Q.12
Which one of the following pairs comprises one scalar quantity and one vector quantity?
A kinetic energy and momentum
C velocity and acceleration
Answer: A
Kinetik energi, potensial energy and work are scalar quantities.
Momentum, velocity, acceleration, weight and force are vector.
Q.13
Three forces of magnitudes 5 N, 4 N and 3 N are in equilibrium.
Assuming that sin 370 = 0.6, what is the angle between the 5 N force and the 3 N force?
A 370 B 530 C 900 D 1270
Answer: D
Sketch a possible vector diagram of forces:
Q.14
Ben, Sam and Jim are pushing a large object. Ben exerts a force of 400 N at 600 to 0x while Sam exerts a force of 200 N at a direction of 300 to 0x, as shown in the given diagram.
Which one of the following sketches shows the magnitude and direction of the smallest force that Jim shoud exert such that the resultant of the three forces acts along 0x?
Answer: C
Sketch vector diagram of force:
For the resultant of all 3 forces to be along 0x, the vector representing Jim’s force should end in 0x.
For Jim’s force to be smallest, it should be perpendicular to 0x.
Q.15
The given diagram shows a horizontal force F applied to a body of mass m on a smooth plane inclined at an angle θ to the horizontal.
What is the magnitude of the resultant force acting on the body?
A F cosθ – mg sinθ B F sinθ + mg cosθ
C F sinθ – mg cosθ D F cosθ + mg sinθ
Answer: A
Deduce that, in the given context, motion is possible only along plane surface,
Resultant force must be along plane surface.
Sketch vector diagram of forces (with components along plane surface).
Resultant along plane surface, upward +ve,
= F cosθ – mg sinθ
Q.16
On a smooth horizontal surface, a body of mass 2 kg is moving with a speed of 1.41 m s-1 in a nort-easterly direction, when a force of 0.2 N acting in a westerly direction is applied to the body for 10 s. What is resultant speed of the body?
A 0.41 m s-1 in a north-easterly direction B 1.00 m s-1 in a northerly direction
C 1.41 m s-1 in a north-westerly direction D 2.24 m s-1 in a direction 63.40 east of north.
Answer: B
Change in momentum of body (westerly),
∆p = force x time = (0.2) x (10) = 2.0 kg m s-1
Change in velocity (westerly),
∆v = ∆p/mass = (2.0)/(2.0) = 1.0 m s-1
Sketch vector diagram of velocities:
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