Dynamics Flashcards by John Lim

What are the eight types of forces?

Friction
Air resistance
Normal force
Tension
Upthrust
Weight
Electrostatic force
Magnetic force

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What is friction? Is it a contact force or non contact force?

The description of friction: When two surfaces are in contact, they exert force on each other. The component parallel to surfaces is the friction. it acts in a direction so as to resist relative motion of tendency of motion between the surfaces.

When surfaces come into contact, the molecules of the two surfaces will attract each other. In some cases, the unevenness of the surfaces causes them to interlock with each other. Both above result in a resistive force when we try to side one surface over the other. This resistive force is friction and always results in the loss of energy in the moving objects. Frictional forces always opposes the relative motion or the tendency of motion of the surfaces.

Friction is a contact force.

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What is air resistance? Is it a contact force or non contact force?

Air resistance description:

Resistive forced exerted by air on an object moving through it. Air resistance increases we speed increase, and is zero when the object is at rest.

An object moving through a fluid (i.e. a gas or liquid) encounters a resistive force because it collides with the fluid particles that get in its way. This resistive force is known as the drag force. If the fluid is air, this drag force is known as the air resistance. The faster the object moves, the harder it knocks onto the air molecules and the larger the air resistance it experiences. Apart from the speed of the object, the shape and size of the object also affect the amount of air resistance it experiences. For example, the big surface area of a parachute increases the air resistance it experiences.

Air resistance is a contact force.

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What is a normal force? Is normal force a contact force or non contact force?

Normal force description:

Perpendicular component of the contact force. It acts outward perpendicular to the surfaces.

Normal force is a contact force.

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What is weight? Is weight a contact or non contact force?

Weight description:

Gravitational force exerted by the Earth on an object.

Weight is a non contact force.

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What is tension? Is tension a contact force or non contact force?

Tension description:

Pulling force acting along a string or a rod.

Tension is a contact force.

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What is electrostatic force? Is electrostatic force a contact or non contact force?

Electrostatic force description:

Force exerted by electric charges on each other.

Electrostatic force is a non contact force.

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What is magnetic force? Is magnetic force a contact or non contact force?

Magnetic force description:

Force exerted by magnets on magnetic materials such as iron and nickel. The magnetic force originated from moving charges.

Magnetic force is a non contact force.

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Why do we draw free-body diagrams? How do you draw a free-body diagram?

We draw a free-body diagrams to help visualise the forces acting on an object.

Forces acting on the object are drawn as arrows that originated from its point of action. All forces must be labelled clearly. The length of the arrows represents the magnitude of the force.

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Does a ball that is thrown in the air have any normal force?

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Since force is a vector quantity, the resultant force is the _________ of all the forces acting on the object.

Since force is a vector quantity, the resultant force is the vector sum of all the forces acting on the object.

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What is the the Newton’s First law of motion?

Newton’s first law of motion states that an object at rest will remain at rest and an object in motion will continue in motion at constant velocity in the absence of a resultant force acting on it.

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What is Newton’s second law of motion?

Newton’s second law of motion states that the rate of change of momentum of a body is directly proportional to the resultant force acting on it and the direction of the resultant force is in the direction of the change in momentum.

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What does this key phrase “rate of change” mean?

It means how fast a quantity (in this case, momentum) changes with time.

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What does this key phrase “direction of change” mean?

This means that since momentum is a vector quantity, the change in momentum is also a vector and has a direction.

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In equation form, Newton’s seconds law is ___________. Explain the formula.

The formula is resultant force = mass times acceleration

F net = ma

Provided m is a constant.

F net refers to the resultant force acting on the body. Newton’s seconds law tells us that when the forces acting on an object are unbalanced so that the resultant force is non-zero, the object will accelerate with an acceleration give by (F net ÷m)

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A falling rock of mass 10.0kg, weighing 98.1N experiences air resistance of 31.0N.

Calculate the acceleration of rock.

F (net) = ma

Weight - air resistance = ma

98.1-31.0 =(10.0)a

a= 6.71 ms to the power of -2 for s downwards

Note that the direction of acceleration is always the same as that of the net force.

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What should you take note when drawing free body diagrams?

Note these marking points:

length (it will vary if there are two force acting on opposite directions and one force is larger than another therefore causing the object to move in a certain direction)

Direction (the direction of the line should be drawn accurately by reading the question properly, ask yourself if the question states that “ignoring the effects of…”, covering the other object if there is another and look at the forces left. Take not that weight always points downwards form the centre of the object vertically downwards. For normal force, it is from the origin to outside of the object, with the same length as the line for the weight since they have the same magnitude —> it should act outward perpendicular to the surfaces meaning it should touch the surface extend outwards through the origin to outside of the object.

Origin

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What is Newton’s third law of motion?

Newton’s third law of motion states that if object A exerts a force on object B, the object B exerts an equal and opposite force on object A.

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Action-reaction pair of forces must satisfy _______________.

Action-reaction pair of forces must satisfy the condition of

same type
Equal in magnitude
Act in opposite directions
Act on different bodies

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How should the action reaction pair statements be written as?

_____ force on object A by object B (please specify the force and the object clearly by looking for it in the question)
_____ force on object B by object A

Both of these statements have to be present.

Are these action-reaction pairs?

A box resting in equilibrium on a table top experiences two forces- its weight and the normal force from the table.

The two forces are equal and opposite but since they both act on the same body, they are not an action-reaction pair.

The force exerted by the wall on the person (F person) is equal in magnitude and opposite in direction to ______________.

The force exerted by the person on the wall (F wall).

The force exerted by the ground on the person (F person) is equal in magnitude and opposite in direction to _________.

the force exerted by the person on the ground (F ground).

When will friction usually occur?

When an object is moving. Therefore if the question requires you to identify the force and the object is not moving, do not identify the force as frictional force.

The diagram shows a cuboid initially at rest on a rough horizontal surface. A student exerts a force F on the cuboid. Identify the type of force.

Contact force. We don’t know if it is friction or normal force. Therefore, it will be safer to write contact force.

The figure below shows two blocks of wood, P and Q, with masses 1.50kg and 2.00kg, respectively. They are connected by a light inelastic and mass-less string and are pulled by a force of 14.0N to the right. Is the acceleration of P and Q the same or different? If it is different does P have a higher acceleration or Q? Why?

P and Q must have the same acceleration. If Q has a higher acceleration, the string will snap, if P has a faster acceleration, the string will slack however these situations did not occur, therefore P and Q are travelling at constant acceleration. (It is found in pg 8 of the notes)

When 3 coplanar forces acting at a point are in __________, they can be represented in magnitude and direction by the adjacent sides of a _________ taken in order.

When 3 coplanar forces acting at a point are in equilibrium, they can be represented in magnitude and direction by the adjacent sides of a triangle taken in order.

When more than 3 coplanar forces acting at a point are in equilibrium, they formed a ___________ when represented in a vector diagram, using the head-to-tail method.

closed polygon

Any vector can be resolved along any ________________. This results in __________ of the vector. The component of a vector is the influence of that vector in a given direction. Perpendicular components are _________________________________.

Any vector can be resolved along any two perpendicular directions. This results in two perpendicular components of the vector. The component of a vector is the influence of that vector in a given direction. Perpendicular components are independent of each other.

The mass of an object is a measure of? What is the definition of mass?

The amount of substance in the object. Mass is a measure of the inertia of an object, which is the resistance of a body to change its state of rest or motion.

Objects of different masses subjected to the same force will have accelerations ______________________.

Objects of different masses subjected to the same force will have accelerations which are inversely proportional to their masses, from Newton’s second law (Fnet = ma)

What is the SI unit of mass?

kilogram (kg)

What is a gravitational field?

A gravitational field is a region in which a mass experiences a force due to gravitational attraction.

Since the Earth exerts a force of 9.81N on each kilogram of mass on its surface, the weight W of an object is given by ___________.

W = mg Where m is the mass of the object and g = 9.81 N kg to the power of -1 is the gravitational field strength on the surface of the Earth. For comparison, the gravitational field strength on the Moon’s surface is about 1.63 N kg to the power of 01. So a 10 kg mass will weigh 98.1 N on the Earth and 16.3 N on the Moon.

The weight of an object is the ______________________.

Gravitational force exerted on the object by the Earth.

What are the differences between mass and weight of a body?

Mass is a scalar quantity while weight is a vector quantity. mass is independent of gravitational field strength of planet while weight is dependent of gravitational field strength of planet. The unit of mass is kg while the unit of weight is N. The measuring instrument for mass is mass balance while the measuring instrument for weight is spring balance.

Why is it wrong to say that the weighing balance measures the weight?

Why is it wrong to say that the spring balance measures gravitational force?

What is gravitational field strength?

gravitational field strength, g, is gravitational force per unit mass placed at that point.

What is the formula for gravitational field strength?

g = weight/mass Hence, weight = mass x g

For a free-falling object, ________, therefore _____________.

For a free falling object, the only force acting on it is its weight. Weight = mass x acceleration Acceleration = weight/ mass = mass x g/ mass = g

Explain terminal velocity.

The speed of a falling object increases due to a downward acceleration. When air resistance is not negligible, the falling object also experiences an increasing resistive force. Eventually, the amount of air resistance becomes the same as the weight of the object, and the velocity of the object ceases to increase. The final velocity attained is known as the terminal velocity.

Describe how the velocity-time graph will look like when an objects falls without air resistance and when an object falls with air resistance.

For the velocity-time graph that falls without air resistance: - a straight line going to the “northeast” direction. For the velocity-time graph that falls without air resistance: - an curve that increases at a decreasing rate - and comes to an eventual horizontal line which is the ****terminal velocity****

What should the free body diagram of friction be when a person is walking/running towards the right on the ground?

friction on shoe (label of arrow) to the right This is because the direction in which the shoe rubs against the ground is to the left so since frictional forces always opposes the relative motion, friction on shoe will be to the right.

What are the positive effects of friction?

1. Walking 2. Holding an object 3. Braking

What are the negative effects of friction?

1. Loss in useful energy 2. Generate heat (where heat is not desired) 3. Wear and tear

What are the methods to reduce friction?

1. Ball bearings 2. Lubricants 3. Air layer

What is static friction?

Friction can occur even when the two surfaces are not moving relative to each other. For example, imagine you are exerting a small force on a heavy stationary box. Initially, when the force applied on the box is small, the box remains stationary. Thus the friction is equal to the applied force. This is called the static friction. However, if you increase the applied force on a heavy box, the box will eventually start moving.

What is kinetic friction?

Kinetic friction occurs when the two surfaces are moving relative to each other. This value is constant regardless of the velocity of the surface, and it is smaller in magnitude than the static friction.

Friction depends on _____________ and is not affected by ___________.

Friction depends on - the nature of the surfaces in contact - how tightly the surfaces are pressed together. Friction is not affected by the area of the surfaces in contact.