Topic 5 - Forces

Question 1

What two qualities do vectors have? Name 2 vectors.

Answer 1

Magnitude and direction. Velocity, momentum, force etc.

Question 2

What qualities do scalars have? Name two scalars.

Answer 2

Scalars have only magnitude and no direction. Mass, speed, time etc.

Question 3

What is the difference between a contact and a non contact force?

Answer 3

A contact force needs to be touching something to have an effect. A non-contact force can perform a push or pull without touching something.

Question 4

What is an interacting pair and which of Newton’s laws is this equivalent to?

Answer 4

An interacting pair is a pair of equal and opposite forces acting on two interacting objects. This is Newton’s third law.

Question 5

What is gravitational force and what 2 ways does it affect objects?

Answer 5

Gravitational force is the force of attraction between masses. It affects us because the planet has a gravitational force which makes things fall to the ground and it also gives objects a mass.

Question 6

What is the difference between weight and mass?

Answer 6

Mass is the amount of ‘stuff’ in an object, whereas weight is the force of gravity acting upon an object.

Question 7

What unit is and how is weight measured? What unit is mass measured in?

Answer 7

Weight is measured in newtons using a spring newtonmeter. Mass is measured in kilograms.

Question 8

What is the equation linking mass, weight and gravitational field strength?

Answer 8

Weight = mass x gravitational field strength
Weight is in N
Mass is in Kg
Gravitational field strength is in N/Kg

Question 9

Are weight and mass directly proportional? How can you write this?

Answer 9

They are. It is written as W(directly proportional sign)m

The directly proportional sign looks like an infinity symbol with a chunk missing from the right.

Question 10

What is a free body diagram and what does it show?

Answer 10

A free body diagram is a diagram showing the forces acting on and around an object. The size of the arrows shows the size of the forces.

Question 11

What is a resultant force?

Answer 11

It is a single force acting on an object, determined by adding forces in the same direction and taking away forces acting in the opposite direction (for example).

Question 12

What happens when a force moves through object at a distance and what is the equation you can use to find this out?

Answer 12

Energy is transferred and work is done. You can find out what work has been done through the equation W=Fs.
W is work done in Joules
F is force in N
s is distance in m

Question 13

How would you use a scale drawing to find the resultant force?

Answer 13

First you would draw the forces. Then draw a straight line from the start of the first force to the end of the last one (the resultant force). Finally, measure this distance to find the magnitude and angle of the force.

Question 14

What is an equilibrium?

Answer 14

Where the resultant force acting on an object is 0.

Question 15

How do you resolve a force?

Answer 15

Draw it on a scale grid to scale, and then work it out the same way you would work out the gradient on a graph.

Question 16

What does it mean if an object is elastically deformed? What energy stores is the energy converted to?

Answer 16

If it is elastically deformed, it can go back to its original shape after being stretched. This makes it an elastic object. When work is done on it, all of the energy is transferred to the elastic potential energy store.

Question 17

What does it mean if an object is inelastically deformed?

Answer 17

It means that it can’t go back to its original shape after it has been stretched.

Question 18

What equation shows that extension is directly proportional to force?

Answer 18

F = ke
F is force in N
K is spring constant in N/m
E is extension in m

Question 19

What is the limit of proportionality?

Answer 19

It is where an object stops being elastically deformed and becomes inelastically deformed. On an extension/force graph this is where the graph curves off.

Question 20

What is a moment?

Answer 20

A turning effect of a force, causing an object to rotate.

Question 21

How is a moment calculated?

Answer 21

M = Fd
M is moment in Nm
F is force in N
d is distance in m

Question 22

How is the force of a moment on a spanner most effective?

Answer 22

When it is applied perpendicular to the object. Other angles make a smaller distance, thus a smaller moment.

Question 23

How is an object balanced about a pivot?

Answer 23

When the clockwise and anti-clockwise forces are equal.

Question 24

How do levers make applying work easier?

Answer 24

They increase the distance between the pivot and the force applied. Using the equation, this means that you need less force to get the same effect.

Question 25

Why will a larger gear cause a larger moment? Do large gears move faster or slower than small gears?

Answer 25

Because the distance to the pivot is greater. They move slower.

Question 26

What is the equation for calculating the pressure of the surface of a fluid?

Answer 26

P = F/A
P is pressure in Pa
F is force normal (at a right angle) to the surface in N
A is surface area in m squared

Question 27

If an object is more dense, does it have more or less particles in a space? In a liquid, how does density change as it gets deeper?

Answer 27

It has more particles. As you get deeper into a liquid, the density increases because there are more particles above it.

Question 28

What is the equation for pressure in a liquid using density?

Answer 28

P = hρg
P is pressure in Pa
H is height in m
ρ is density in kg/m cubed
G is gravitational field strength in N/kg

Question 29

Why does upthrust occur? How large is the force of upthrust?

Answer 29

Upthrust occurs because when an object is submerged in a liquid pressure acts on it in all directions, and as you get deeper this increases, meaning there is a larger force at the bottom than the top of the object. This creates the resultant force upthrust, which is equal to the size of the submerged object.

Question 30

What is the relationship with weight to upthrust?

Answer 30

If an object floats, the upthrust is equal to the weight. If an object sinks, the upthrust is less than the weight.

Question 31

What is the relationship between density, weight and upthrust?

Answer 31

A less dense object displaces it’s volume (equivalent to the same weight of water) before the whole object is submerged, meaning upthrust keeps it floating. A more dense object displaces it’s volume when it is fully submerged, so upthrust can’t keep it floating and it sinks.

Question 32

How is atmospheric pressure created and as you get further from earth does it increase or decrease?

Answer 32

It is created by air particles colliding with surfaces. As you get further from Earth this pressure decreases because it is less dense and there are less surfaces to collide with. There are also less air particles at high altitudes so the weight behind it decreases so there is less pressure.

Question 33

Why is distance scalar and displacement a vector?

Answer 33

Distance only has magnitude. Displacement also measures the direction in a straight line from the finish to end points. Therefore, if you walk one step west and one east, the displacement is 0 steps but the distance is 2 steps.

Question 34

Give the formula for calculating a constant speed or an average speed.

Answer 34

S = vt
S is distance travelled in m
v is speed in m/s
t is time in seconds

Question 35

How can an object have a constant speed with a changing velocity?

Answer 35

The object changed direction whilst travelling at the same speed, such as travelling at 2m/s in a circle.

Question 36

What are the 6 speeds you need to know?

Answer 36

Walking: 1.5m/s
Running: 3m/s
Cycling: 6m/s
Car: 25m/s
Train: 55m/s
Plane: 250m/s

Question 37

What is acceleration?

Answer 37

The change in velocity in a certain amount of time.

Question 38

What is the equation for acceleration?

Answer 38

A = ΔV / t
A is acceleration in m/s squared
ΔV is change in velocity in m/s
t is time in s

Question 39

What is uniform acceleration?

Answer 39

Constant acceleration.

Question 40

What is the equation for uniform acceleration?

Answer 40

V squared - u squared = 2as
V is final velocity in m/s
U is initial velocity in m/s
A is acceleration in m/s squared
S is distance in m

Question 41

What is drag?

Answer 41

The force of friction in a liquid or gas or on a solid.

Question 42

Will a car have to do more or less work against drag as it speeds up?

Answer 42

More. Drag increases with speed, so the car has to work faster to get to a steady speed.

Question 43

What is terminal velocity?

Answer 43

When an object falls, it speeds up because gravity is greater than the friction acting against it. Eventually speed increases and drag increases, so the force of friction becomes equal to the acceleration, creating 0 resultant forces and meaning the object has reached the fastest speed it will fall, or its terminal velocity.

Question 44

What factors does terminal velocity rely on?

Answer 44

Air resistance.
Drag vs weight.
The shape and area of the object. An example is a skydiver falling and then increasing their area so that W = mg but the terminal velocity is much slower.

Question 45

What is Newton’s First Law?

Answer 45

A resultant force is needed to slow something down, start it or speed it up.
“A resultant force of 0 on a stationary object means it won’t move. A resultant force of 0 on a moving object will mean it keeps going at a constant velocity.”

Question 46

What is Newton’s Second Law?

Answer 46

Resultant force is directly proportional to acceleration.

Question 47

What is the formula for Newton’s Second Law?

Answer 47

F = ma
F = resultant force in N
M is mass in kg
A is acceleration in m/s squared

Question 48

What is Newton’s Third Law?

Answer 48

When two objects interact, the forces exerted on each other are equal and opposite.

Question 49

What is inertia and how is it calculated?

Answer 49

Inertia is the tendency to continue at the same state of motion, like in Newton’s First Law. It’s calculated with the ratio of force over acceleration.

Question 50

What is the stopping distance?

Answer 50

Stopping distance = thinking distance + braking distance.

Question 51

What factors affect thinking distance?

Answer 51

Speed and reaction time.

Question 52

What factors affect braking distance and what is the average braking speed?

Answer 52

Factors are speed, road surface, tyre condition and brake condition. The average braking speed is 14m at 30mph, 55m at 60mph and 75m at 70mph.

Question 53

How does a car brake work?

Answer 53

Brake pads are pressed against the wheels, creating friction and causing work to be done, meaning the kinetic energy goes to the thermal energy store and the wheels heat up. A faster vehicle needs more braking time and this fast deceleration can be dangerous because it can cause skidding and can damage the tires.

Question 54

What is the typical reaction time and what can affect it?

Answer 54

It is between 0.2 and 0.9 seconds and can be affected by drinking alcohol, tiredness, distractions and drugs.

Question 55

Why does the speed of a car affect the braking distance more than the thinking distance?

Answer 55

Because as the car speeds up, the brakes will be the same as the car’s kinetic energy, meaning as speed increases kinetic energy increases four-fold. This means the braking distance needed increases by four-fold.

Question 56

What is the equation for momentum?

Answer 56

P = mv
P is momentum in kg m/s
M is mass in kg
V is velocity in m/s

Question 57

What is the conservation of momentum?

Answer 57

The conservation of momentum means the momentum stays the same before and after an object has moved in a closed system. If momentum before an event is 0, momentum after an event will be 0 because momentum in different directions cancel out.

Question 58

Give the equation that forces cause a change in momentum.

Answer 58

F = mΔv / Δt
F is force in N
MΔv is change in momentum in kg m/s
Δt is change in time in s