P2 - Forces

Question 1

Define a vector quantity and give examples of some

Answer 1

They have magnitude and direction

Examples: force, velocity, displacement, acceleration, momentum

Question 2

Define scalar quantity and give examples of some

Answer 2

They have magnitude but no direction

Examples: speed, distance, mass, temperature, time

Question 3

What are vectors usually represented as?

Answer 3

An arrow on a diagram

Question 4

Define a contact force

Answer 4

When two objects have to be touching in order for a force to act.
Examples: friction, air resistance

Question 5

Define non-contact force

Answer 5

When the objects do not need to be touching for the force to act
Examples: magnetic force, gravitational force, electrostatic force

Question 6

What is an interaction pair?

Answer 6

A pair of forces that are equal and opposite and act on two interacting objects (Newton’s Third Law)

Question 7

What is the difference between weight, mass and gravity?

Answer 7

Gravity = the force of attraction between masses (gives everything a weight)
Mass = amount of ‘stuff’ in an object (same value anywhere in the universe)
Weight = the force acting on an object due to gravity (so its different on different planets)

Question 8

What is the equation that links weight (force), mass, and gravitational field strength?

Answer 8

Weight (force) (N) = Mass (kg) x Gravitational Field Strength (N/kg)

Question 9

What is a free body diagram?

Answer 9

Shows all the forces acting on an object with different sized arrows to show strong/weak forces

Question 10

Define resultant force

Answer 10

The overall force on a point or object. You have horizontal and vertical force that have resultant force in a specific direction depending on which side has a stronger force.

Question 11

What does it mean when work is done?

Answer 11

When a force moves an object through a distance, energy is transferred meaning work is done on the object.

Question 12

What is the equation for work done?

Answer 12

Work done (J) = Force (N) x Distance (m)

Question 13

How do you use scale drawings to find resultant forces?

Answer 13

Draw the scale drawing then measure the distance from start to finish and measure the angle to find the bearing.

Question 14

What does it mean if something is elastically deformed?

Answer 14

It can go back to its original shape and length after the force has been removed meaning it is an elastic object (like a spring)

Question 15

What is the elastic potential energy store of an object?

Answer 15

The energy transferred when work is done to stretch or compress an object.

Question 16

What is the equation for extension and spring constant?

Answer 16

Force (N) = Spring Constant (N/m) x Extension (m)

Question 17

What is the limit of proportionality on an extension-force graph?

Answer 17

The maximum force meaning that the extension is no longer proportional to force

Question 18

How can you investigate the link between force and extension? [PRACTICAL]

Answer 18

Equipment: weighted stand, fix millimetre ruler and spring on with clamp, add masses to spring, tape to mark end of spring

• measure natural length of spring
• add mass to spring + record new length of spring (extension is change in length)
• repeat by adding more mass
• when there is big increase in extension this means you have reached the spring’s limit of proportionality.
• draw graph (should be straight line that then curves at end)

Question 19

What is the equation for elastic potential energy?

Answer 19

Elastic Potential energy(J) = 1/2 x spring constant (N/m) x extension (m)

Question 20

What’s the equation for distance?

Answer 20

Distance (m) = speed (m/s) x time (s)

Question 21

What is the equation for acceleration?

Answer 21

Change in velocity (m/s) = Acceleration (m/s^2) x time (s)

Question 22

What’s the difference between acceleration and deceleration?

Answer 22

Deceleration is negative acceleration

Question 23

What is the equation for uniform acceleration?

Answer 23

V^2 - u^2 = 2as

Final velocity (m/s) = v
Initial velocity (m/s) = u
Acceleration (m/s^2) = a
Distance (m) = s

Question 24

What do different lines on a distance time graph mean?

Answer 24

Gradient = speed (steeper is faster)
Flat line = stationary
Straight diagonal lines = constant speed
Line curving upwards = accelerating 
Line curving downwards = decelerating

Question 25

What do the lines on a velocity-time graph mean?

Answer 25

Gradient = acceleration (steeper is faster) Flat line = steady speed Straight diagonal lines = constant acceleration/deceleration Line curving upwards = increasing acceleration Line curving downwards = decreasing acceleration Area under graph = distance travelled

Question 26

What does it mean when falling objects reach terminal velocity?

Answer 26

When something first falls gravity is the stronger force meaning it accelerates. As the speed increases so does friction meaning it starts to slow it down until gravity and friction are equal and it falls at constant speed. This means it has reached its terminal velocity.

Question 27

What are Newton’s three laws of motion?

Answer 27

1. You need a resultant force for something to start moving or change direction (one force has to be stronger) 2. The larger the resultant force the faster it will move (F=ma) 3. When two objects interact they exert equal and opposite forces onto each other.

Question 28

How can you investigate the effect of mass and force on acceleration? [PRACTICAL]

Answer 28

Equipment: trolley with card in middle (interrupts light gate), light gate connected to data logger, pulley at edge of table, hook with mass hanging from pulley, trolley attached to pulley on table behind starting line. Effect of mass = add mass to trolley, put behind starting line, release, record acceleration, keep mass on hook the same throughout Effect of force = all mass on trolley then one at time transfer to hook, put trolley behind starting line, release, then record acceleration

Question 29

What is the equation for stopping distance?

Answer 29

Stopping distance = thinking distance + braking distance

Question 30

What is the thinking distance affected by?

Answer 30

Speed - faster you’re going the further you will travel in time to react Reaction time - longer reaction time means longer thinking distance

Question 31

What is the braking distance affected by?

Answer 31

Speed - faster it’s going the longer it takes for it to stop Weather + road surface - whether it’s wet, icy, there are leaves, oil or less grip of tyres Condition of tyres - if they are bald then they can skid Condition of brakes - worn or faulty means not enough force will be applied

Question 32

How can you measure reaction times using the ruler drop test?

Answer 32

- someone holds ruler for you - you have finger and thumb in line with 0cm - they drop it without warning and you catch ruler with thumb and finger - where you caught it is the distance fallen - acceleration is roughly 9.8 m/s^2 - use v^2 = u^2 + 2as to find final velocity - then use change in velocity/acceleration = time (rearrange v/t=a

Question 33

What is the equation for momentum?

Answer 33

Momentum (kg m/s) = mass (kg) x velocity (m/s)

Question 34

Define conservation of momentum

Answer 34

The total momentum before an event is the same as after the event (e.g. during a collision) If a moving ball with momentum hit a stationary ball, the stationary ball would start moving and the moving ball would move but slower than before. If you add up the momentum from both balls, it is the same as the beginning. Momentum before = momentum after

Question 35

What is the equation for change in momentum?

Answer 35

Force (N) = change in momentum (kg m/s) / change in time (s)

Question 36

What causes a change in momentum?

Answer 36

When a non-zero resultant force act on a moving object it causes the velocity to change meaning there is a change in momentum. The force causing the change is equal to the rate of change of momentum.