David

David 16 Aug 2016 - 2

Question

The figure below shows the path of ray X as it passes through a thin converging lens AB.

Draw light rays to find the position of the image formed and determine the focal length of the lens AB.

Answer

Firstly, ray X is used to find the position of the base of the image. Since ray X comes from the base of the object, it will form the base of the image. Hence, the image is located along the blue line shown below:

Next, the ray from the tip of the object that passes through the optical centre is drawn.

The intersection point of the ray and the blue line is the location of the tip of the image.

The blue line is removed and a ray parallel to the principal axis from the tip of the object is added. This ray bends at the lens and reaches the tip of the image. The focal point can be found and the focal length can now be obtained.

The focal length is 10.0 cm.

David 16 Aug 2016 - 1

Question

A rectangular block of wood rests on the ground as shown in the diagram below.


Which of the following is the easiest way for a force F to topple the block?

Answer

For each block we identify the pivot point and the perpendicular distance of force F from the pivot point.


We can see that the perpendicular distance for D is the greatest. Force F will produce the greatest moment for D, hence it will topple the easiest.

Answer is D.

David 08 May 2016 - 1

Question

To avoid an accident the brakes of a car travelling at 30 ms-1 were applied suddenly. If the driver has a mass of 80 kg and it takes 3.0 s for the car to decelerate to rest what is the average force exerted by the safety belt on the driver during deceleration?

Answer

We first calculate the acceleration of the driver.

The force by the safety belt provides the force that causes the deceleration.

Hence, the average force exerted by the safety belt on the driver is 800 N.

David 07 May 2016 - 5

Question

38. The figure below shows a permanent magnet suspended from a spring placed right on top of an electromagnet.

What will happen to the permanent magnet once the switch S is turned on?

Answer

When the switch is closed, current will flow in the direction as shown by the arrows on the wires in the diagram below. Based on the right hand-grip rule, the magnetic field produced by the current will be as shown below.

This means that the top of the iron core will be a magnetic North pole and repel the North pole of the permanent magnet. Hence, the permanent magnet will be pushed upwards.

David 07 May 2016 - 4

Question

A stone is thrown vertically upwards with a velocity of 5.0 m/s. After time t, it reaches the original position. Neglecting air resistance, what is the time required for the ball to reach the highest position?

Answer

By conservation of energy, the final velocity of the stone will be 5.0 m/s downwards. We also know that the acceleration of the stone is constant at 10m/s2 downwards. Hence, we can sketch a velocity-time graph.

The displacement upwards from 0 to 0.5 t must be equal to the displacement downwards from 0.5 t to t since the stone returns to its original position.

By symmetry, the highest point must be at 0.5 t.

Hence, the ball takes 0.5t to reach the highest point.

David 07 May 2016 - 3

Question

12. A pendulum bob is released from rest at position X which is at a height h above its lowest position Y and then passes through position Z. Position Z is 0.4h above Y. Assuming air resistance is negligible, find the ratio of the speed of the bob at position Y to that at position Z.

Answer

By Principle of conservation of energy, the energy at points X, Y and Z are all the same. This means that we can express the speeds at Y and Z in terms of the gravitational field strength, g, and h.

Now, we can find the ratio of the speed at Y to the speed at Z.

Hence, the ratio is 1.29 : 1.

David 07 May 2016 - 2

Question

11. A block of mass 2.0 kg is released from rest at point P as shown in the figure below. It is found that the block finally comes to rest at point Q.

The length of the track PQ is 2.5 m What us the average value of frictional force acting on the block when it is moving from P to Q?

Answer

All the initial energy of the block was lost to work done against friction.

The average frictional force is 4.0 N.