Problem Solving Strategy for Motion in Two Dimensions

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Motion in Two Dimensions is a very important topic in Physics. In this post, I am going to demonstrate my strategies for solving this kind of problems. First, we revise the formula we have learnt :

  • v = u + at
  • v^2-u^2=2as
  • s=ut+\frac{1}{2}at^2

These formulas are useful. However, it may not be the best choice to solve problems using only formulas. In fact, equations would be tedious if you do not know how to apply it well.

Before introducing my strategy, I’ll share another useful but not commonly used formula

\displaystyle y = \frac{\sec^2{\theta}}{2v^2}g\text{ }x^2 + x\tan \theta

This formula is used to calculate the displacement of the particle at the y-axis for given displacement at the x-axis. The derivation of the formula is actually pretty simple.

The partial displacement at the y-axis and x-axis is respectively

\begin{aligned} \displaystyle y &= u_y^2t+\frac{1}{2}gt^2 \\ x &= u_xt \end{aligned}

Substitute t to the first equation

\begin{aligned} \displaystyle y &= \frac{u_y}{u_x}x+\frac{1}{2}g\bigg ( \frac{x}{v \cos \theta} \bigg )^2 \\ &= x\tan\theta+\frac{1}{2v^2\cos^2\theta}g\text{ }x^2 \\ &=\frac{\sec^2\theta}{2v^2}g\text{ }x^2+x\tan\theta\end{aligned}

Example Problem 0.1 :

A canon ball is being launched at speed 10\text{ ms}^{-1} and 60^\circ in respect to the horizontal. If the target is at height 1\text{ m}, what is the horizontal distance should the canon be placed?

g = -10\text{ ms}^{-2}

Solution 0.1 :

Apply the formula above, we have

\begin{aligned} \displaystyle y &= \frac{\sec^2\theta}{2v^2}g\text{ }x^2+x\tan\theta \\ 1 &= \frac{\sec^2 60^\circ}{2(10)^2}g\text{ }x^2+x\tan 60^\circ \\ 1 &= \frac{1}{5}x^2+\sqrt{3}x \end{aligned}

Solve the quadratic equation we get x = 0.62\text{ m} or x = 8.04\text{ m}.

Strategy 1 : Graphic ( 图解法 )

Graph is a useful tool that is commonly ignored by most students. Solving problems using graphics can help you visualize the problem. The best graph, in my opinion, is the v-t graph, because it can show us two things:

  • Displacement – area under the function
  • Acceleration – slope of the function

Example Problem 1.1 :

Screenshot 2015-01-20 at 22.31.28

Consider the free fall of a particle at position A with height H. Given that AB = BC and the time for the particle to travel from B to C is 2\text{ s}.

  1. What is the time traveled from A to B, in seconds?
  2. What is the height H, in meters?

Solution 1.1

Practice Problem 1.2 :

A particle at rest starts moving with acceleration a_1. Until it reaches the speed v, it immediately decelerates with a_2 until it stops. The total time consumed is t. Prove that:

  1. The displacement of the particle is \frac{1}{2}vt.

Discuss your solution here.

一个静止的质点以 a_1 的加速度加速至 v. 过后,马上以减速度 a_2 减速至静止为止。这整个过程共用了 t 时间。试证:

  1. 质点的位移为 \frac{1}{2}vt.

Strategy 2 : Relative Motion ( 相对运动 )

This strategy is often used to solve problems that has more than two moving objects involved. The formula of relative motion is :

v_a' = v_a - v_b

Once you got the idea, the formula is not necessary to be memorized.

Example Problem 2.1 :

A boat can travel at a speed of 3\text{ ms}^{-1} on still water. A boatman wants to cross a river whilst covering the shortest possible distance. In what direction should he row with respect to the bank if the speed of the water is

  1. 2\text{ ms}^{-1}
  2. 4\text{ ms}^{-1}

Assume that the speed of the water is the same everywhere.

Solution 2.1

Practice Problem 2.2 :

There are two particles at points A(0,0) and B(40,0) respectively. Particle A moves upwards at speed 3\text{ ms}^{-1} while particle B moves towards left at speed 4\text{ ms}^{-1}. What is the minimum distance can they get between them?

Submit your answer here.

Practice Problem 2.2 :

A sandstorm 100\text{ m} wide is heading towards Adam at speed 2\text{ ms}^{-1} from distance 1000\text{ m} away. Since Adam is infamous for his laziness, he wants to cross the sandstorm running as slow as possible. What is the minimum speed should Adam run without being hit by the sandstorm?

Practice Problem 2.3 :

Three small snails are each at a vertex of an equilateral triangle of side 60\text{ cm}. The first sets out towards the second, the second towards the third and the third towards the first, with a uniform speed of 5\text{ cm min}^{-1} . During their motion each of them always heads towards its respective target snail. How much time has elapsed, and what distance do the snails cover, before they meet?

Practice Problem 2.4 :

A small ball falling vertically with constant velocity v elastically strikes a massive inclined cart moving with a horizontal velocity v. What is the rebound velocity of the ball? ( The velocity of the cart doesn’t change after the collision )

Submit your answer here.

Practice Problem 2.5 :

Two ships A and B are originally at a distance d from each other depart at the same time from a straight coastline. Ship A moves along a straight line perpendicular to the shore while ship B constantly heads for ship A, having at the each moment same as the latter. After a sufficiently great interval of time the second ship will be following the first one at a certain distance x. Find x.

Submit your answer here.

Strategy 3 : Energy Conservation ( 功能原理 ) :

The Energy Conservation is useful especially when dealing with projectile motion. Energy can be changed from one form to another without losing its total internal energy. In this case, the internal energy of a particle is consists of its kinetic energy and potential energy.

\begin{aligned} \displaystyle E &= E_k + E_p \\ \\ &= \frac{1}{2}mv^2 + mgh \end{aligned}

Practice Problem 3.1

A ball with mass 1\text{ kg} is thrown upwards with speed 30\text{ m/s}. It is given that when the ball reaches its highest point, the work done by surrounding air on the ball is 100\text{ J}. What is the highest point can the ball reaches?

Practice Problem 3.2 :

Christopher accidentally fell inside a cylindrical well of depth 15 meters and radius 8 meters. Jumping from the center of the bottom of the well, what is the minimum launch speed that Christopher needs to achieve, in order to jump out of the well?

Submit your answer here.

Practice Problem 3.3 :

A ball slides along a friction-less ramp of length L. As it reaches the bottom, it hits the board and slides up along the slope again. If the speed of the ball after the collision is \frac{4}{5} times the speed of the ball just before the collision, find the total distance traveled by the ball when it comes to a complete stop.

Submit your answer here.

That’s all for now. If you have a good strategy, share it with me! Good Luck Problem Solving!

Problem Solving Strategy for Motion in Two Dimensions

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