IB Physics SL A.2 Forces & momentum Question Bank

Practise modelling forces with free-body diagrams, applying momentum and impulse relations and connecting angular motion to centripetal acceleration.

Exam points

draw a free-body diagram, resolve forces and apply Newton's laws to equilibrium or acceleration
calculate normal, friction, tension, elastic, drag or buoyant forces from the stated force model
conserve signed momentum in collisions or explosions and calculate unknown masses or velocities
use impulse or force-time data to calculate momentum change, contact time or average force
link angular and linear speed, then identify the real force providing centripetal acceleration

Covered syllabus points

A.2.1—Newton’s three laws of motion A.2.2—Forces as interactions between bodies A.2.3—Free-body diagrams A.2.4—Resultant force from diagrams A.2.5—Contact forces A.2.6—Normal force A.2.7—Frictional force A.2.8—Tension A.2.9—Hooke’s law restoring force A.2.10—Viscous drag A.2.11—Buoyancy A.2.12—Field forces A.2.13—Weight A.2.14—Electric force Fe A.2.15—Magnetic force Fm A.2.16—Linear momentum conservation A.2.17—Impulse A.2.18—Impulse-momentum change A.2.19—Newton’s second law forms A.2.20—Elastic and inelastic collisions A.2.21—Explosions A.2.22—Collision energy A.2.23—Centripetal acceleration A.2.24—Centripetal force A.2.25—Direction change in circular motion A.2.26—Angular and linear speed

[Maximum number: 6]

(a)

B3. This question is in two parts. Part 1 is about a collision. Part 2 is about electric current and resistance.
Part 1 A collision
Two identical blocks of mass 0.17 kg and length 0.050 m are travelling towards each other along a straight line through their centres as shown below. Assume that the surface is frictionless.

The initial distance between the centres of the blocks is 0.900 m and both blocks are moving at a speed of $0.18 \mathrm{~m} \mathrm{~s}^{-1}$ relative to the surface.

[ 6 ]

(i)

As a result of the collision, the blocks reverse their direction of motion and travel at the same speed as each other. During the collision, 20 % of the kinetic energy of the blocks is given off as thermal energy to the surroundings.

[ 2 ]

(i)

State and explain whether the collision is elastic or inelastic.

[ 2 ]

(ii)

State Newton's third law of motion.

[ 1 ]

(iii)

During the collision of the blocks, the magnitude of the force that block A exerts on block B is $F_{\mathrm{AB}}$ and the magnitude of the force that block B exerts on block A is $F_{\mathrm{BA}}$ . On the diagram below, draw labelled arrows to represent the magnitude and direction of the forces $F_{\mathrm{AB}}$ and $F_{\mathrm{BA}}$ .

[ 3 ]

[Maximum number: 1]

An elevator (lift) and its load accelerate vertically upwards.

Figure for Question A.2 Forces and momentum question 2 — IB Physics SL

Which statement is correct in this situation?

The net force on the load is zero.

The tension in the cable is equal but opposite to the combined weight of the elevator and its load.

The normal reaction force on the load is equal but opposite to the force on the elevator from the load.

The elevator and its load are in translational equilibrium.

[Maximum number: 7]

A cylindrical cork of height H and cross-sectional area A is floating stationary in water. Its depth below the water surface is D.

Figure for Question A.2 Forces and momentum question 3 — IB Physics SL

(a)

Draw and label the forces acting on the cork.

[ 1 ]

Weight and Buoyancy drawn in proper directions (by eye) and correctly identified

Allow any sensible identification of the forces (e.g. mg for $F_{g}$ )

Do not allow "gravity" for weight.

(b)

Show that

\frac{D}{H}=\frac{\rho_{\mathrm{c}}}{\rho_{\mathrm{w}}}

where $\rho_{\mathrm{c}}$ is the density of the cork and $\rho_{\mathrm{w}}$ is the density of water.

[ 2 ]

Weight $=\rho_{c} \cdot A \cdot H \cdot g$

OR

Buoyancy $=\rho_{w} \cdot A \cdot D \cdot g$ $\rho_{c} \cdot A \cdot H \cdot g=\rho_{w} \cdot A \cdot D \cdot g$

<<algebraic manipulation to show the relationship>>

(c)

An icebreaking ship is designed to withstand a collision with an iceberg, a large partially submerged body of ice freely floating in water. The designers model the shape of the iceberg as a cylinder with an approximate cross-sectional area of $4200 \mathrm{~m}^{2}$ and height above sea level of 32 m .

The following data are available:

\begin{aligned} \rho_{\text {ice }} & =920 \mathrm{~kg} \mathrm{~m}^{-3} \\ \rho_{\text {seawater }} & =1030 \mathrm{~kg} \mathrm{~m}^{-3} \end{aligned}

[ 4 ]

(i)

Show that the mass of the iceberg is about $1.2 \times 10^{9} \mathrm{~kg}$ .

The designers assume that the mass of the ship is about $\frac{1}{40}$ the mass of the iceberg and is moving at $12 \mathrm{~m} \mathrm{~s}^{-1}$ when it collides with the iceberg. They stick together after the collision.

[ 2 ]

$\begin{aligned} \text { «(H -32) /H = 920 / } 1030 \text { so } \text { » } \mathrm{H}=300 \text { 《m》 } \text { OR } \mathrm{D}=268 \text { "m" } \mathrm{m}=920 \times 300 \times 4200 \text { OR } 1.16 \times 10^{9} \text { "kg" } \end{aligned}$

Must see either full substitution or
answer to 3 or more significant
figures.

(ii)

Calculate the speed of the ship after the collision.

Ice in a still lake will usually form in a single layer on the surface.

[ 2 ]

$\begin{aligned} \frac{1}{40} m 12=\left(\frac{1}{40} m+m\right) v v=0.29\left\langle\mathrm{~m} \mathrm{~s}^{-1}\right\rangle \end{aligned}$

All question bank results loaded

Covered syllabus points

A.2 Forces and momentum question 1

Question (a)

Question (i)

Question (i)

Question (ii)

Question (iii)

A.2 Forces and momentum question 2

A.2 Forces and momentum question 3

Question (a)

Question (b)

Question (c)

Question (i)

Question (ii)