3.2 Forces in Action

Question 1

What is a force? (2)

Answer 1

An interaction between two objects involving a push or pull

A vector with direction in space

Question 2

What are the four fundamental forces? (4)

Answer 2

Gravitational
Electromagnetic
Strong nuclear
Weak nuclear

Question 3

What force acts on an object in water, allowing it to float?

Answer 3

Upthrust

Question 4

What force acts on an object when it is pulled by a string?

Answer 4

Tension

Question 5

What force acts between a surface and an object when they are in contact with each other?

Answer 5

Normal contact force

Question 6

What is friction?

Answer 6

Particles colliding with a moving object, causing kinetic energy to be lost as thermal energy

Question 7

What is a free body diagram?

Answer 7

A diagram using a dot to represent an object and arrows to scale to show the direction and magnitude of the forces acting on it

Question 8

What is a plumb line?

Answer 8

A mass suspended by a length of string, allowing straight, vertical lines to be drawn

Question 9

How is the centre of mass of a 2D object found? (4)

Answer 9

Choose a point on the object and allow it to swing freely from a pin before coming to rest
Using a plumb line, draw a vertical line through the pin
Repeat this twice more, creating a point where all three lines cross
The point where they cross is the centre of mass

Question 10

What is drag?

Answer 10

The frictional force experienced by an object travelling through a fluid

Question 11

What factors affect drag? (5)

Answer 11

Speed of the object
Cross-sectional area
Shape of the object
Roughness/texture of the object
Density of the fluid that is travelled through

Question 12

How is drag force and speed linked?

Answer 12

Drag force is proportional to speed^2

Question 13

What are the forces on an object in the instant that it falls? (2)

Answer 13

No drag

Weight (acceleration = g)

Question 14

What happens as a falling object’s speed increases? (2)

Answer 14

Drag force increases

Instantaneous acceleration decreases (less than g)

Question 15

What happens when an object reaches terminal velocity? (2)

Answer 15

Drag is equal and opposite to weight

Acceleration is zero so speed is constant

Question 16

What is the moment of a force?

Answer 16

The turning effect of a force about a pivot

Moment = force x perpendicular distance from pivot

Question 17

What is a couple?

Answer 17

A pair of equal but opposite forces acting parallel and on different lines
They cause a turning effect but no translational movement

Question 18

What is the torque of a couple? (2)

Answer 18

The moment of a couple

Torque = one of the forces x perpendicular distance between forces

Question 19

What is the principle of moments? (2)

Answer 19

When a body is in equilibrium, the net force acting on it is zero and its net moment is zero
The sum of clockwise moments = the sum of anti-clockwise moments

Question 20

What is the centre of gravity?

Answer 20

The point at which the entire weight of the object can be said to act

Question 21

What is the difference between a torque and a moment?

Answer 21

A moment can have a non-zero resultant force, thereby causing movement whereas a torque always has a resultant force of zero

Question 22

What is density? (2)

Answer 22

Mass per unit volume

Density = mass / volume

Question 23

What is pressure? (2)

Answer 23

Force per unit area (acting perpendicular to the surface)

Pressure = force / area

Question 24

What is the difference between solid and liquid pressure? (2)

Answer 24

Solid pressure - the smaller the area, the higher the pressure
Liquid pressure - in a fluid at rest, pressure at any point acts equally in all directions and increases with depth

Question 25

What is Archimedes’ Principle?

Answer 25

The upthrust that a floating or submerged object feels is equal to the weight of displaced fluid

Question 26

How is the upthrust on an object in a fluid calculated?

Answer 26

Pressure = height of liquid above x density of liquid x acceleration due to gravity

Question 27

How is the density of an object calculated? (2)

Answer 27

Use a Eureka can - volume of water displaced = volume of object
Use a balance to find the mass of the object