Law of Motion

A heavy truck has high inertia. Is it easy or difficult to start it moving or to stop it moving?

• To start moving : Easy and To stop moving : Easy

• To start moving : Easy and To stop moving : Difficult

• To start moving : Difficult and To stop moving : Easy

• To start moving : Difficult and To stop moving : Difficult

Answer explanation

The inertia of a body is its reluctance to start moving, and its reluctance to stop once it has begun moving. Heavy objects have high inertia. Due to high inertia, to start moving : difficult and to stop moving : difficult.

A car is moving in a straight line on a level road. Its engine provides a forward force on the car. A second force of equal size acts on the car due to resistive forces. Which statement describes what happens?

• The car changes direction.

• The car moves at a constant speed.

• The car slows down.

• The car speeds up.

Answer explanation

The car is moving in a straight line on a level road. So the car is moving at a constant speed.

A box of mass 2.0 kg is pulled across a horizontal floor by a force of 6.0N. The frictional force acting on the box is 1.0N. What is the acceleration of the box?

• 0.40 ms^-2

• 2.5 ms^-2

• 3.0 ms^-2

• 3.5 ms^-2

Answer explanation

Horizontal force and frictional force are opposite directions.

Resultant force on the box, F = Horizontal force - frictional force

F= 6-1 = 5N

F = ma

a = F/m = 5/2 = 2.5 ms^-2

An object accelerates from 10 m / s to 30 m / s in 4.0 seconds. The accelerating force is 150N. What is the mass of the object?

• 0.033 kg

• 5.0 kg

• 7.5 kg

• 30 kg

Answer explanation

$v_0=10ms^{-1}$

$v\ =30ms^{-1}$
$t=4s$
$a=\frac{v\ -v_0}{t}=\frac{30-10}{4}=5ms^{-2}$
F = 150 N
F = ma
$m=\frac{F}{a}=\frac{150}{5}=30\ kg$

When a block of wood of mass 2 kg is pushed along a horizontal surface with a force of 6 N, if moves with a constant velocity of 3 m/s. If the block is pushed with a force of 10 N, the block will move with a/an _________.

• Velocity of 2 ms^-1

• Velocity of 5 ms^-1

• Acceleration of 2 ms^-2

• Acceleration of 5 ms^-2

A car travels along a horizontal road at a constant speed. Three horizontal forces act on the car. The diagram shows two of these three forces.What is the size and direction of the third force acting on the car?

• 1200 N backwards

• 1200 N forwards

• 1800 N backwards

• 1800 N forwards

Answer explanation

If the car travels at a constant speed, forwards force and backwards force are equal.

So, third force + resistance force = force from engine

third force = 1500 - 300 = 1200 N backwards

Which statement is true?

• Your weight would be the same on the Earth and on the Moon.

• Your mass would be the same on the Earth and on the Moon.

• Your weight would be more on the Moon than on the Earth.

• Your mass would be less on the Moon than on the Earth.

Answer explanation

Weight differ but mass does not change on the earth or on the moon.

The gravitational field strength on the Moon is 1.6 N/ kg. An astronaut has a mass of 75kg. What is the weight of the astronaut on the Moon?

• 47N

• 75N

• 120N

• 750N

Answer explanation

m = 75 kg

g on the moon = 1.6 N/kg

w=mg

w= 75 x 1.6 = 120 N

Which statement about the mass and the weight of an object is correct?

• They are both affected by changes in the acceleration of free fall.

• They are both forces.

• They have different units.

• Weight is calculated by dividing mass by the acceleration of free fall.

An astronaut orbits the Earth in a space station. Which is a vector quantity?

• the mass of the astronaut

• the speed of the satellite

• the temperature inside the satellite

• the weight of the astronaut

Answer explanation

Mass, temperature and speed are vector quantities.