Physics - Forces 1

What is a force?

A push or pull that acts on an object due to its interaction with another object

What is a vector quantity?

A measure of both magnitude and direction

Is force a scalar or vector quantity?

Vector

What is a contact force?

A force that requires touch to have an effect

What is speed?

How quickly something is moving

What is the speed formula triangle?

Distance
Speed | time

How to calculate speed from a distance time graph?

Rise/run

What is the gradient of any line on a distance time graph?

Speed

What does a straight line on a distance time graph represent?

A constant speed

What does a horizontal line mean?

Object is stationary

What does a steep slope mean?

Faster speed

What does the graph look like if speed is increasing or decreasing (object accelerating/decelerating)?

A curve

How do you calculate the speed of the curve?

Draw a tangent to it, then calculate the rise/run

What axis is time always on?

X axis

What is the typical speed of:
Walker
Runner
Cyclist
Car
Passenger Aeroplane
Sound

(m/s)
1.5
3
6
10-30
200-250
330-340

How can light gates be used to measure time?

How can light gates be used to measure speed

What do non contact forces have?

A field of influence

What happens to the influence as the objects get further apart?

It weakens

Give 5 examples of scalar quantities

Speed
Distance
Mass
Temperature
Time

Give 5 examples of vector quantities

Force
Velocity
Displacement
Momentum
Acceleration

What does -2km east mean?

2km west

What is a free body diagram?

A simple diagram which shows all the forces acting on an object using force arrows

True/false: If forces act in opposite directions, they are added on to each other

False - they cancel each other out

What is the resultant force?

The overall force acting on an object, taking into account all of the different forces acting on it

If a resultant force acts on an object, what two properties are affected by it?

Speed and direction

If all the forces on an object balance out, then the object is in…

Equilibrium

Describe a person jumping out of a parachute

When the person first steps out of the plane, the only thing that is acting on them is their weight, downwards, resultant force is downwards. This causes them to accelerate downwards. As soon as they start falling, they encounter air resistance, which acts upwards. By this point they won’t have accelerated much, so the velocity is still quite low. This means the air resistance is also quite low. The resultant force is still downwards, so they continue accelerating downwards. However, as their velocity increases, so does air resistance. Because their weight is constant, this means that, as the air resistance increases, the rate of acceleration decreases (not the speed). Eventually, the velocity increases so much that the air resistance increases to the size of the weight. This means that the resultant force is 0N and the object has reached terminal velocity, so it stops accelerating. At this point, the skydiver may put up their parachute, massively increasing their surface area, and creating a huge amount of air resistance. This means that the resultant force is now upwards, causing the diver to deccelerate. As they slow down, their velocity decreases, causing their air resistance to decrease. Eventually, the air resistance will equal the weight again, causing the net force to be zero. This means that the skydiver reaches terminal velocity again, except, this time, much lower than before. They continue falling at the same speed until they land

What is air resistance caused by

An object colliding with air particles

What two things is air resistance determined by

Speed (faster = more collisions) and surface area (bigger = more collisions) of the object

How to find the resultant force using a scale diagram? (eg. Bob is cycling north with a force of 4N, there is a strong wind blowing him east with a force of 3N)

Draw it
Place the arrows tip to tail
Draw a diagonal line between them
Measure the line
To find the direction, measure the angle from north

What does it mean if the arrows all match up perfectly?

0 resultant force

How to resolve vectors? (eg. Bob pushes a toy car up a ramp with a force of 5N, what is the upwards and right forces)

Draw the resultant force
Draw a horizontal and vertical lines
Measure the lines

What is the velocity triangle?

Displacement
Velocity | Time

What is acceleration measured in

m/s^2 (don’t actually square anything)

What is the acceleration triangle?

Change in velocity*
acceleration | time

*in practice this is simply the final velocity - the initial velocity

True or False: Speed can be negative

False - velocity can be negative as it is a vector, but speed is scalar so has no direction

True or False: Acceleration can be negative

True - means you are decelerating

What is the alternate acceleration equation?

2 x acceleration x distance =final velocity squared - initial velocity squared

What is the acceleration of any object that is dropped?

9.8m/s^2

A ball is dropped from an unknown height above the ground. The speed of the ball just before it hits the ground is 7m/s. Calculate the height from which the ball is dropped. Ignore air resistance.

2as=v^2 - u^2
S x 2 x 9.8 = 49 - 0
S = 49
19.6
S = 2.5 metres

How to calculate acceleration from a velocity time graph?

Change in velocity
Change in time

What do horizontal lines mean on a velocity time graph?

The object is neither accelerating nor decelerating (this means that you only have to look at the y axis to find velocity)

What does it mean if there is a curve on a velocity time graph

Rate of acceleration is increasing/decreasing

How to find the distance from a velocity time graph?

Find the area of the shape under the line (if it is a curve, you will only ever be asked to estimate, in which case just count the squares)

What does Newton's First Law state?

There must be a resultant force for an object’s movement to change (inertia)

What does newton's first law mean for stationary objects

They stay stationary

What does newton's first law mean for moving objects

They continue moving at the same rate

What does Newton’s second law state?

Any non-zero force acting on an object will cause it to accelerate

If an object has unbalanced forces acting on it towards the right, what does that mean if it is:
Stationary
Moving right
Moving left
Moving in any other direction

Starts moving
Speeds up
Slows down (potentially stop)
Change direction

*a change in direction is considered acceleration because acceleration is affected in change in velocity, and velocity is affected by both speed and direction

What is Newton's third law?

When two objects interact, they exert an equal and opposite force on each other

If you push a box with a force of 100N, what is the force the box is pushing on you and what is it called

100N, normal contact force

In the above situation, do you move or does the box move?

It could be either, or both, depending on the mass of you and the mass of the box

What is the force, mass, acceleration triangle

F
M A

*this means that you need a high force or a small mass to make an object move

What is stopping distance?

The minimum distance required to stop a vehicle in an emergency

How to calculate stopping distance

Stopping distance = thinking distance + braking distance

What is thinking distance?

How far the car travels in the time it takes the driver to brake after seeing the hazard

What can affect the thinking distance?

How fast the car is travelling
Reaction time
Alcohol consumption
Tiredness
Distractions

What is braking distance?

The distance taken to stop under the braking force (once the brakes have been applied)

What affects braking distance?

Speed
Car
Mass
Condition of the brakes
Condition of tyres
Road
wet/icy
grit

True or false: thinking distance increases proportionally with speed?

True

True or false: braking distance increases proportionally with speed?

False, the stopping distance increases more