Single PX only set 2

A sound wave is travelling through water.

What is a possible speed for the wave?

• 150 m/s

• 300 m/s

• 1500 m/s

• 5000 m/s

Answer explanation

Useful speed to recall - sound travels ~ 340 m/s in air, ~1500 m/s in water

A force F is applied to a spanner, as shown.

Which action increases the moment of F about the centre of the nut?

• apply the force F to the end of the spanner handle

• apply the force F parallel to the spanner handle

• spray oil on the nut

• use a shorter spanner

Answer explanation

Moment = force x perpendicular distance from the pivot to the line of action of the force

Bats produce ultrasound waves to navigate.

What is a possible frequency range for these waves?

• 0–20 Hz

• 20 Hz–2000 Hz

• 2 kHz–20 kHz

• 20 kHz–120 kHz

Answer explanation

Ultrasound is sound with a frequency > 20 kHz

The diagram shows a transformer that has an output voltage of 12 V.

How many turns of wire are in the secondary coil?

• 12

• 20

• 50

• 20 000

Answer explanation

N_s = N_p x V_s ÷ V_p

An athlete with mass 70 kg trains by performing press-ups with a load on his back. The diagram shows the perpendicular distances involved. The centre of mass of the athlete is C_M and the centre of mass of the load he is carrying is C_L. (An object's weight acts through its centre of mass)

The mass of the load is 6.0 kg. What is the upward force exerted by his two arms?

• 54 N

• 76 N

• 540 N

• 760 N

Answer explanation

anti clockwise moment about foot:

(0.9 m x 700 N) + (1.2 m x 60 N) = 702 Nm

clockwise moment about foot = ? N x 1.3 m

? = 702 Nm ÷ 1.3 m = 540 N

Two people X and Y sit on a see-saw, as shown.

X weighs 400 N and sits 1.5 m from the pivot. pivot

The weight of the see-saw is 300 N and acts through the pivot.

When the see-saw is balanced, the pivot pushes up on the see-saw with a force of 900 N.

What is the weight of person Y and how far from the pivot is he sitting?

• Weight of Y=200 N

  Distance from pivot = 1.5 m

• Weight of Y=200 N

  Distance from pivot = 3.0 m

• Weight of Y=400 N

  Distance from pivot = 1.5 m

• Weight of Y=400 N

  Distance from pivot = 3.0 m

• 

Answer explanation

Moment of X = 400 N x 1.5 m = 600 Nm

Moment of Y = 600 Nm = 200 N x 3m

(OR 400 N x 1.5m)

BUT: Total downward force = 900 N (to balance upward force) so Y's weight must be 200 N

Air in a sealed syringe is slowly compressed by moving the piston. The temperature of the air stays the same.

Which statement about the air is correct?

• The pressure of the air decreases because its molecules now travel more slowly.

• The pressure of the air decreases because the area of the syringe walls is now smaller.

• The pressure of the air increases because its molecules now hit the syringe walls more frequently.

• The pressure of the air increases because its molecules now travel more quickly.

Answer explanation

In a  closed system where the temperature stays the same, the  pressure of a gas increases as the volume decreases.

The number of molecules and their average energy do not change. The force of each collision with the sides of the container (AT 90^o TO THE SURFACE) stays the same BUT the number of frequency of collisions increases because the surface area of the container is smaller so the pressure increases.

What is the approximate wavelength in air of the highest frequency sound that can be heard by a normal healthy person?

• 0.02 m

• 60 m

• 20 000 m

• 7 000 000 m

Answer explanation

340 m/s ÷ 20 000 Hz = 0. 017 m ~ 0.02 m

Light from a torch is incident on a plane mirror. The angle of incidence is 38^o. What is the angle of reflection?

• 38^o

• 52^o

• 76^o

• 142^o

Answer explanation

Angle of reflection = Angle of incidence

The diagrams each show a ray of light from an object O passing through a thin converging lens. The principal focuses in each diagram are labelled F.

Which diagrams are correct?

• 1 & 2

• 2 only

• 1 & 3

• 3 only

Answer explanation

Ray of light incident on the centre of the lens continue in a straight line.

Rays parallel to the principal axis before the lens go through principal focus.

Rays that pass through the principal focus before the lens go parallel to the principal axis.