Forces Flashcards
Definition of force
A force is a push or pull upon an object resulting from the object’s interaction with another object.
Types of non-contact forces
Gravitational force, electric force and magnetic force
Definition of force field
A force field is a a region in which an object experiences a force.
Definition of non-contact forces
Non-contact forces are types of forces in which the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite the physical separation.
Where does electric force exist?
Electric force exists between two charges. It can be attractive or repulsive.
Where does gravitational force exist?
Gravitational force exists between any two masses and is always attractive. i.e. weight
Where does magnetic force exist?
Magnetic force exists between two magnets. It can be attractive or repulsive.
Definition of contact forces
Contact forces are types of forces that exist in the two interacting objects which are physically in contact with each other.
Types of contact forces
Normal contact force, frictional force, tension, viscous forces and upthrust
Magnitude of normal contact force
Depends on how much force the object is exerting on the surface
Direction of normal contact force
Always perpendicular to the contact surface and its direction is through the object of interest
Definition of normal contact force
The normal contact force is the perpendicular force exerted by a surface on an object in contact with it
Definition of frictional force
Frictional force is the force exerted by the surface on the object as an object moves across it or makes an effort to move across it.
Definition of reaction force
The reaction force is the resultant force of both frictional force and normal contact force.
Magnitude of friction
Depends on the roughness of the contact surfaces
Direction of friction
Opposite direction to the relative motion of the object and parallel to the surface.
Definition of tension
Tension is the force which is transmitted through an object, when it is pulled tight by forces acting at each end.
For a force in equilibrium, what is the tension
The tension is constant.
Formula of Hooke’s law
F = kx
F is the load/force exerted by the spring(N)
k is the spring constant(N m^-1)
x is the extension/compression of the spring(m)
Magnitude of tension
Varies according to the extension for springs
Definition of pulling force
The pulling force due to an extended spring is also the tension exerted by the extended spring.
Direction of tension
It is always along the string, rope or cable and away from the object.
Definition of Hooke’s law
Hooke’s law states that the magnitude of force F exerted by a spring on a body attached to the spring is proportional to the extension x of the spring from equilibrium, provided the limit of proportionally of the spring is not exceeded\
What is the graph of force exerted on a spring and the extension of the spring?
Constant gradient(proportional) and decreasing gradient(where it doesn’t obey Hooke’s law)
Definition of viscosity
Property of a fluid which tends to resist relative motion within itself
Definition of viscous force
Viscous force is the resistive force that an object experiences when it moves through a fluid.
Magnitude of viscous force
Increases with velocity, and is dependent on type of flow of liquid
Direction of viscous force
Opposite to the motion of the object
Definition of centre of gravity
The centre of gravity of an object is the point through which the entire weight of a body acts.
Definition of the moment of a force
Moment of a force about a point is the turning effect of the force about that point and is equal to the product of the force and the perpendicular distance from its line of action to the point
Formula of moment of a force
T = Fd
How do you find the perpendicular distance for the moment?
- Extend the line of action of force past the pivot
- Draw a perpendicular line from the line of action of force to pivot
What is the torque of a couple?
The torque of a couple is the product of one of the forces and the perpendicular distance between the lines of action of the forces.
Definition of equilibrium
Equilibrium of a body occurs when there is no resultant force in any direction and there is no resultant torque about any point.
What is a couple(torque) ?
A couple is a pair of forces of equal magnitude but opposite in direction acting on an object where their lines of action do not coincide, producing only rotation.
What are the two conditions to be fulfilled for an object to be in equilibrium?
1) No resultant force in any direction
2) No resultant force at any point
What are the conditions for the body to be in translational equilibrium?
1) Summation of forces acting on the object is 0
2) The vector addition of the forces yields a closed polygon.
What does the rotational equilibrium of an object mean?
It means that the object is not rotating or rotating at a constant angular velocity.
What does the principle of moments state?
The principle of moments states that for an object to be in rotational equilibrium, the sum of clockwise moments about any point must equal the sum of anti-clockwise moments about that pivot.
What is static equilibrium of a body?
Static equilibrium of a body is a state when that body is at rest.
Conditions required for static equilibrium
1)No resultant force in any direction(Fx = 0, Fy = 0)
2)No resultant torque at any point(Ft = 0)
3)For static equilibrium, object is stationary.
Steps involving static equilibrium problems
1)Draw a free-body diagram.
2)Set up moment equation.
Ft = 0
Taking moment about any point,
Sum of clockwise = Sum of anti-clockwise
3)Set up the 2 force equations
Fx = 0, Fy = 0
4)Solve the equations to find the unknown forces.
What are the conditions to find the common point for all lines of actions?
1) When the net torque is 0
2) Rotational equilibrium
Definition of pressure
Pressure is defined as force per unit area.
Definition of upthrust
The pressure difference at the top and bottom surfaces give rise to net upward force. The net upward force on the body by the liquid is called the upthrust.
What does the Archimedes’ Principle state?
The Archimedes’ Principle states that the upthrust acting on the body, partially or totally immersed in a fluid is equal in magnitude and opposite in direction to the weight of the fluid displaced by the body.
Formula for upthrust
U = pvg
What is the magnitude and direction of upthrust?
Magnitude: Equivalent to weight of displaced fluid
Direction: Upwards
What does the law of flotation state?
The law of flotation states that the weight of the fluid displaced by an object is equal to the weight of that object when the object floats in equilibrium.
