This page covers the following topics:
1. Types of forces
2. Free-body diagrams
A force is a push or pull acting on an object due to its interaction with another object. It is a vector quantity and is measured in newtons (N). A force can be either a contact or a non-contact one, depending on whether the interacting objects are touching. A contact force is a force that requires objects to be touching to be exerted. A non-contact force is a force that can be exerted without objects touching.
Normal reaction force arises for objects placed on a surface and is perpendicular to a surface. Tension is a force that commonly exists in ropes and pulls objects in the direction of the side the rope is attached to. Friction is a force that opposes the motion of an object, usually due to interaction with a medium. Friction in a liquid is called drag, while in air it is called air resistance.
Gravitational force is a force between two objects that have mass. Usually gravitational forces between two objects is going to be significant when at least one of them is heavy, like the Earth or the Sun. Gravitational force near the surface of such heavy object is called weight. An electrical force is an attraction or repulsion between two charged particles. A magnetic force is an attraction or repulsion between two poles.
A free-body force diagram is a diagram which shows the relative magnitude and direction of all the forces acting on an object. The object is drawn as a dot and all forces acting on it are drawn as arrows pointing away from it. Each force must be labelled with the name of the force. Both normal reaction force and friction arise from a surface, thus their vectors start from where an object touches the surface. Weight is caused by the mass of the object, thus an arrow representing weight starts at the centre of mass.
Forces can be split into two or more vectors as long as they add up to the original vector. To find the resultant force between two vectors, we can draw the two vectors one after the other, that is the second starts where the first ends. To get an accurate answer, the lengths of the vectors have to be proportional to the sizes of the forces. The resultant force is the vector that joins the start and the finish points. You can determine whether an object is in equilibrium, by drawing all the forces acting on it one after the other. If the last vector ends where the first started, then the object is in equilibrium.
A ball is being pulled towards the left across a table as in the free-body diagram provided. Name forces A, B, C, D.
Horizontally towards the left there is going to be a pull (D) and towards the right - friction (B). Vertically the weight of the ball (C) is going to be balanced by the normal reaction force (A).
A = normal reaction force
B = friction
C = weight
D = pull
A mass is placed on an inclined plane. Provide names and the directions of the forces that would you represent in a free-body diagram for the mass.
Normal reaction force would be acting perpendicular to the inclined plane. Friction arising from the surface wold be acting parallel to the surface. Weight would be acting on the climber downwards.
normal reaction force perpendicular to the surface, friction parallel to the surface, downwards weight
A board is placed on a support at one end as shown in the image. A box is put at the other end of the board. State the names and types of forces acting on the board.
A contact normal reaction force is exerted by the support on the board. A non-contact weight of the board is exerted on the board by the Earth. A contact push force exerted by the box on the board.
contact normal reaction from the support, non-contact weight from the Earth, contact push force from the box
A climber is partially holding on one of the rocks as well as is attached to a rope. Provide names and the directions of the forces that would you represent in a free-body diagram for the climber.
The climber is partially held by the normal reaction force. Normal reaction force would likely be acting upwards and/or horizontally. Tension arising from the rope wold be acting upwards. Weight would be acting on the climber downwards. Friction could be acting upwards and/or horizontally.
upwards/horizontal normal reaction force, upwards tension, downwards weight, upwards/horizontal friction
What type of force is friction and in which direction does it act?
Friction is a contact force exerted on a moving object and it acts in the direction opposing the direction of the motion of the object.
contact, opposite to object's motion
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