Phy SPA - Dynamics (Free Body Diagram & Friction)

At the end of the lesson, you should be able to:

a) Identify forces acting on an object and draw

free body diagram(s) representing the forces

acting on the object (for cases involving

forces acting in at most 2 dimensions)

b) Explain the effects of friction on the

motion of a body

• A push or a pull – represented by arrows

• S.I. Unit: newton (N)Contact Forces – surfaces in touch with each other

Normal Reaction Friction Tension

Push exerted by a

surface on an object

pressing on it – always

perpendicular to the

surface.

Force that opposes or

tends to oppose motion

between surfaces in

contact

Pull exerted by a

stretched spring, string

or rope on an object

attached to it.

N

T

f

Non-Contact Forces

Gravitational Force Electric Force Magnetic Force

Pull exerted by Earth’s

gravity on any object

(i.e. weight)

The attractive (pull) or

repulsive (push) forces

between electric

charges.

The attractive (pull) or

repulsive (push) forces

between magnets.

W

+ -Attractive electric forces

between unlike charges.

+ +

Repulsive electric forces

between like charges.

S N S N

Attractive magnetic

forces between unlike

poles.

S NN S

Repulsive magnetic

forces between like

charges.

Effects of a force

Move a body at

rest

Increase the

speed of a moving

body

Decrease the

speed of a moving

body

Change direction

of a moving body

Throwing a ball

Kicking a rolling

ball to make it

move faster

Using your feet to

cushion a rolling

ball slowly to a

stop

Kicking a rolling

ball back to its

original position

• A diagram which shows all the forces acting

on a body

• Consists of a sketch of a body and arrows

representing the forces acting on it.

Weight

Normal reaction force

Tension

Friction

• Weight of a body is the gravitational force

exerted by the Earth on the body.

• Always acts at the centre of gravity of a body.

• Directed towards the centre of the Earth

W

• Normal reaction force is the force exerted by

the contact surface on the body

Acts perpendicularly to the contact surface

• Two forces act on the body:

the Normal reaction force and the weight.

W

N

• The tension is directed along the length of the string and pulls equally on the support and the body.

• But this is not a free body diagram!There are 3 bodies: The support, the string and the body.

T

T

• Isolate the body as a free-body

• Free-body diagram of the body consists of the

tension and weight only.

T

W

• The free-body diagram of the crate would consist

of

• The weight of the crate,

• The tension force exerted on the crate,

• The contact force of the floor of the crate.

• Weight and contact force are equal. No

resultant force acting vertically.

• Resultant force of crate is due to the tension

force.

Useful or Useless?

At the end of the lesson, you should be able to:

• Explain the effects of friction on the motion of

a body

• Friction is the contact force that oppose or tends to oppose motion between surfaces in contact.

• Represented by small f

• Result from irregularities of surfaces

• Must be drawn along the surface of contact

• Slows down or stops an object in motion

F

f

• Air resistance, or drag, is the friction

between a moving object and the air it is

moving through.

• Slows vehicles down and uses more fuel

• Streamlining helps overcome air resistance

• To move forward, you place one foot in front of you and push you other foot backwards on the ground.

• The friction between your shoe and the ground prevents slipping.

• As the wheel turns, friction is generated between the wheel and the road.

• At point of contact, wheel is moving backwards.

• Friction acts in the opposite direction, providing the forward force that drives the car.

• Without friction, the wheels will spin without

helping the car to move forward.

E.g. Car stuck in mud

• Wear and tear of surfaces

e.g. Soles of shoes

The frictional force between two surfaces on a horizontal

plane changes with several factors.

• Nature of the surfaces in contact: the rougher the

surface the greater is the friction

• Proportional to the force pressing the surfaces

together

• in the case of horizontal movement, the pressing force is the

weight.

• Independent of the area of contact!

• A larger area does not mean greater friction!

Complete the questions on the last page of the notes.

We will discuss the answer in class.