Paper 1 Dynamics Answers

Q	Ans	Brief reason
1	B	$\sum {\vec{F}}_{\mathrm{ext}}=\mathrm{d}\vec{p}/\mathrm{d}t$.
2	B	$a=F/m=12/3.0=4.0\mathrm{m}{\mathrm{s}}^{-2}$.
3	A	Constant velocity means zero acceleration.
4	C	Same interaction, forces on different bodies.
5	B	$p=mv=0.20(15)=3.0\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-1}$.
6	B	Impulse equals change in momentum.
7	B	Area under $F$-$t$ gives impulse.
8	C	Momentum conservation requires negligible resultant external impulse.
9	A	$v=0.50(2.0)/(2.00)=0.50\mathrm{m}{\mathrm{s}}^{-1}$.
10	C	Elastic collisions conserve both.
11	C	$N-mg=ma$, so $N>mg$.
12	C	Apparent weight is normal contact force.
13	B	Internal forces cancel for the combined system.
14	B	Resultant force and acceleration oppose velocity.
15	A	$\mathrm{N}=\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-2}$.
16	A	$F_{\mathrm{avg}}\Delta t=\Delta p$.
17	B	Momentum may still be conserved; kinetic energy is not.
18	C	Third-law partners are equal and opposite.
19	B	Momentum is vector mass times velocity.
20	B	Reversal makes $\Delta \vec{p}={\vec{p}}_f-{\vec{p}}_i$ large.