- Messages
- 142
- Reaction score
- 131
- Points
- 43
June 2012, p13
Question 10
Considering forces acting on the barrel:
resultant = weight of barrel - T
120a =120g - T (eqn 1)
a=acceleration of barrel, T=tension in rope
considering forces on man:
since man moves upwards, the net resultant force is upwards:
80a = T - 80g (eqn 2)
Both the man and the barrel have the same acceleration, as they are connected by the pulley system.
hence we add eqn 1 and eqn 2:
120 a + 80 a = 120g -T + T - 80g
200 a = 40 g
a = 0.2 g ms^-2
now we can use equation of motion : v^2 = u^2 + 2as
we are required to find the man's speed when he is level with the barrel, at this point, the displacement of the man(and of barrel btw) is 18/2, i.e it is 9 m. They meet halfway.
so: s=9, a= 0.2g, u = 0
replace in eqn of motion and you have the answer
Question 14
Upthrust is an upward force resulting from the difference pressures acting at the bottom and at the top of a body.
difference in pressure = pressure at bottom - pressure at top
since: pressure= force/area
force = pressure x area
in this case:
upthrust = difference in pressure x area
Hence the answer is C
We do not consider the weight as the upthrust is not a resultant force, it's just one of the forces acting on a body immersed in fluid, along with the weight and drag force.
Question 17:
First of all, find the mass: density = mass/volume
2.0 × 10^3= mass/ (0.60^3)
mass = 432 kg
The simply apply the formula for gravitational potential energy: P.E = mgh
June 2010, p13:
Question 17
efficiency = useful output energy/total input energy
The total input energy here is the work done by the force F in moving the car up the slope.
total input energy= Fs
The useful output energy is the increase in gravitational potential energy
useful output energy = mg Δh
Δh = vertical height covered by car = s sin α
useful output energy = m x g x (s sin α)
efficiency = m x g x (s sin α) / Fs
as "s" is common to both the denominator and numerator, it is eliminated, and the answer is D
