Dynamics

Lesson ObjectivesAt 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

What are forces?• A push or a pull – represented by arrows• S.I. Unit: newton (N)

Contact Forces – force touching the objectNormal 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

What are forces?Non-Contact Forces – force not touching the object

Gravitational Force Electric Force Magnetic ForcePull 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 NAttractive magnetic forces between unlike poles.

S NN SRepulsive magnetic forces between like charges.

Effects of Forces on MotionEffects 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 ballKicking 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

Representation of Forces

Lesson ObjectivesAt the end of the lesson, you should be able to:• 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)

Free Body Diagrams• 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.WeightNormal reaction forceTensionFriction

Weight (W)• 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

(always vertically downwards)

W

Normal Reaction force (N)• Normal reaction force is the force

exerted by the contact surface on the bodyActs perpendicularly to the contact surface

• Two forces act on the body: the Normal reaction force and the weight.

W

N

Tension (T)• 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 – rod, string, object

T

T

Tension (T)• Isolate the body as a free-body• Free-body diagram of the body consists

of the tension and weight only.

T

W

Combining all 3 kinds of forces

• 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.

Combining all 3 kinds of forces

• Weight and contact force are equal. No resultant force acting vertically.• Resultant force of crate is due to the

tension force.

Useful or Useless?Friction

Lesson ObjectivesAt the end of the lesson, you should be able to:• Explain the effects of friction on the

motion of a body

Friction• 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

Ff

Friction• 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

Friction (Positive effects)

• 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.

Friction (Positive effects)• 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.

Friction (Positive effects)

• Without friction, the wheels will spin without helping the car to move forward.E.g. Car stuck in mud

Friction (Negative effects)• Wear and tear of surfaces

e.g. Soles of shoes

Factors that affect frictional forceThe 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!

AssignmentComplete Worksheet 1in pencil.

We will discuss the answer in class.