SP2 Motion and Forces

Question 1

What is resultant force?

Answer 1

The total force that results from 2 or more forces acting upon a single object.

If the forces are acting in the same direction, we add them to find the resultant force.

If the forces are acting in opposite directions, we subtract them to find the resultant force.

We take direction into account.

Question 2

What is Newton’s first law?

Answer 2

If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity (spped and direction).

Question 3

Describe circular motion uing Newton’s first law to help.

Answer 3

1. Velocity is both the speed and direction of an object.
2. If an object is travelling in circular orbit (at a constant speed), it is constantly changing direction, so is constantly changing velocity.
3. This means it’s accelerating
4. From Newton’s first law, this means there must be a resultant force acting on it.
5. This force acts towards the centre of the circle
6. The force that keeps something moving in a circle is called a centripetal force.

Question 4

What formula describes Newton’s second law?

Answer 4

F = m x a
Resultant Force (N) = Mass (kg) x Acceleration (m/s²)

Question 5

What is inertia? How is it linked to Newton’s first law?

Answer 5

The tendency for motion to remain unchanged.

Newton’s first law says that until acted on by a resultant force, objects at rest stay at rest, and objects moving at a constant velocity will stay moving at that velocity. This tendency to keep moving with the same velocity is called inertia.

Question 6

What is an object’s inertial mass? How can it be found, linking to Newton’s second law?

Answer 6

An object’s inertial mass measures how difficult it is to change the velocity of an object.

Newtons’ second law: F = m x a
Inertial mass: m = F / a (it is rearranged)

Question 7

What is Newton’s third law?

Answer 7

When 2 objects interact, the forces they exert on each other are equal (magnitude) and opposite (direction).

Action - Reaction forces.

Question 8

What is mass?

Answer 8

The quantity of matter there is in an object.
A scalar quantity.
Measure in kg using a mass balance.

Question 9

What is weight?

Answer 9

The force acting on an object due to gravity (the pull of the gravitational force on the object).
A vector quantity.
A force measured in N using a newton meter.

Question 10

What does weight depend on?
And thus,
Weight (N) =
W =

Answer 10

Mass and Gravitational field strength (the weight of an object changes depending on the object’s location e.g. stronger if it’s closer to the mass causing the field)

= Mass (kg) x Grav. Field Strength (N/kg)
= m x g

Question 11

What is momentum?

Answer 11

A measure of the tendency of an object to keep moving, or how hard it is to stop it moving.
A vector quantity.

Question 12

Momentum (kg m/s) =
p =

Answer 12

mass (kg) x velocity (m/s)
m x v

Question 13

What is conservation of momentum?

Answer 13

Where, in a closed system, the total momentum before is the same as after.

Momentum before = Momentum after

Question 14

Forces cause changes in momentum.
Newton’s second law can explain this, giving us which formula?

Answer 14

Force (N) = Change in momentum (kg m/s) / Time (s)

F = (mv - mu) / t

Question 15

What is reaction time?

Answer 15

How quickly one reacts

Question 16

How can you measure reaction times?

Answer 16

The Ruler Drop Test:
1. Sit with you arm resting on the edge of a table (to stop you moving it during the test)
2. Get someone to hold a ruler above your hand, so that the zero mark lines up with your thumb and forefinger.
3. Without giving any warning, the person holding the ruler should drop it
4. Try to catch the ruler as quickly as possible
5. The measruement on the ruler where it was caught it how far the ruler dropped in the time it takes you to react
6. Do multiple repeats and calculate the mean reaction time
7, (To calculate how long the ruler falls for, us v² - u² = 2ax)

Question 17

What is stopping distance?

Answer 17

The distance it takes to stop a car

Question 18

What is thinking distance?

Answer 18

The distance the car travels in the driver’s reaction time (the time between noticing the hazard and applying the brakes)

Question 19

What is braking distance?

Answer 19

The distance taken to stop once the brakes have been applied

Question 20

Stopping distance =

Answer 20

Thinking distance + Braking distance

Question 21

What is thinking distance affected by?

Answer 21

1. Speed - the faster you are going the further you will travel in the time you take to react
2. Reaction time - longer reaction time = longer thinking distance. This can be affected by tiredness, drugs, alcohol. Distractions also affect your ability to react.

Question 22

What is braking distance affected by?

Answer 22

1. Speed - the faster a vehicle travels, the longer it takes to stop
2. Friction between tyres and the road - more likely to skid if the road is dirty, icy, wet or tyres are bald
3. Quality of brakes - if they are worn or faulty, they won’t be able to apply as much force as well-maintained brakes.
4. Mass of car - greater mass = slower stop

Question 23

Safety features in cars include:

Why are they necessary?

Answer 23

1. sealt belts
2. air bags
3. crumple zones

They are designed to increase collision times, which reduces the force, so reduces the risk on injury.

(Reducing the force makes the deceleration less large, and less dangerous)

Question 24

What is work done?

Answer 24

The energy transferred by a force acting over a distance

Question 25

Work done (J) =
E =

Answer 25

Force (N) x Distance moved in the direction of the force (m)
F x d

Question 26

What is kinetic energy?

Answer 26

The energy stored in a moving object. When a vehicle stops, the KE is transferred to other energy stores by the braking force

Question 27

Kinetic energy (J) =
KE =

Answer 27

0.5 x Mass (kg) x Velocity ² (m/s)²
0.5 x m x v²