CP2 - Motion and Forces

Vector

A quantity that has magnitude and direction.

Scalar

A quantity that only has magnitude.

How to show size and direction of a force on a diagram?

With an arrow - length represents size, direction represents direction.

How to calculate resultant forces?

Add together all forces in one plane and treat the two planes separately.

Total force acting on an object

Resultant force.

Name for resultant force greater than zero

Unbalanced.

Name for resultant force equalling zero

Balanced.

What happens to the velocity of an object when the resultant force equals zero?

Stays the same.

What happens to an object if the forces acting on it are unbalanced?

Accelerates or decelerates.

Newton’s first law

An object will move at a constant speed and direction unless there is a resultant force acting upon it.

What direction does centripetal force pull?

Towards the centre of the circle.

What causes the centripetal on the moon orbiting the earth?

The force of gravity due to the earth’s gravitational field.

Mass

The amount of matter in an object.

Weight

Force applied to an object by a gravitational field.

Earth gravitational field strength

9.81N/kg can round to 10N/kg.

Two factors affecting weight

Mass and gravitational field strength.

Weight equation

Weight = mass x gravitational field strength

Unit of force

Newtons (N)

Word for change of velocity

Acceleration or deceleration.

Factors affecting acceleration

Mass and resultant force applied.

Equation form of Newton’s second law

Force = mass x acceleration

Inertial mass

The force applied to an object divided by the acceleration that force produces.

Newton’s third law

For every action force there is an equal and opposite reaction force

Equilibrium situation

When nothing is moving, therefore all the forces on an object are balanced.

Action and reaction pair of a person leaning on a wall.

Force of person on wall, force of wall pushing back on person.

Factors affecting momentum

Mass and velocity

Momentum equation

Momentum = mass x velocity

Conservation of momentum

Momentum before a collision = momentum after a collision.

How to calculate the force needed to change momentum?

Force = change in momentum / time

How to calculate stopping distance?

Stopping distance = thinking distance + braking distance

Factors affecting thinking distance

Being tired, taking drugs, drinking alcohol, using a mobile phone, speed of vehicle.

Factors affecting braking distance

Worn brakes, icy road, wet road, mass of vehicle, speed of vehicle, potholes etc. in road.

Typical human reaction time

0.25s

What is the equation we use to calculate the force acting when there is a collision?

Force = mass ( change in velocity ) / time

What does a large deceleration mean?

A very fast negative change in velocity, eg in a crash.

Dangers of a large deceleration

Injure or kill people and can damage vehicles.

Why does a large deceleration cause danger?

It means a large force will act on the people and vehicle.

How to decrease force in a collision?

Increase the time it takes to stop, eg. With crumple zones and seatbelts.

Typical road collision force

100,000N to 1,000,000N

How to tell if you are looking at balanced forces versus action-reaction pairs?

Balanced forces have all the forces acting on one object.

Action-reaction pairs are acting on two different objects.

Why is an object changing velocity when moving in a circle?

Velocity is a vector with direction and in a circle the direction is constantly changing.

What are the two forces that act on an object moving in a circle?

Centripetal and centrifugal force.