3.2 Forces in action

Question 1

Net force (N) =

Answer 1

Mass(Kg) x acceleration (ms-2)

Question 2

Tension

Answer 2

If a string is pulled tight, tension is the force pulling equally on the object at wither end of the string

Question 3

Normal contact force

Answer 3

if an object exerts a force on a surface, the surface exerts an equal but opposite force on the object. The force acts perp to the surface

Question 4

Weight=

Answer 4

Mass x Gravity

Question 5

Frcition

Answer 5

if an object is moving it usually has a friction force acting on it in the opposite direction to motion.

Question 6

Centre of gravity

Answer 6

The point where the entire weight of an object seems to act

Question 7

centre of mass

Answer 7

The point where the entire mass of an object seems to be concentrated

Question 8

How do you find the centre of mass for a regular object?

Answer 8

centre of mass is at the centre where the lines of symmetry cross, halfway through the thickness of the object

Question 9

How do you find the centre of mass for an irregular object?

Answer 9

1. Hang the object from a point freely
2. draw a vertical line downwards from the point of suspension (use plumb bob to get exactly vertical)
3. Hang object from different point
4. Draw another vertical line down
5. Centre of mass is where the lines cross

Question 10

An object will be nice and stable if?

Answer 10

Low centre of mass and wide base area

Question 11

When does an object topple over?

Answer 11

if a vertical line drawn downwards from its centre of mass falls outside its base area. This is because of the moments about the pivot

Question 12

What are the 2 main types of friction?

Answer 12

Contact friction between solid surfaces

fluid friction

Question 13

What factor effect drag for an object travelling through air?

Answer 13

Velocity
density of fluid/ viscosity of fluid
shape and size of object

Question 14

What are the 3 things to remember about friction?

Answer 14

They act in the opposite direction to the motion of the object

They can never speed things up or start something to move

They convert kinetic energy into heat energy

Question 15

Describe the 3 main stages of reaching terminal velocity for a car

Answer 15

1. The car accelerates from rest using a constant driving force
2. As the velocity increases, the resistive forces increases, this reduces the resultant force on the car and hence reduces its acceleration
3. Eventually the car reaches a velocity at which the resistive forces are equal to the driving forces. There is now no resultant force and no acceleration, so the car carries on at constant velocity

Question 16

What’s the graph for terminal velocity like?

Answer 16

Velocity against time graph start of positive steep then levels of. (Like the side of a Christmas pudding)

Acceleration time graph starts up high has negative steep gradient and levels of at the bottom (it has a slight backwards s shape to it)

Question 17

Describe a parachutist falling?

Answer 17

1. Sky diver leaves plane and will accelerate until air resistance = weight
2. He will then be travelling at terminal velocity
3. They will open the parachute immediately increasing air resistance so it is now n=bigger then than his weight
4. This slows him down until his speed has dropped enough for the air resistance to be equal to his weight again. This new terminal velocity is small enough so you don’t die

Question 18

Describe the velocity time graph for a parachutist

Answer 18

Steep positive gradient slowly levelling off, (Terminal velocity initially) then parachute opens so steep negative gradient and then levels of (New terminal velocity)

Question 19

How do you measure the terminal velocity of a ball bearing?

Answer 19

1. Put elastic bands around a tube of viscous liquid at fixed distances using a ruler
2. Drop the ball bearing into the tube, and use a stopwatch to record the time at which it reaches each elastic band. Record results in a table
3. Repeat few times to reduce random error effects, Use magnet to remove ball from tube
4. Calculate the times taken by the ball bearing to travel between consecutive elastic bands and calculate the average velocity between each pair of elastic bands
5. You should find the avg velocity increases at first, then stays constant.

Question 20

Density=

Answer 20

Mass/volume

Question 21

A solid object will float on a fluid if?

Answer 21

it has a lower density than the fluid`

Question 22

Pressure=

Answer 22

Force/Area

Height x density x acceleration due to gravity

Question 23

Newton

Answer 23

Force required to give 1 Kg mass an acceleration of 1 m/s

Question 24

Upthrust

Answer 24

upwards force that fluids exert on objects that are completely or partially submerged in the fluid. Caused as the top and bottom of a submerged object are at different depths

Question 25

Archimedes principle

Answer 25

an object completely or partially submerged in a fluid experiences upthrust = to the weight of the fluid displaced

Question 26

How do submarines make use of Archimedes principle?

Answer 26

To sink, large tanks are filled with water to increases the weight of the submarine so that it exceeds the upthrust.
To rise to the surface, the tanks are filled with compressed air to reduced the weight so that its less then upthrust.

Question 27

Moment of a force (Nm)=

Answer 27

Force (N) x Perpendicular distance from pivot (m0

Question 28

What’s the principle of moments?

Answer 28

For a body to be in equilibrium, the sum of the clockwise moments about a point is = to the sum of the anticlockwise moments about the same point

Question 29

In a lever a _____force acts against a ____force by means of a ____object rotating around a _____

Answer 29

In a lever a effort force acts against a Load force by means of a Rigid object rotating around a Pivot

Question 30

A Couple

Answer 30

Pair of forces of equal size that act parallel to each other but in opposite directions.

Question 31

What does a couple produce?

Answer 31

Torque (a turning force)

Question 32

Torque of a couple (Nm)=

Answer 32

size of one of the forces (N) x perpendicular distance between the forces (m)

Question 33

The DVD is dropped from rest. The circular face remains horizontal as it falls. The DVD does
not reach terminal velocity before it hits the ground.
Describe and explain how the acceleration of the DVD varies from the instant it is dropped
until just before it hits the ground

Answer 33

At the start, acceleration = g (because there is no drag)

Drag increases (as its speed increases / accelerates)

net force decreases or net force < weight

(As it falls) acceleration decreases / (As it falls) acceleration < g

Question 34

why cant f=ma not be applied to particles travelling at speeds close to speed of light?

Answer 34

The mass of particles increases at its speed gets

closer to the speed of light

Question 35

state 2 conditions that apply when an object is in equilibirum

Answer 35

Net force = 0

Net moment / torque = 0