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From Mechanics

Theory

Exercises

Key Points

In all collisions, where no external forces act, momentum will be conserved and we can apply

mAvA+mBvB=mAuA+mBuB

or

mAvA+mBvB=mAuA+mBuB

Example 16.1

A bullet of mass 40 grams is travelling horizontally at 250 ms−1. It hits a wooden trolley that is at rest. The bullet and trolley then move together at 10 ms−1.Assume that the bullet and trolley move along a straight line. Find the mass of the trolley.

Solution

Before the collision:

uB=250 and uT=0

After the collision:

vB=vT=10

Also the mass of the bullet should be converted to kg:

mB=0.04

Using conservation of momentum gives:

mBuB+mTuT=mBvB+mTvT0.04250+mT0=0.0410+mT1010=0.4+10mTmT=1010−0.4=0.96 kg

Example 16.2

A van, of mass 2.5 tonnes, drives directly into the back of a stationary car, of mass 1.5 tonnes. The van was travelling at 12 ms−1 and both vehicles move together along a straight line after the collision. Find the speed of the vehicles after the collision.

Solution

Before the collision: uV=12 and uC=0

After the collision: vV=vC=v

The masses should be converted to kilograms:

mV=2500 and mC=1500

Using conservation of momentum gives:

mVuV+mCuC=mVvV+mCvC250012+15000=2500v+1500v30000=4000vv=400030000=7.5 ms-1

Example 16.3

Two particles, A and B of mass m and 3m are moving towards each other with speeds of 4u and u respectively along a straight line. They collide and coalesce. Describe how the motion of each particle changes during the collision.

Solution

Before the collision: \displaystyle {{u}_{A}}=4u and \displaystyle {{u}_{B}}=-u

After the collision: \displaystyle {{v}_{A}}={{v}_{B}}=v

Using conservation of momentum gives:

\displaystyle \begin{align} & {{m}_{A}}{{u}_{A}}+{{m}_{B}}{{u}_{B}}={{m}_{A}}{{v}_{A}}+{{m}_{B}}{{v}_{B}} \\ & m\times 4u+3m\times (-u)=mv+3mv \\ & mu=4mv \\ & v=\frac{mu}{4mu}=\frac{u}{4} \end{align}

Example 16.4

A particle, A, of mass 2 kg has velocity \displaystyle (4\mathbf{i}+2\mathbf{j})\text{ m}{{\text{s}}^{\text{-1}}} . It collides with a second particle, B, of mass 3 kg and velocity \displaystyle (2\mathbf{i}-4\mathbf{j})\text{ m}{{\text{s}}^{\text{-1}}} . If the particles coalesce during the collision, find their final velocity.

Solution

Before the collision: \displaystyle {{\mathbf{u}}_{A}}=4\mathbf{i}+2\mathbf{j} and \displaystyle {{\mathbf{u}}_{B}}=2\mathbf{i}-4\mathbf{j}

After the collision: \displaystyle {{\mathbf{v}}_{A}}={{\mathbf{v}}_{B}}=\mathbf{v}

The masses are defined: \displaystyle {{m}_{A}}=2 and \displaystyle {{m}_{B}}=3

Using conservation of momentum gives:

\displaystyle \begin{align} & {{m}_{A}}{{\mathbf{u}}_{A}}+{{m}_{B}}{{\mathbf{u}}_{B}}={{m}_{A}}{{\mathbf{v}}_{A}}+{{m}_{B}}{{\mathbf{v}}_{B}} \\ & 2\times (4\mathbf{i}+2\mathbf{j})+3\times (2\mathbf{i}-4\mathbf{j})=2\mathbf{v}+3\mathbf{v} \\ & 8\mathbf{i}+4\mathbf{j}+6\mathbf{i}-12\mathbf{j}=5\mathbf{v} \\ & 14\mathbf{i}-8\mathbf{j}=5\mathbf{v} \\ & \mathbf{v}=\frac{14\mathbf{i}-8\mathbf{j}}{5}=2\textrm{.}8\mathbf{i}-1\textrm{.}6\mathbf{j} \end{align}

Example 16.5

A car, of mass 1.2 tonnes, is travelling at 15 \displaystyle \text{m}{{\text{s}}^{-1}}, when it is hit by a van, of mass 1.4 tonnes, travelling at right angles to the path of the first car. After the collision the two vehicles move together at an angle of 20\displaystyle {}^\circ to the original motion of the car. Find the speed of the heavier van just before the collision.

Solution

This diagram shows the velocities before the collision.

\displaystyle {{\mathbf{u}}_{C}}=15\mathbf{i} and \displaystyle {{\mathbf{u}}_{V}}=U\mathbf{j}

This diagram shows the velocity after the collision.

\displaystyle {{\mathbf{v}}_{C}}={{\mathbf{v}}_{V}}=V\cos 20{}^\circ \mathbf{i}+V\sin 20{}^\circ \mathbf{j}

Using conservation of momentum gives:

\displaystyle \begin{align} & {{m}_{C}}{{\mathbf{u}}_{C}}+{{m}_{V}}{{\mathbf{u}}_{V}}={{m}_{C}}{{\mathbf{v}}_{C}}+{{m}_{V}}{{\mathbf{v}}_{V}} \\ & 1200\times 15\mathbf{i}+1400\times U\mathbf{j}=2600(V\cos 20{}^\circ \mathbf{i}+V\sin 20{}^\circ \mathbf{j}) \end{align}

Considering the \displaystyle \mathbf{i} component gives:

\displaystyle \begin{align} & 1200\times 15=2600V\cos 20{}^\circ \\ & V=\frac{1200\times 15}{2600\cos 20{}^\circ }=\frac{180}{26\cos 20{}^\circ }=7\textrm{.}36\text{ m}{{\text{s}}^{\text{-1}}} \end{align}

Considering the \displaystyle \mathbf{j} component gives:

\displaystyle \begin{align} & 1400U=2600\times \frac{180}{26\cos 20{}^\circ }\times \sin 20{}^\circ \\ & U=\frac{2600\times 180}{1400\times 26}\tan 20{}^\circ =4\textrm{.}68\text{ m}{{\text{s}}^{\text{-1}}} \end{align}