一堆physics問題,請大家幫下手, 謝謝! (15分)

1. As no diagram is given, I presume the underlined object is the cricket ball only

It is obvious that a net force is present. The force comes from the impact with the ground and is pointing vertically upward.
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2. The passengers feel lighter.
The reason is because of inertia. The car goes down suddenly along the arc after passing over the highest point. Passengers inside tends to remain at the higest point by inertia, thus exerting less force onto their seats. The normal reaction on each passenger is reduced and the passenger feels less weight.
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3. Let m be the mass of the bob and g be the acceleration due to gravity
By resolving the tension T on the string vertically and horizontally, we have:

Vertically,
T.cos(theta) = mg ----------------- (1)
Horizontally,
T.sin(theta) = m.r.w^2 ------------ (2)
where r is the radius of the circle
w is the angular speed of revolution

But w = 2.pi/P, where pi =3.14159.....
thus, T.sin(theta) = m.r.(2.pi/P)^2 ---------- (3)

(3)/(1): sin(theta)/cos(theta) = (r/g).[2.pi/P]^2

since from geometry, sin(theta) = r/L
i.e. r = L.sin(theta)

thus (3) becomes, sin(theta)/cos(theta) = [L.sin(theta)/g].[2.pi/P]^2
ie. cos(theta) = (g/L).[P/2.pi]^2

first difference: the magnitude of acceleration is constant in a circular motion, but not in the case of SHM

second difference: the speed is constant in circular motion, but varies with time in SHM