TOPIC 5: Forces

Question 1

What is the difference between speed and velocity?

Answer 1

Speed is a scalar quantity (which only have magnitude and do not have direction)
Velocity is a vector quantity (which has direction and magnitude)

Question 2

What is displacement?

Answer 2

• the distance an object moves in a straight line from a starting point to a finishing point
• vector quantity

Question 3

What is distance?

Answer 3

• how far an object moves
• scalar quantity

Question 4

What is a force?

Answer 4

a push or pull that acts on an object when it interacts with another object

Question 5

What are contact forces?

Answer 5

• happen when two objects are physically touching
• example: friction

Question 6

What are non-contact forces?

Answer 6

• happen when objects are not touching
• example: gravitational force

Question 7

What is the mass of an object?

Answer 7

a measure of the amount of matter it contains

Question 8

What is the centre of mass?

Answer 8

the point through which an object’s weight appears to act

Question 9

What is weight?

Answer 9

the force acting on an object due to gravity. The weight of an object depends on the gravitational field strength

Question 10

What is the resultant force?

Answer 10

• The sum of all forces acting on an object
• Causes the change in an object’s motion

Question 11

Newton’s 1st law:

Answer 11

Says that the velocity of an object will only change if a resultant force is acting on an object. This applies to a stationary or moving object.

Question 12

Newton’s 3rd law:

Answer 12

Says that whenever 2 objects interact, the forces that they apply to each other are equal and opposite

(If a hand pushes on a table, the table with push back with an equal force but in the opposite direction)

Question 13

What is a free body force diagram?

Answer 13

a diagram showing the forces acting on an object (shown as vectors)

Question 14

Newton’s 2nd Law:

Answer 14

Says that the acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of an object

Question 15

Why does more than one force need to applied to change the shape of an object?

Answer 15

Because if only one force was applied then the object would simply move in the direction of that force

Question 16

What is elastic deformation?

Answer 16

An elastically deformed object will return to its original shape when the force stops
Example: springs

Question 17

What is inelastic deformation?

Answer 17

An inelastically deformed object will not return to its orginal shape when the force stops
Example: after a car has crashed into a tree, it will not return to its orginal shape

Question 18

What is the limit of proportionality?

Answer 18

The point at which Hooke’s law breaks down

Question 19

What is Hooke’s Law:

Answer 19

The extension of a spring is directly proportional to the force applied, provided that the limit of proportionality is not exceeded

Question 20

What is the increased length in a spring called when it is stretched?

Answer 20

the extension

Question 21

Hooke’s Law required practical:

Answer 21

1) set up apparatus (spring, ruler, known mass)
2) measure the original length of the spring
3) hang different masses on the spring and measure the length of the spring each time
4) plot a graph with extension on the spring on the x axis and force on the y axis

Question 22

What does the Hooke’s law required practical tell us?

Answer 22

• ## adding masses to the spring increases the downwards force as each mass has a weight

Question 23

What is the spring constant?

Answer 23

it measures how stiff a spring is

Question 24

When is work done on a spring?

Answer 24

When a force stretches a spring or compresses another object, work is done. When work is done, energy is transferred into an elastic potential energy store

Question 25

The effect of air resistance:

Answer 25

- is a frictional force that opposes the motion of objects moving quickly through air - slows down a falling object - the force due to air resistance increases as the speed of the falling object increases

Question 26

What is terminal velocity?

Answer 26

the fastest an object can fall

Question 27

What is the conservation of momentum?

Answer 27

- momentum cannot be created or destroyed - if two objects collide, the sum of momentum before the collision is equal to the sum of momentum after the collision

Question 28

What is stopping distance?

Answer 28

- The distance it takes a car to stop in an emergency - The greater the speed of the vehicle, the greater the stopping distance

Question 29

What is the thinking distance?

Answer 29

the distance travelled between when the driver realises they need to brake and why they apply the brakes

Question 30

What is the braking distance?

Answer 30

the distance the car travels between the driver applying the brakes and the car stopping

Question 31

Factors affecting thinking distance (reaction time):

Answer 31

- distractions - tiredness - drugs or alcohol

Question 32

Factors affecting braking distance:

Answer 32

- road conditions - initial car speed - the condition of the car

Question 33

Work done when braking:

Answer 33

1) when we push the brake pedal, brake pads are pressed onto the wheels 2) this contact causes friction which causes work to be done 3) the work done between the brakes and the wheels converts energy from kinetic energy in the wheels to thermal energy in the brakes 4) the temperature of the brakes increases

Question 34

What are the dangers of large decelerations?

Answer 34

- can cause brakes to overheat or the car to skid - a larger deceleration with transfer more stopping force to passengers