1© Manhattan Press (H.K.) Ltd. Weight Normal reaction Normal reaction 1.2 Different types of forces Tension Tension Elastic force Elastic force Friction

© Manhattan Press (H.K.) Ltd. 1

• WeightWeight

• • Normal reactionNormal reaction

1.2 Different 1.2 Different

types of forcestypes of forces

• • TensionTension

• • Elastic forceElastic force• • Friction (frictional force)Friction (frictional force)


Weight (W)

1.2 Different types of forces (SB p. 40)

The weight of a body (on the earth) is actually the gravitational attractive force acting on the body by the earth.

Towards earth’s centre


Normal reaction (R)


When a body is placed on the ground (or an inclined plane), the force exerted vertically upwards by the ground on the body is called normal reaction.

Balance downward force


Tension (T)


Tension is a pulling force acting along the string when a body is pulled by an inextensible string.

Direct away from bodiesGo to

More to Know 4More to Know 4


Elastic force (Fe)


When an elastic body (e.g. spring) is stretched or compressed, an elastic force (Fe ) is set up along it in order to restore to its original length.

Restoring force – opposes the deformation


Elastic force (Fe)


Hooke’s Law: Fe = ke

k – force constant, e – extension (or compression) of body


Elastic force (Fe)


e.g.

applied force is balanced by elastic force

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If no applied force, elastic force causes the spring to restore to its original length


Friction (frictional force)


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Friction is a force that always acts to oppose motion.

relative motion between 2 surfaces

friction acts along 2 rough contact surfaces




1. Static friction

Friction v.s. applied force




1. Static friction

Static friction – occurs between surfaces (at

rest)

Static friction increases to prevent motion

Limiting friction – maximum static friction

fL = s R

s = coefficient of static friction

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Common Error




2. Kinetic friction Kinetic friction – occurs between surfaces (moving)

fk = k R

k = coefficient of kinetic friction




Note:

1. Both limiting friction (fL) and kinetic friction (fk) are directly proportional to the normal reaction (R).

2. μk is often less than μL, so less force is needed to keep the block moving than to start moving it.

3. Both fL and fk are independent of the surface areas of the contact surfaces.

4. fk does not depend on the speed of relative motion.Go to

Example 1Example 1

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Example 2Example 2


End


Tension

The attraction exerted among the particles in the stretched string is the cause of tension.

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Spring deformity

In Fig. 1.10, if the applied force ( F ) gets too big, the spring deforms. The spring has passed its elastic limit and will not go back to its original length.

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Car tyre

A car travels on a road. If the road is wet, the wheels of the car may lose their grip. Hence, it is important to have sufficient gaps between the tread on a car tyre to increase friction. The gaps help to drain away water from the area of contact between the tyre and the road. A film of water between the tyre and the road will be disastrous when the brakes are applied. Return to

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It is often mistaken that the wider tyres produce more friction. The friction is independent of the contact area. The larger contact area reduces pressure and thus reduces wear, besides heat can be conducted away faster.

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Q:Q: The figure shows a wooden block B at rest on another block A. Both blocks are connected by a string which goes round a fixed smooth pulley. The kinetic friction between block A and the table is 14 N and that between blocks B and A is 10 N. What is the horizontal force P which is required to move block A with uniform velocity?

Solution



Solution:Solution:

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Let T = tension in the string.

When A moves with uniform velocity, B also moves with the same speed but in the opposite direction. The forces acting on A and B are shown in the figure above.

Consider forces acting on B,

Tension T = Friction f2 = 10 N

Consider forces acting on A,

P = f2 + T + f1 = 10 + 10 + 14 = 34 N



Q:Q: A metal block of mass 4 kg is on a plane inclined at an angle of 30° to the horizontal. The block slides down the plane from rest and travels a distance of 1.0 m in 2.0 s. (Assume g = 9.81 m s−2.)(a) What is the friction between the block and the inclined plane when the block is sliding?(b) What is the coefficient of kinetic friction

(μk)?

Solution



Solution:Solution:

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(a) s = ut +at2/2

1.0 = 0 +a(2.0)2

∴ a = 0.5 m s−2

F = ma

mg sinθ − f = ma

∴ f = m ( g sinθ − a)

= 4 (9.81 sin30° − 0.5)

= 17.62 N

(b) R = mg cosθ

= 4 × 9.81 × cos30°

= 33.98 N

fk = μk R

μk = 17.62 / 33.98

= 0.52

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1© Manhattan Press (H.K.) Ltd. Weight Normal reaction Normal reaction 1.2 Different types of forces Tension Tension Elastic force Elastic force Friction