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

2 © Manhattan Press (H.K.) Ltd. 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

3 © Manhattan Press (H.K.) Ltd. Normal reaction (R) 1.2 Different types of forces (SB p. 40) 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

4 © Manhattan Press (H.K.) Ltd. Tension (T) 1.2 Different types of forces (SB p. 41) Tension is a pulling force acting along the string when a body is pulled by an inextensible string. Direct away from bodies Go to More to Know 4 More to Know 4

5 © Manhattan Press (H.K.) Ltd. Elastic force (F e ) 1.2 Different types of forces (SB p. 41) When an elastic body (e.g. spring) is stretched or compressed, an elastic force (F e ) is set up along it in order to restore to its original length. Restoring force – opposes the deformation

6 © Manhattan Press (H.K.) Ltd. Elastic force (F e ) 1.2 Different types of forces (SB p. 41) Hooke’s Law:F e = ke k – force constant, e – extension (or compression) of body

7 © Manhattan Press (H.K.) Ltd. Elastic force (F e ) 1.2 Different types of forces (SB p. 42) e.g. applied force is balanced by elastic force Go to More to Know 5 More to Know 5 If no applied force, elastic force causes the spring to restore to its original length

8 © Manhattan Press (H.K.) Ltd. Friction (frictional force) 1.2 Different types of forces (SB p. 42) Go to More to Know 6 More to Know 6 Friction is a force that always acts to oppose motion. relative motion between 2 surfaces friction acts along 2 rough contact surfaces

9 © Manhattan Press (H.K.) Ltd. Friction (frictional force) 1.2 Different types of forces (SB p. 43) 1. Static friction Friction v.s. applied force

10 © Manhattan Press (H.K.) Ltd. Friction (frictional force) 1.2 Different types of forces (SB p. 43) 1. Static friction Static friction – occurs between surfaces (at rest) Static friction increases to prevent motion Limiting friction – maximum static friction f L =  s R  s = coefficient of static friction Go to Common Error

11 © Manhattan Press (H.K.) Ltd. Friction (frictional force) 1.2 Different types of forces (SB p. 44) 2. Kinetic friction Kinetic friction – occurs between surfaces (moving) f k =  k R  k = coefficient of kinetic friction

12 © Manhattan Press (H.K.) Ltd. Friction (frictional force) 1.2 Different types of forces (SB p. 44) Note: 1. Both limiting friction (f L ) and kinetic friction (f k ) are directly proportional to the normal reaction (R). 2. μ k is often less than μ L, so less force is needed to keep the block moving than to start moving it. 3. Both f L and f k are independent of the surface areas of the contact surfaces. 4. f k does not depend on the speed of relative motion. Go to Example 1 Example 1 Go to Example 2 Example 2

13 © Manhattan Press (H.K.) Ltd. End

14 © Manhattan Press (H.K.) Ltd. Tension The attraction exerted among the particles in the stretched string is the cause of tension. Return to Text 1.2 Different types of forces (SB p. 41)

15 © Manhattan Press (H.K.) Ltd. 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. Return to Text 1.2 Different types of forces (SB p. 42)

16 © Manhattan Press (H.K.) Ltd. 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 Text 1.2 Different types of forces (SB p. 42)

17 © Manhattan Press (H.K.) Ltd. 1.2 Different types of forces (SB p. 43) 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. Return to Text

18 © Manhattan Press (H.K.) Ltd. 1.2 Different types of forces (SB p. 44) 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

19 © Manhattan Press (H.K.) Ltd. 1.2 Different types of forces (SB p. 44) Solution: Return to Text 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 f 2 = 10 N Consider forces acting on A, P = f 2 + T + f 1 = = 34 N

20 © Manhattan Press (H.K.) Ltd. 1.2 Different types of forces (SB p. 45) 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 (μ k )? Solution

21 © Manhattan Press (H.K.) Ltd. 1.2 Different types of forces (SB p. 45) Solution: Return to Text (a) s = ut +at 2 /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) = N (b) R = mg cosθ = 4 × 9.81 × cos30° = N f k = μ k R μ k = / = 0.52