Chapter 4 - forces in motion

Question 1

6 forces that could be on a free body diagram

Answer 1

Weight, friction, tension, drag, normal force, human force

Question 2

What to do with the arrows on a free body diagram?

Answer 2

Point them all away from the centre of mass

Question 3

Free body diagram

Answer 3

Shows the forces acting on an object from its centre of mass

Question 4

How to find the acceleration down the ramp where you know the angle of the ramp from the ground

Answer 4

1. Draw the acceleration due to gravity acting downwards from the object on the ramp
2. Make a right angle triangle with g, the ramp and a line perpendicular to the ramp so that g is the hypotenuse
3. The angle between g and the ramp is (90 - the angle from the ground to the ramp)
4. Use trig to find the component

Question 5

Turning Moment

Answer 5

Force x perpendicular distance
Nm = N x m
The force must be resolved to be perpendicular to the surface

Question 6

Principle of moments

Answer 6

Clockwise moment = anti-clockwise moment when a body is in equilibrium

Question 7

How to find the force acting on each of 2 pivots?

Answer 7

1. Find the moment around the other pivot

2. Divide by the distance between them to find the reaction force

Question 8

A force with a component upwards

Answer 8

A lift force

Question 9

A force with a component forwards

Answer 9

A thrust

Question 10

A force with a component backwards

Answer 10

A drag force

Question 11

What does trebling the speed do to the drag force?

Answer 11

It multiplies it by 9

Drag force is directly proportional to speed squared

Question 12

Graph of v/t of a falling object with no parachute

Answer 12

Increases velocity with decreasing acceleration until it plateaus at terminal velocity

Question 13

Density

Answer 13

Mass (kg) / Volume (m^3)

Question 14

Pressure

Answer 14

Force (N) / Area (m^2)

Question 15

How to make an object rotate around a point without accelerating in a line?

Answer 15

Apply two equal and opposite forces, a “couple”

Question 16

Turning effect of a couple

Answer 16

The torque

Question 17

Torque equation

Answer 17

Average force x distance between forces

Question 18

Pressure in a fluid

Answer 18

depth x density x acceleration due to gravity

Question 19

In which direction(s) does pressure in a fluid act?

Answer 19

Equally in all directions

Question 20

Archimedes’s Principle

Answer 20

If an object is partially or fully submerged, the upthrust is equal to the weight of fluid displaced.

Question 21

How to find the upthrust on an object?

Answer 21

The difference between the forces at the top and bottom of the object
Change in depth x density x acceleration due to gravity x area

Question 22

Torque

Answer 22

One of the forces multiplied by the perpendicular distance between them

Question 23

How goes gps work?

Answer 23

• uses radio waves emitted by multiple satellites
• Multiply the speed of light by the time taken for the receiver to receive them to find the distance from each satellite
• there is a circle of points that distance from each satellite, the intersection of the circles is its location

Question 24

Resultant force of a couple

Answer 24

0N

Question 25

What does a balance measure?

Answer 25

The normal contact force