General overview (motion and forces)

What is a scalar quantity?

A quantity with magnitude but not direction

What is a vector quantity?

A quantity with a magnitude and a direction

Examples of scalar quantities

Speed, distance, time, mass, energy

Examples of vector quantities

Velocity, displacement, acceleration, force, momentum

What is velocity?

Speed in a stated direction

What is the formula for speed?

Speed=distance/time

What does the gradient represent on a displacement-time graph?

Velocity

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

Acceleration

What is the formula for acceleration?

Acceleration=change in velocity/time

Laboratory methods for determining the speed of objects

For constant or average speeds: measure the distance travelled and time how long it takes to go that distance with a stopwatch, then use the speed=distance/time formula

For speed at a specific point: use a light gate at the point you want to measure the speed at. It will measure the speed the object passes through it at.

Typical speed of wind

5-7m/s

Typical speed of sound

340m/s

Typical speed of walking

1.4m/s

Typical speed of running

3m/s

Typical speed for cycling

4m/s

Typical speed for trains

125mph

Typical speed for a plane

900km/h

What is Newton’s first law?

An object has a constant velocity unless acted on by a resultant force. If the resultant force is zero, the object stays at a constant speed or rest if the object is not already moving. If the resultant force is not zero, the speed or direction of the object will change

What is Newton’s second law?

Force=mass x acceleration

What is the formula for weight?

Weight=gravitational field strength x mass

How is weight measured?

Using a Newton meter

What is the relationship between the weight of an object and the gravitational field strength?

The greater the gravitational field strength, the greater the weight of an object

Why do objects in circular motion have a constant speed but a changing velocity?

In circular motion, direction is always changing. Velocity is speed in a particular direction, so a change in direction means that velocity changes. This means that acceleration can happen while speed remains the same

What force acts on an object in a circular motion?

Centripetal force

What is inertial mass?

A measure of how difficult it is to change the velocity of an object

Inertial mass=force/acceleration

What is Newton’s third law?

Every action has an equal and opposite reaction

Apply Newton’s third law to a book on a table In equilibrium

The weight of the book is equal to the reaction force on the book by the table

Apply Newton’s third law to a rocket taking off

The force of gases being ejected from the rocket is equal to the force that lifts the rocket from the ground

Apply Newton’s third law to marbles colliding

The force of one marble on the other is equal to the force of the other marble on the first

What is the formula for momentum?

Momentum=mass x velocity

What is the theory of the conservation of momentum?

Momentum is always conserved in a closed system, when there are no external forces. The total momentum before is equal to the total momentum after

Explain what happens when two marbles collide

Before two marbles collide, each of them have a momentum. When they collide, the momentum is equally distributed between the two marbles so they move in the opposite direction but with the same momentum

What is the formula for Newton’s second law in collision?

Force=change in momentum/time

What is the range for typical reaction times?

0.2-0.9 seconds

What is the formula for stopping distance?

Stopping distance=thinking distance + braking distance

What factors affect a vehicle’s stopping distance?

Mass of vehicle, speed of vehicle, driver’s reaction time, state of brakes on vehicle, state of road, amount of friction between tyres and road (weather conditions - wet or icy roads will cause less friction between the tyres and the road)

What factors affect the thinking distance?

Drugs, alcohol, distractions (children, phone), tiredness

Explain the dangers caused by large decelerations

Large decelerations cause the brakes to overheat due to a large increase in friction between the brakes and the tyres of the vehicle. The driver may lose control of the vehicle

Why is the braking force directly proportional to the initial velocity squared?

The work done to stop a vehicle is equal to the initial kinetic energy of the vehicle. Since all of the kinetic energy has to be transferred for the friction to stop, the braking distance is directly proportional to the initial velocity²