151

B: NEWTON’S LAWS

Unit 2: Free body diagrams

Unit 3: Newton’s laws of motion

Unit 1: Forces

Doc Scientia - IEB Senior Physics Textbook and Workbook Book 1

UNIT 1: Forces

1.1 Gravitational force 1.6.1 Factors

1.2 Normal force 1.6.2 Coefficient

1.3 Applied force 1.6.3 Reduced

1.4 Tension 1.6.4 Static friction

1.5 Air resistance 1.6.5 Kinetic friction

1.6 Friction 1.6.6 Application

Unit 1: Forces

Someone accidentally knocks over a tower of food tins in a shop. A force is exerted on the previouslystationary tins and they now move. A tin hits the floor and dents due to the force of the floor acting on the tin. A force:• is a pull or a push;• is a vector (note that the direction in which the force acts is very important);• has the symbol F;• is measured in newton (N);• can be represented by an arrow like any other vector quantity. - The length of the arrow represents the size, or magnitude, of the force. - The arrowhead points in the direction in which the force acts.

Contact and non-contact forces Forces

Non-contact:act over a distance.

Examples:1. Magnetic force2. Electrostatic force3. Gravitational force (Fg)

Contact: objects are in contact with each other.

Examples:1. Applied force2. Friction3. Normal force4. Tension5. Air friction6. Compression

If a unit that has been named after a person is written in words, only lower case letters are used, e.g. newton. When it is abbreviated, we use a capital letter, e.g. N.

QUICK FACTS

NEWTON’S LAWSB

Doc Scientia - IEB Senior Physics Textbook and Workbook Book 1152

The normal is always perpendicular to the surface, even if the object is on an inclined surface. It acts on the object resting on or against it.The normal force is a supporting force. It is equal in magnitude, but in the opposite direction to the force of the object acting on the contact surface.

1.1 Gravitational force

Weight is calculated as follows:Fg = mgg = gravitational accelerationClose to Earth’s surface, the approximate value of g = 9,8 m⋅s-2.

1.2 Normal force (FN)

1.3 Applied force

A person or object exerts a force on another object/person.The applied force is:• in the same line – for example forwards or backwards – as the direction of movement; or• at an angle to the direction of the movement. The components of the force can be calculated in the

direction of movement.

Weight (Fg) is the gravitational force that Earth exerts on any object on or near its surface.

GRAVITATIONAL FORCE

Normal force (FN) is the perpendicular force exerted by a surface on an object in contact with it.

NORMAL FORCE

Fg

FT = T

FN

f

An object must be in contact with a surface to experience a normal force.Objects falling through the air do not experience normal force.

QUICK FACTS

Components of a force acting atan angle to the horizontal.

Fx = F cosθFy = F sinθ

REMEMBER

Fx

Fy

Components of the gravitational force parallel and perpendicularto the slope:

Fg⊥ = Fg cosθFgll = Fg sinθ

REMEMBER

Fgll

Fg⊥

1Unit

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FORCES

1.4 Tension (FT or T)

When a cable or rope is pulled, tension is exerted on it.The tension in a rope is constant throughout, but the direction is not only in one direction.The mass of any rope/cable will be considered to be negligible, unless there is an instruction to take itinto consideration.

If you need to take the mass into consideration, the gravitational force will act on the centre of gravity of the rope.

1.5 Air resistance (Fair or Ff)

A resistant force exerted by the collisions of air particles on a moving object.Air friction always acts in the opposite direction to movement.

1.6 Friction force (f or Ff)

When an object moves, or tries to move on a surface. The contact surface exerts a frictional force on the object.

The force is always:• in the opposite direction to the movement, or attempted

movement;• parallel to the surface on which the object makes contact.

Friction is a contact force.It occurs when two objects touch each other and try to move over each other.A surface which looks smooth, has uneven particles at microscopic level.The surface of solids is fairly rough.When two objects move over each other (or try to move over each other), these rough parts of the surfaces get hooked onto each other. Friction acts against movement.

Friction (Ff) due to a surface is the force that opposes the motion of an object and acts parallel to the surface with which the object is in contact.

FRICTION FORCE

Frictional forces act parallel to the plane of motion, but are always in the opposite direction to the motion.

QUICK FACTS

Friction force

Static friction

Only for two objects at restrelative to one another.

Kinetic friction

Only for two objects in motionrelative to one another.

NEWTON’S LAWSB

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1.6.1 Factors that influence the magnitude of the frictional force

Factors that influence friction:• normal force;• surface type.

1. The normal forceThe larger the forces that the two surfaces exert on each other perpendicular to the surface, the larger the friction.The normal is an indication of this. To calculate friction, we need to know what the normal is. The frictional force is directly proportional to the normal force ∴ f ∝ FN.

The frictional force is independent of the velocity at which the object moves.

QUICK FACTS

Examples

Rajesh pushes a crate across a rough surface, as indicated in the diagram. The applied force is labelled FRajesh and the friction is labelled f. At first, his force is not large enough to overcome thefriction, so he keeps increasing it.

Study the following table to understand how the friction varies:

Magnitude of appliedforce FRajesh (N) Crate Magnitude of

friction force f (N)Type of

friction force

40 At rest. 40 Static

60 At rest. 60 Static

80At the point of starting to move.

80 Static

82Accelerates to the right.

60 Kinetic

85Accelerates to the right.

60 Kinetic

Initially, the static frictional force is equal to the applied force and the crate is not moving.As the applied force increases, the static friction increases equally, to resist the movement.From the table it is clear that the static friction force reaches a maximum magnitude of 80 N.When Rajesh applies a force larger than the maximum frictional force, the crate starts to move and accelerate. The kinetic friction is now smaller than the maximum value of the static friction.As soon as the crate is in motion, the frictional force is smaller.

FN

FRajesh

Fg= 98 N

10 kg f