physics module F4 (force and motion 2)

8/9/2019 physics module F4 (force and motion 2)

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2.5 UNDERSTANDING THE EFFECTS OF A FORCE [««../28 x 100 = .««% ]

A student is able to :

y describe the effects of balanced forces acting on an object

y describe the effects of unbalanced forces acting on an object

y determine the relationship between force, mass and acceleration i.e F = ma

y Solve problems using F = ma A. Answer the following questions.

1. What are the effects of a force when acting on an object? A force can«.

(i) (1) ««««... a stationary object

(ii) (2) «««««a moving object

(iii) change the (3) «««««. / (4) ««««« of an object

(iv) change the (5) ««««« / (6) «««««. of an object

2. Force is a (7) ( scalar / vector ) quantity .

3. The S.I. unit for force is (8) ««««...

B. Fill in the blanks with appropriate answers for balanced and unbalanced forces.

accelerate / decelerate constant velocity zero rest

1. Forces acting on an object are said balanced when the net force is (9) ««««

2. When the forces acting on an object are balanced, it is either at (10) «««««.. or

moving with (11) «««««. «««««««.

3. An object will (12) «««««««««««.. if the forces acting on it are not balanced.

C. Draw the graph and state the relationship between acceleration, a, force, F and mass, m.

1.

(13)

(m is constant)

(14)

(F is constant)

a

F

a

m

1



14

2. Relationship :(15)

a is (16) ««««««««««« to F

Relationship :(17)

a is (18) «««««««««««. to m

3. Combine the relationship:(19)

(20)

(21) F = k ; k is constant (k = 1)

(22) F =

4. The relationship between F , m and a is derived from Newton¶s (23) «««««« law

of (24)«««««««..

D. Solve the following.

1. A force, F is required to move an object of mass 1000 kg with an acceleration of 3 m s-2. Calculate F when(i) object is on a smooth surface(ii) object is on a surface where the frictional force is 200 N

Solution :

(i) (25)

(ii) (26)

FF

aa == 3 m s-2

mm == 1000 kkgg

w w a a

a w

w F



15

2. A block of mass 20 kg is pulled along the ground by a force, F of 60 N. The

frictional force is 10 N. Calculate the acceleration of the block.

Solution: (27)

3. A car of mass 1200 kg which is travelling at 90 km h-1 comes to a stop in adistance of 50 m when the brakes are applied. What is the average braking forceof the car?

Solution: (28)

F



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2.6 ANALYSING IMPULSE AND IMPULSIVE FORCE [ «««.. / 25 x 100 = «««.% ]


y Explain what an impulsive force is

y Give examples of situations involving impulsive forces

y Define impulse as a change of momentum ie Ft = mv-mu

y Define impulsive force as the rate of change of momentum in a collision or explosioni.e F = mv - mu

t

y Explain the effect of increasing or decreasing time of impact on the magnitude of theimpulsive force

y Describe situations where an impulsive force needs to be reduced and suggest waysto reduce it

y Describe situations where an impulsive force is beneficial

y Solve problems involving impulsive forces

A. Define impulse and impulsive force by completing the following table.

Situation Explanation

A ball of mass, m is kickedwith a force, F. The time of contact is t . The ballaccelerates from u to v .

From : F = ma

(1) F = m ¹¹ º

¸©©ª

¨

(2) F =t

mv ± mu = change of (3) «««««««««

Impulsive force

Ft

1w

F mumv w

(4) F =t

t

u

v= (5) «««««««««««««««««

@Impulsive force is defined as the (6) ........................

««««««««««««««. in a collision or

explosion.

Impulse= force x time

Impulse = Ft = (7) ««««««««

= change of (8) ««««««««..

SI unit for impulse is (9) ««« or (10) «««..



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B. 1. Fill in the blanks with appropriate answers.

Impulsive force is a (11) ««««« force which acts over a very (12) ««««.. time

interval during (13) ««««««««.. and (14)«««««««««.

2. By using the figures given, determine whether the impulsive force acting is large or

small.

(15) (16) (17)

C. Answer the following questions.

1. Two eggs of the same mass are released from the same height, as shown in thefigure below. Egg A falls on a concrete floor while egg B falls on a thick folded towel.

Using Figure A and Figure B, compare the conditions of the eggs after the fall andexplain the relationship between the material where the eggs landed and the force actingon the eggs upon landing.

(18) (19) (20)

Figure A Figure B



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Answer:

conditions of the eggs after the fall : (21) «««««««««««««««««

«««««««««««««««««««««««««««««««««««

The time of impact for hard surface is (22) «««««««««and thus produce(23) «««««««« impulsive force

2. A tennis ball of mass 45 g travels at a velocity of 70 m s ±1 immediately after it is struckby a racket. The time of contact between the tennis ball and the racket is 0.5 ms.Calculate (a) impulse and (b) impulsive force exerted on the ball.

Solution :(a) (24)

(b) (25)



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2.7 BEING AWARE OF THE NEED FOR SAFETY FEATURES IN VEHICLES

[ «.«./15 x 100 = «..«%]


y Describe the importance of safety features in vehicles

Safety features Importance

Padded dashboardTo cushion an impact and increases the (7) «««« «««««.of collision so the (8) ««««.. ««««««produced is therebyreduced.

Head rest (9)

(10) To prevent the windscreen from shattering.

Automatic air bag (11)

Safety seat belt (12)

(13) To minimize the force acting from a side- on collision.

(14) To prevent wheel lock and skidding, thus contributing to safer braking

(15) To increase the time interval of impact so that the resultantimpulsive force is reduced.

(3) «««««««««««««««««(ABS)

Shatter proof (2) «««««..

Side impact bars

Automatic air (4)«««

Head (5) ««......

Reinforced passenger compartment

Crumple zones

Safety seat (6) «««...

Padded (1) «««««.



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2.8 UNDERSTANDING GRAVITY [ «««« / 26 x 100 = «««. % ]


y Explain acceleration due to gravity

y State what a gravitational field is

y Define gravitational field strength

y Determine the value of acceleration due to gravityy Define weight (W) as the product of mass (m) and acceleration due to gravity (g) i.e

W = mg

y Solve problems involving acceleration due to gravity

A. Complete the following by fill in the appropriate answer.

1. An object will fall to the surface of the earth because it is pulled down by the

(1) «««««««.««««««««..

2. The pull or force of gravity also known as (2) ««««««««««««««««

3. When an object falls without encountering any resistance and under the force of

(3) «««. only, the object is said to be (4) ««««««««.

4. All objects (5)««« freely with the same (6) «««.«.

acceleration regardless of their (7) ««« and (8) ««..

5. The constant (9) «««««.. of objects falling freely due

to the force of (10) «««.. is known as (11) ««««««

«««««, g.

6. Practically, a free (12) ««. can only take place in

(13) ««««. where there is no air resistance.

7. The (14) «««««««. field of the earth is the region around the earth where

the gravitational force acts.

8. The gravitational field strength is defined as the (15) ««««««. ««««««.

acting on 1 kg mass.9. When an object falling freely, acceleration due to gravity, g = (16) «««««

An object at the surface of the earth experiences gravitational force, g =

(17) «««.

vacuum

stone

feather



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B. State the differences between weight and mass in the table below.

Weight Mass

1. the gravitational force act on the object.

2. (19)

3. vector quantity

4. (21)

5. SI unit : Newton (N)

1. (18)

2. constant quantity everywhere

3. (20)

4. base quantity, mass = m

5. (22)

C. Solve the problem below

1. A coconut falls from rest and hit the ground after 1.2 s.(a) What is the velocity of the coconut just before hits theground? (23)(b) Calculate the height of the coconut before it fall. (24)

4. A stone is thrown vertically upwards with a velocity of 20 m s-1. If g = 10 m s-2,calculate(a) the maximum height reached (25)(b) the time taken for the stone to return to its original position. (26)

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2.9 ANALYSING FORCES IN EQUILIBRIUM [ ««« / 25 x 100 = «««« %]


y Describe situations where forces are in equilibrium

y State what a resultant force is

y Add two forces to determine the resultant force

y Resolve a force into the effective component forcesy Solve problems involving forces in equilibrium

A. Fill in the blanks.

1. Forces that act on an object are said to be in (1) ««««««.. when the object

is (2) ««««««««. or is moving at (3) ««««««« velocity.

2. The net force that acts on an object when two or more forces act on it is known

as the (4) ««««««« ««««««.

3. When (5) «««««««. is reached, the resultant force acting on the object

is (6) ««««

4. Newton¶s (7) ««««. law of motion states that to every (8) ««««. there is

an equal but opposite (9) ««««««..

5. A force can be resolved into component which are (10) ««««««« to one

another.

Horizontal component Fx = (11)

Vertical component Fy = (12)

B. In the table below, mark and label all the forces that act on the object.

1. A box is at rest on a table.(13) 2. An object that is suspended by a rope.(14)



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3. An airplane flying at a constant velocity.(15)

4. A trolley being pushed at a constantvelocity (16)

5.

Solution :(a) (17)

A box of mass 4 kg is being pulled by a forceof 60 N at an angle of 30 0 to the surface of afloor. Calculate(a) the component of the force which causesthe box to move forward.(b) the acceleration of the box if the frictionalforce between the floor and the box is 5 N,

(b) (18)

6. (19) A man pushes a box of mass 40 kg is up aninclined plane. The inclined plane makes an angleof 30 0 with the horizontal floor. The frictional forceacting between the inclined plane and the box is120 N. If the man pushes the box with a force of 400 N, calculate the acceleration of the box.

Solution : (20)

300

60 N

4 kg



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7.

Solution :(a) (21)

(b) (22)

A 4 kg trolley is connected by a rope to a load of mass 6 kg. The friction between the table and thetrolley is 15 N. The load is then released.

Assuming that the pulley is smooth and the rope isof negligible mass, find(a) the resultant force that act on the system

(b) the acceleration of the system(c) the tension of the rope

(c) (23)

8. Two loads of mass 3 kg and 5 kg are connected by arope which passes through a smooth pulley. If thesystem is released from rest, calculate the accelerationof the 5 kg load.

Solution : (24)

3 Forces in equilibrium

9. A block of mass 6 kg is suspended vertically by astring tied at O to the string AOB. If the system is inequilibrium, draw a vector diagram (triangle of forces) and hence determine the tension of stringOA and OB.

Solution : (25)

6 kg

4 kg Smooth pulley

3 kg

5 kg

300A

OB

6 kg

physics module F4 (force and motion 2)

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