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6.4 Daily life examples of force
Skiing squirrel Tension in a string Normal reaction Friction Check-point 3 Weight — the pull of gravity Check-point 4 1 2 3 ? 4 ? 6.4 Daily life examples of force
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Skiing squirrel There are different forces acting on this ‘skier’.
Can you identify them? 6.4 Daily life examples of force
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1 Tension in a string Tension – force in a string – either left/right
(depending on what it acts on) tension Tension can act in opposite directions. 6.4 Daily life examples of force
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2 Normal reaction The force acting on a boy by the surface in contact with him is normal reaction. Any force to balance the weight? normal reaction weight An upward force provided by the surface acting perpendicular (normal) to the boy Net force acting on the boy = 0 6.4 Daily life examples of force
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2 Normal reaction No matter he stands at A (a horizontal surface) or
B (an inclined surface) normal reaction normal reaction the normal reaction acting on boy is perpendicular (normal) to the road surface. 6.4 Daily life examples of force
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a Where does friction come from?
When the block moves, the tiny bumps catch each other friction (opposing motion) bumps of surfaces Likewise… v v friction friction Friction arises when object slides (or tends to slide) over another object. 6.4 Daily life examples of force
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b Reducing friction and using friction
Friction opposes or prevent the motion of an object. People find ways to reduce it, for example: 6.4 Daily life examples of force
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Experiment 6e Gently push them and observe their motion.
Frictionless motion Gently push them and observe their motion. 6.4 Daily life examples of force
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Experiment 6e Frictionless motion The ring puck, balloon punk and vehicle on linear air track all travel at a nearly constant speed along a straight line. Video 6.4 Daily life examples of force
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b Reducing friction and using friction
However, friction is essential in our daily life. For example: 6.4 Daily life examples of force
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3 Friction 6.4 Daily life examples of force
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c Fluid friction Fluid: gas/liquid Moving in liquid friction
(e.g. air resistance) Moving speed , fluid friction 6.4 Daily life examples of force
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e.g. fast-moving car experiences large air resistance
c Fluid friction moving speed , fluid friction e.g. fast-moving car experiences large air resistance streamlining the car body air passes car body smoothly friction 6.4 Daily life examples of force
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c Fluid friction For the ball-bearing: At beginning, vball-bearing = 0
force(s) acting on it: weight • weight • air resistance net force: downward Accelerates: downwards 6.4 Daily life examples of force
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but at a decreasing value
c Fluid friction For the ball-bearing: v gradually force(s) acting on it: • weight (constant) • air resistance () air resistance net force: downward v Accelerates: downwards weight but at a decreasing value 6.4 Daily life examples of force
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When air resistance = weight,
c Fluid friction For the ball-bearing: v air resistance When air resistance = weight, net force = acceleration = air resistance Newton’s 1st law v move at constant speed such v is called terminal speed. weight 6.4 Daily life examples of force
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Experiment 6f 1 Drop a ball-bearing and observe its motion.
Motion affected by fluid friction 1 Drop a ball-bearing and observe its motion. 2 Release a parachute and watch its motion. 6.4 Daily life examples of force
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Experiment 6f Motion affected by fluid friction We get… Both ball-bearing and the parachute fall with constant speeds after accelerating for a short time. Video Video Simulation 6.4 Daily life examples of force
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d How air resistance affects the motion of free falling objects
is similar to a skydiver taking a free fall 6.4 Daily life examples of force
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d How air resistance affects the motion of free falling objects
v air resistance air resistance When air resistance = weight v net force = 0 weight move at constant (terminal) speed (as high as 60 m s–1) 6.4 Daily life examples of force
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d How air resistance affects the motion of free falling objects
Video 6.4 Daily life examples of force
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d How air resistance affects the motion of free falling objects
v / m s1 fair = mg, a = 0, vterminal open parachute, fair greatly 60 a fair fair > mg, ve a v , fair (<mg), +ve a v , fair mg fair = mg, v'terminal 2 t / s dive, v = 0, air resistance (fair) = 0 6.4 Daily life examples of force
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3 Friction E 6.4 Daily life examples of force
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Check-point 3 A trolley is... 2 A book rests on...
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A trolley is connected to a block by a string.
Check-point 3 – Q1 A trolley is connected to a block by a string. (a) In the figure, draw the directions of tension acting on the trolley and acting on the block. tension tension 6.4 Daily life examples of force
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The acceleration of the trolley = = ( ) 4/1 4 m s-2 downwards
Check-point 3 – Q1 (b) If the tension in the string is 4 N, what is the acceleration of the block? The acceleration of the block is (equal to/ greater than/smaller than) that of the trolley. By F = , ma The acceleration of the trolley = = ( ) 4/1 4 m s-2 downwards 6.4 Daily life examples of force
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A book rests on an inclined plane.
Check-point 3 – Q2 A book rests on an inclined plane. Which of the following is the correct free-body diagram of the book? 6.4 Daily life examples of force
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Which of the following is the correct free-body diagram of the book?
Check-point 3 – Q2 Which of the following is the correct free-body diagram of the book? A B C D 6.4 Daily life examples of force
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4 Weight the pull of gravity
Weight is the gravitational force acting on an object by the Earth. Measured in newton (N) if an object falls freely, its weight W gives it an acceleration of g. Applying F = ma , W = mg 6.4 Daily life examples of force
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4 Weight the pull of gravity
W = mg Taking g to be 10 m s2, the weight of a 1-kg mass is W = 1 kg 10 m s2 = 10 N 6.4 Daily life examples of force
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4 Weight the pull of gravity
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4 Weight the pull of gravity
a Mass and weight Mass Weight a measure of an object’s inertia gravitational force change from place to place, depends on g same in everywhere SI unit: kg SI unit: N symbol: m symbol: W 6.4 Daily life examples of force
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a Mass and weight Gravitational force (g) is different on the surfaces of Earth, the Moon and other planets: 6.4 Daily life examples of force
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a Mass and weight Video Simulation 6.4 Daily life examples of force
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According to Newton’s 2nd law,
a Mass and weight According to Newton’s 2nd law, the gravitational force ⇒ acceleration weight on the Moon = 1/6 weight on the earth Simulation 6.4 Daily life examples of force
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b Beam balance & spring balance
The weights of 2 different masses, m1 & m2, in the same place: W1 = m1g & W2 = m2g Weights of different objects are proportional to their masses in a particular place. 6.4 Daily life examples of force
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b Beam balance & spring balance
Beam balance can measure mass by comparing weight of an unknown mass with weight of known standard mass. beam balance unknown mass standard masses 6.4 Daily life examples of force
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b Beam balance & spring balance
A spring balance measures weight but NOT mass. It gives different values of weight of the same object at different places. spring balance 6.4 Daily life examples of force
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c Apparent change in weight in a lift
When you travel in a fast lift, you can feel a loss or gain of weight. How does this happen? 6.4 Daily life examples of force
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c Apparent change in weight in a lift
Video Simulation 6.4 Daily life examples of force
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c Apparent change in weight in a lift
There are 2 forces acting on the girl: unchange weight W & normal force R by the lift floor – can be measured by a weighing scale W R – scale reading shows the weight felt by the girl, i.e. the girl's apparent weight may change 6.4 Daily life examples of force
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c Apparent change in weight in a lift
R changes when travelling in the lift. 10/F G/F decelerates downwards R > W rest at G/F rest at 10/F R = W accelerates downwards R = W uniform velocity R < W R = W 6.4 Daily life examples of force
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c Apparent change in weight in a lift
By Newton's 2nd law, the acceleration of the lift and the girl can be found if R is known. W R 6.4 Daily life examples of force
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4 Weight the pull of gravity
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Check-point 4 Mark and label all forces... 2 The force pulling the...
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A squirrel of 1.2 kg is skiing on water.
Check-point 4 – Q1 A squirrel of 1.2 kg is skiing on water. Mark and label all forces acting on the squirrel. normal reaction tension friction weight 6.4 Daily life examples of force
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Check-point 4 – Q2 If the force pulling the squirrel is 5 N, the friction of water is 3 N. What is the acceleration of the squirrel? normal reaction tension friction weight By F = ma , 5 – 3 = 2 a a = 1.67 m s2 (towards right) 6.4 Daily life examples of force
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The End 6.4 Daily life examples of force
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Example 4 Ave. acceleration of cyclist = 2 m s–2
Acceleration of a cyclist Ave. acceleration of cyclist = 2 m s–2 Mass of cyclist and bicycle = 84 kg What is the average friction acting on him and the bicycle? If his average driving force is 140 N, 6.4 Daily life examples of force
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Example 4 Take the direction to the right as positive.
Acceleration of a cyclist Take the direction to the right as positive. Net force acting on the cyclist = 140 N − f By F = ma, 140 − f = 84 × 2 f = −28 N (28 N towards the left) 6.4 Daily life examples of force
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opposes the fall of raindrop
Example 5 E Raindrop Forces A and B act on a raindrop as it falls to the ground. B air resistance What are A and B ? Why does it fall with a constant speed? weight A = B, but opposite A net force on raindrop = 0 By Newton’s 1st law, it falls at constant speed. 6.4 Daily life examples of force
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Example 6 Thrust of Long March 2F rocket At lift-off, Long March 2F rocket (carried with Shenzhou 6 spacecraft) has an lift-off thrust of N. Its total initial mass is kg. What is the initial acceleration of the rocket and the spacecraft? 6.4 Daily life examples of force
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Example 6 By Newton’s 2nd law of motion, F = ma U − mg = ma
Thrust of Long March 2F rocket Thrust U Rocket's (with spacecraft) mass = kg By Newton’s 2nd law of motion, F = ma U − mg = ma − = a a = 1.72 m s2 (upwards) mg 6.4 Daily life examples of force
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Example 7 Find the acceleration of the lift in (a) 0-5 s, (b) 20-25 s.
Apparent change in weight in a lift scale reading/N time/s 5 20 25 450 500 550 28 mass of girl = 50 kg when lift is stationary on 10/F, reading of weighing scale = 500 N lift takes 25 s to travel to G/F Find the acceleration of the lift in (a) 0-5 s, (b) s. 6.4 Daily life examples of force
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Example 7 (a) Take downwards as +ve. During 0-5 s, R < W and
Apparent change in weight in a lift (a) Take downwards as +ve. During 0-5 s, R < W and W R = ma 500 450 = 50a a = 1 m s2 The lift accelerates downwards at 1 m s2. 6.4 Daily life examples of force
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Example 7 (b) During 20-25 s, W < R and W R = ma
Apparent change in weight in a lift (b) During s, W < R and W R = ma 500 550 = 50a a = 1 m s2 The lift decelerates downwards at 1 m s2. 6.4 Daily life examples of force
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