5.6 Forces and motion

Question 1

Does a distance quantity require a specific direction?

Answer 1

No specific direction is required as it is a scalar quantity

Question 2

If an object moves 3 meters to the left, and then 3 meters back to its initial position, what is the object’s total displacement?

Answer 2

The object has zero displacement. It is a vector quantity so it also involves direction. The object starts and ends at the same point.

Question 3

State a typical value for the speed of sound

Answer 3

330 m/s

Question 4

What is a typical value for a human walking speed?

Answer 4

1.5 m/s

Question 5

What is a typical value for human running speed?

Answer 5

3 m/s

Question 6

What is a typical value for human cycling speed?

Answer 6

6 m/s

Question 7

State the equation linking distance, speed and time. Give appropriate units.

Answer 7

Distance = Speed x Time

Distance (m), Speed (m/s), Time (s)

Question 8

Why can an object travelling at a constant speed in a circle not have a constant velocity? (Higher)

Answer 8

Speed is a scalar quantity.

Velocity is a vector quantity which means it can only be constant if the direction is constant. In circular motion, the direction is constantly changing.

Question 9

How can speed be calculated from a distance-time graph?

Answer 9

The speed is equal to the gradient of the graph

Question 10

What must be done to calculate speed at a given time on a distance-time graph for an accelerating object? (Higher)

Answer 10

Drawing a tangent to the curve at the required time.

Calculating the gradient of the tangent.

Question 11

State the equation for the average acceleration of an object. Give appropriate units.

Answer 11

Acceleration = Change in velocity / Time taken

Acceleration (m/s^2), Velocity (m/s), Time (s)

Question 12

How can the distance travelled by an object be calculated from a velocity-time graph?

Answer 12

It is equal to the area under the graph

Question 13

Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface

Answer 13

9.8 m/s^2

Question 14

What can be said about the resultant force acting on an object when it is falling at terminal velocity?

Answer 14

The resultant force is zero. When at terminal velocity, the object is moving at constant speed - it’s not accelerating.

Question 15

State Newton’s first law for a stationary object

Answer 15

If the resultant force on a stationary object is zero, the object will remain at rest.

Question 16

State Newton’s first law for a moving object

Answer 16

If the resultant on a moving object is zero, the object will remain at constant velocity (same speed in the same direction).

Question 17

What can be said about the braking forces and driving forces when a car is travelling at constant velocity?

Answer 17

The braking forces are equal to the driving forces

Question 18

If an object changes direction but remains at a constant speed, is there a resultant force?

Answer 18

Since there is a change in direction, there is a change in velocity and so there must be a resultant force.

Question 19

What is inertia? (Higher)

Answer 19

The tendency of an object to continue in its state of rest or uniform motion.

Question 20

State the defining equation for Newton’s second law

Answer 20

Resultant force = Mass x Acceleration

F = ma

Question 21

State Newton’s second law in words

Answer 21

An object’s acceleration is directly proportional to the resultant force acting on it and is inversely proportional to its mass

Question 22

What is inertial mass? (Higher)

Answer 22

A measure of how difficult it is to change a given object’s velocity. The ratio of force over acceleration.

Question 23

What is the symbol used to represent an approximate value?

Answer 23

~

Question 24

State Newton’s third law

Answer 24

Whenever two objects interact, the forces that they exert on each other are equal and opposite.

Question 25

What is the stopping distance of a vehicle equal to?

Answer 25

The sum of thinking distance and braking distance.

Question 26

For a given distance, if the vehicle’s speed is increased, what can be said about its stopping distance?

Answer 26

The stopping distance is increased with an increase in speed

Question 27

Give a typical range of values for human reaction time

Answer 27

0.2 - 0.9 seconds

Question 28

Give three factors which can affect a driver’s reaction time

Answer 28

● Tiredness
● Drugs
● Alcohol

Question 29

Give two factors which may affect braking distance

Answer 29

● Adverse (wet/icy) road conditions

● Poor tyre/brake conditions

Question 30

Describe the energy transfers that take place when a car applies its brakes

Answer 30

Work is done by the friction force between the brakes and wheel. The kinetic energy of the wheel is converted to heat and dissipated to the surroundings through the brake discs.

Question 31

To stop a car in any given distance, if it’s velocity is increased, what must happen to the braking force applied?

Answer 31

The braking force must also be increased

Question 32

State two consequences of a vehicle undergoing very large decelerations

Answer 32

● Kinetic energy converted to heat is very high causing brakes to overheat

● Loss of control of the vehicle