Paper 1 Forces Answers

Q	Ans	Brief reason
1	B	A free-body diagram shows external forces on one chosen body.
2	C	Third-law partners act on different bodies within the same interaction.
3	C	$W=mg=2.50(9.81)=24.5\mathrm{N}$.
4	B	$F_x=40\cos 30^{\circ }=34.6\mathrm{N}$.
5	A	Translational equilibrium requires $\sum \vec{F}=0$.
6	B	Moment $=Fd=12(0.25)=3.0\mathrm{N}\mathrm{m}$.
7	C	Pressure $=F/A$, so unit is $\mathrm{N}{\mathrm{m}}^{-2}$.
8	B	$F=kx$ within the proportional region.
9	B	Gradient of $F$ against $x$ is $k$.
10	B	Archimedes’ principle.
11	B	Drag increases with speed until resultant force is zero.
12	B	Force has magnitude and direction.
13	C	A couple gives a turning effect with no resultant force.
14	B	Weight can be treated as acting through the centre of gravity.
15	B	Lower centre of gravity and wider base increase stability.
16	A	$p=\rho gh$ for gauge pressure at depth.
17	C	Normal means perpendicular to the surface.
18	C	Static friction responds to impending relative motion.
19	A	Perpendicular distance from pivot is zero.
20	B	Components are a vector decomposition, not extra interactions.