces are called balanced forces. Here, the net force acting on the body is zero.
→ Unbalanced force: If two or more forces act on a body such that their resultant is not zero, the resultant is an unbalanced force. It produces acceleration in the body.
→ Inertia: The tendency of an object to remain in its existing state is called its inertia.
→ The effects of a force acting on a body:
- A force can set a body in motion. For example, if a ball at rest on the floor is pushed, it rolls on the floor.
- A force can stop a moving body. For example, a moving bicycle can be brought to rest by application of brakes.
- A force acting on a body can change the speed of the body. For example, when brakes are applied to a moving bicycle, its speed decreases due to the friction between the brakeshoes and the rim of the tyre.
- A force can change the direction of motion of the body. For example, in uniform circular motion of a body, the direction of motion of the body keeps on changing due to the applied force.
- A force can change the speed as well as the direction of motion of the body. For example, when a ball bowled by a bowler is hit by a batsman, there occurs a change in the speed as well as the direction of motion of the ball.
- A force can change the shape and size of the body on which it acts. For example, when a rubber ball is pressed, it gets deformed and hence no longer remains spherical. Also, there can be a decrease in its volume.
The force exerted perpendicularly on a unit area is called pressure. The SI unit of pressure is N/m2. It is called the pascal (Pa). In atmospheric science, the unit for pressure is called the bar.
1 bar = 105 Pa.
The air pressure at the sea level is called 1 atmosphere.
1 atmosphere = 101 × 103 Pa.
→ At a depth h below the free surface of a liquid, the pressure exerted by the liquid is hρg 11.
The upward force acting on the object in water or other fluid or gas is called the buoyant force. It depends on the volume of the object immersed in the fluid (V), the density of the fluid (ρl) and the acceleration due to gravity (g) at that place. Magnitude of the buoyant force = Vρ1g.
→ Archimedes’ principle: When an object is partially or fully immersed in a fluid, a force of buoyancy acts on it in the upward direction. This force is equal to the weight of the fluid displaced by the object [Note: The two forces mentioned here are equal in magnitude and opposite in direction.]
→ Density = mass/volume. Its SI unit is kg/m3 and CGS unit is g/cm3. 1g/cm3 = 103 kg/m3. Relative density = density of a substance/density of water. It has no unit. Relative density is also called specific gravity.