physics - forces and motion

Speed

Distance travelled per unit time

Acceleration

Change in velocity per unit time

What is the relationship between final speed, initial speed, acceleration and distance moved?

(Final speed)²= (initial speed)²+(2 x acceleration x distance moved)

Velocity

Speed in a given direction

Distance

Measured in meters (m)

Time

Measured in seconds (s)

Gradient ( v graph )

Velocity in a distance-time graph

Stationary

Horizontal line in a distance-time graph

Curved line

Velocity is changing and accelerating in a distance-time graph

Gradient

Acceleration in a velocity-time graph

Rest

Speed is zero in a velocity-time graph

Friction is a force that

Opposes motion

Area under the line

Distance travelled in a velocity-time graph

Scalars

Quantities with just magnitude

Vectors

Quantities with magnitude and direction e.g force

Identify types of forces

E.g gravitational, weight, friction, drag, tension, up thrust, electrostatic

Describe the forces acting on falling objects

Initially the only force is weight as drag is proportional to velocity. So the object accelerates downwards. As it accelerates the velocity so the drag increases as well. meaning there is a smaller resultant force downwards so a smaller acceleration. Until the object reaches a speed where the drag is equal to the weight meaning there is no acceleration, this velocity is know as terminal velocity. 

What is terminal velocity

The maximum speed an object reaches when falling because the forces acting on it become balanced

practical investigate how extension varies with applied force for helical springs, metal wires and rubber bands

1. Set up your apparatus as shown in the

2. Measure the length of your spring without
   any hanging masses.

3. Hang a mass of 100g on the spring

4. Measure the new length of the spring

5. Calculate the extension of the spring

6. Repeat steps 3-5 for increasing the mass
   in increments of 100g

7. Take note of your results in the table.

Force =

Mass x acceleration

Friction

Force between two surfaces that impedes motion

Momentum =

Mass x velocity

Resultant force

Net force acting on an object

Newton's first law

Object has constant velocity unless acted on by a resultant force

Newton's second law

Force equals mass times acceleration

Newton's third law

Every action force has an equal and opposite reaction force

Mass

Measure of how much matter is in an object

Describe the effects of forces between bodies such as changes in speed, shape or direction

Forces can act on a body to change the velocity, so the speed, direction or both.

Or forces can change the shape of a body, stretching it squishing it or twisting it. 

Deformation

Change in size and shape of a body

Elastic deformation

Object returns to its original shape when load is removed

Hooke's law

Force applied to a spring equals spring constant times extension

Limit of proportionality

Point where linear force-extension graph stops obeying Hooke's law

Non-linear force-extension graph

Deformation not following Hooke's law

Moment of a force

Measure of its turning effect

Equilibrium

Sum of clockwise moments equals sum of anticlockwise moments and no resultant force

Momentum

Product of mass and velocity

Force exerted on an object

Change in momentum over time

Thinking distance

Distance travelled between realizing the need to brake and pressing the brakes e.g drugs, alcohol

Braking distance

Distance travelled between pressing the brakes and coming to a stop e.g road conditions, tire conditions

Stopping distance

Sum of thinking distance and braking distance

What is elastic behaviour

The ability of a material to recover its original shape after the forces causing deformation have been removed

Linear force-extension graph

Elastic deformation following Hooke's law

Moment of a force

Measure of its turning effect

Momentum

Product of mass and velocity

Safety features in cars

Increase time taken to come to rest, reducing force

• To reduce the force experienced by the passenger you need to extend the time for a passenger to stop in a collision. As force is the change in momentum divided by time.  

Conservation of momentum

Total momentum before collision equals total momentum afterwards

Force =

Change in momentum/time taken

Recoil speed

Speed of gun after firing a bullet

Moment =

Force x perpendicular distance from pivot

Weight of a body acts through

It’s centre of gravity

Use the principle of moments for a simple system of parallel forces acting in one plane

principle of moments states that when the clockwise moments are equal to the anticlockwise moments a body will be in equilibrium.

Understand how the upward forces on a light beam vary with the position of a heavy object placed on it

when moments are taken from the right hand side as the block is a greater distance the force from the left hand pivot must be bigger to counteract it. The opposite is true for the left hand side. 

Explain why seat belts are used in cars for safety

• These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop

• They are designed to stretch slightly to increase the time for the passenger’s momentum to reach zero and reduce the force on them in a collision

Explain why airbags are used for car safety

• 
  These are deployed at the front on the dashboard and steering wheel when a collision occurs

• They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact

Explain why crumple zones are used in car safety

• These are designed into the exterior of vehicles

• They are at the front and back and are designed to crush or crumple in a controlled way in a collision

• This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions

• The crumple zones increase the time over which the vehicle comes to rest, lowering the impact force on the passengers

Explain why crash mats are used to reduce risk of injury

• They are thick and soft to offer shock absorption of the force created by the person landing on the mat

• When a person lands on a crash mat with a large force, for example, after jumping, the soft landing means their body is in contact with the mat for a longer period of time than if it were otherwise not there

• This increases the contact time over which their momentum is reduced, creating a smaller impact forceand a lower chance of injury