Forces and Motion

What is a scalar and give examples

a quantity that has a magnitude / size only, eg speed, distance, mass, energy

What is a vector and give examples

a quantity that has a magnitude / size as well as a direction, eg displacement, velocity, acceleration, force, weight, momentum

what is velocity

speed in a specified direction

formula for speed. rearrange to give 2 other forms (3) give units too

• v = d/t

• d = vt

• t = d/v

v: speed (m/s)

d: distance (m)

t: time (seconds)

What does the gradient show on a distance-time graph?

the speed

formula for acceleration through time. rearrange to give other forms and give units too

• a = (v-u)/t

• t = (v-u)/a

• v = u + at

• u = v - at

a: acceleration (m/s²)

v: final velocity (m/s)

u: starting velocity (m/s)

t: time taken (seconds)

formula for acceleration through displacement. rearrange to give other forms and give units too.

• a = (v² - u²)/2s

• s = (v² - u²)/2a

• v = √u² + 2as

• u = √v² - 2as

a: acceleration (m/s²)

s: displacement (m)

v: final velocity (m/s)

u: starting velocity (m/s)

What does the gradient on a velocity-time graph show?

the acceleration

How to work out the distance travelled in a velocity-time graph?

calculate the area of the space under the line

speed of bus travelling at 30mph in m/s

13 m/s

speed of walking in m/s

1.5 m/s

speed of cycling in m/s

6 m/s

speed of car at 70mph in m/s

31 m/s

speed of running in m/s

3 m/s

speed of sound in m/s

330 m/s

Newton’s First Law

when the forces acting on an object are balanced / the resultant force is 0N, the object stays at rest / stays at a constant speed

Newton’s Second Law. rearrange to give other forms and give units too.

• F = ma

• m = F/a

• a = F/m

F: resultant force (N)

m: mass (kg)

a: acceleration (m/s²)

what is the difference between mass and weight

mass is the amount of matter in an object, measured in kg. weight is the force acting on an object due to gravity, measured in newtons.

formula for weight. rearrange to give other forms and give units too

• W = mg

• m = W/g

• g = W/m

W: weight (N)

m: mass (kg)

g: gravitational field strength (N/kg)

difference between contact and non-contact forces. give examples of each

contact forces, eg friction, require objects to physically touch. non-contact forces, eg weight, don’t need objects to touch.

explain how an object can reach terminal velocity

an object is dropped and the only force acting on it is weight, downwards. Initial drag is 0, so the object accelerates downwards at 10 m/s²

as the object accelerates, the drag increases. the drag is still less than the weight though, meaning the resultant force is still acting downwards - the object continues to accelerate, but at a decreasing rate as drag increases and the resultant force decreases.

eventually the object’s speed increases enough that the drag equals the weight. as the resultant force is 0, the object no longer accelerates and continues at a constant, maximum speed called terminal velocity