Physics - Forces and Motion

What is weight defined as?

The force acting on an object due to gravitational attraction

Because of weight…

• Objects stay firmly on the ground

• Objects will always fall to the ground

• Satellites are kept in orbit

What is mass measured in and what is its definition?

Mass is measured in kg and it is related to the amount of matter in an object

What is weight measured in and why?

Weight is measured in Newtons as its a force

The weight that an object experiences depends on:

• The object’s mass

• The mass of the planet attracting it

What can weight be measured with?

A Newton meter

What can the centre of mass of an object be defined as?

The point which the weight of an object acts

For a symmetrical object of uniform density where is the centre of mass located?

At the point of symmetry

How can the centre of mass of an irregular shape be found?

By locating its balance point

What is the gravitational field strength on earth?

9.8N/kg

On earth what rate do all free-falling objects accelerate towards earth?

9.8m/s²

On a graph we can see that the weight and mass of an object are…

Directly proportional

What is acceleration defined as?

The rate of change of velocity

How do you work out the change in velocity?

final velocity - initial velocity

What is acceleration measured in?

m/s²

What is change in velocity measured in?

m/s

What is time measured in?

seconds

An object that speeds up is…

accelerating

An object that slows down is…

decelerating

When is acceleration positive?

When an object is speeding up

When is acceleration negative?

When an object is slowing down

What is the typical acceleration for a family car in m/s?

2-3

What is the typical acceleration for a falling object in m/s?

10

What is the typical acceleration for a rocket in m/s?

30

What is the typical acceleration for a formula 1 car in m/s?

50

What is the typical acceleration for a fighter jet in m/s?

90-120

What information do velocity time graphs show

• If the object is moving with a constant acceleration/deceleration

• The magnitude of the acceleration/deceleration

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

constant acceleration

What does the slope of a line on a velocity time graph represent?

the magnitude of acceleration

• A steep slope means large acceleration (or deceleration) i.e. the object's speed changes very quickly

• A gentle slope means small acceleration (or deceleration) - i.e. the object's speed changes very gradually

• A flat line means the acceleration is zero - i.e. the object is moving with a constant velocity

How can the acceleration of a velocity time graph be calculated?

from the gradient of the graph (change in y/change in x)

What does the area under a velocity time graph represent?

the displacement (or distance travelled) by an object

What two forces act when skydivers fall out of a plane?

• weight

• air resistance

Describe the process of terminal velocity when a skydiver jumps out of a plane?

• An object falling through fluid experiences two forces of weight and friction

• At first air resistance is small and the person accelerates

• As the person speeds up, air resistance increases

• After the while the air resistance balances the weight. The person travels and a constant (terminal) velocity

• When the parachute is opened there is a large upward force which causes the person to decelerate

• Eventually the forces balance once more and the person reaches a new, much slower, terminal velocity

What does Newton’s first law of motion state?

Objects will remain at rest or move with constant velocity unless acted on by a resultant force. This means that:

• A stationary object will remain stationary

• An object moving with a constant velocity will continue to move at that constant velocity

• Unless the object is acted on by a resultant force

What does constant velocity mean?

• No change in speed

• No change in direction

What are balanced forces?

Forces that have equal magnitude but are in the opposite direction, so there is no net force

When there is a resultant force acting on an object then Newton’s first law no longer applies and the object can…

• Change direction

• Change speed

• Change shape

What is Newton’s second law?

The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object’s mass

What important principle’s does Newton’s second law explain?

• An object will accelerate (change its velocity) in response to a resultant force

• The bigger this resultant force the larger the acceleration

• For a given force, the greater the object’s mass, the smaller the acceleration experienced

What are all the units for the F=ma equation relating to Newton’s second law

• F is in N

• m is in kg

• a is in m/s²

What are the units ofr the equation for momentum?

• p is in kgm/s

• m is in kg

• v is in m/s

What does the momentum equation mean?

That an object at rest has no momentum

What does momentum do to an object?

Momentum keeps a moving object moving in the same direction, making it difficult to change the direction of an object with a large momentum

What does velocity being a vector mean for the momentum of an object?

That the momentum of an object also depends on the direction of its travel

• This means that momentum can either be positive or negative

• If an object traveling to the right has a positive momentum, an object traveling in the opposite direction (to the left) will have a negative momentum

When will the momentum of an object change?

• When the object accelerates or decelerates

• When the object changes direction

• When the object’s mass changes

What is the principle of the conservation of momentum?

In a closed system, the total momentum before an event is equal to the total momentum after the event

During a collision objects will either

• Collide and move in opposite directions

• collide, stick together and move in the same direction together

What happens when objects move in different directions during a collision?

Each object will have a different velocity depending on its mass and the initial momentum of the system

What happens when objects stick together and move in the same direction together during a collision?

They will have a combined mass and velocity

when a force acts on an object that is moving, or able to move, the object will accelerate (or decelerate). What does this cause?

a change in momentum, more specifically the force is the rate of change of momentum

What are the units for the rate in change of momentum equation?

F is in N

Change in momentum is in kgms-1

Change in time is in s

What is the change in momentum defined as?

final momentum vs initial momentum

Impulse is equal to…

change in momentum

Since force is equal to the rate of change in momentum, how can the force of an impact in a vehicle collision be decreased?

By increasing the contact time over which the collision occurs

• The contact time is the time in which the person is in contact with what they have collided with

What safety devices are vehicles fitted with and why?

• Air bags

• seat belts

• crumple zones

They increase impact time and reduce passenger injury

How does a seat belt reduce injury?

It stretches slightly, increasing stopping time and reducing force

How does an airbag reduce injury?

It cushions the passenger and increases the time over which they stop

How do crumple zones reduce injury?

They crumple during a collision, increasing stopping time and reducing force

How do crash mats reduce injury?

They increase contact time when a person lands, reducing the impact force

Why do playgrounds have cushioned surfaces?

They increase contact time during a fall, reducing the force of impact

What two factors affect the usefulness of safety equipment?

• Mass

• Velocity