Physics - Forces and Motion

Speed

Speed is a scalar quantity that measures how fast an object is travelling

Average Speed

average speed = total distance / total time

The speeds of typical methods of transport

Walking - 1.5m/s, Running - 3m/s, Cycling - 6m/s, Cars - 13 to 30m/s, Trains - 50m/s, Aeroplane -250m/s, Speed of sound - 330m/s

Scalar

A scalar quantity is one which has only magnitude (size) e.g Distance to sun

Vector

A vector quantity has only magnitude (size) and direction e.g force

Examples of scalars

-Distance, time, energy, speed, current, resistance, density, mass, potential difference

Examples of Vectors

-Displacement (distance), velocity (speed), force (weight), pressure, acceleration

Displacement

Distance travelled in a particular direction

Velocity

Velocity is the rate of change of displacement

Acceleration

Acceleration is the rate of change of velocity.

Calculate Acceleration

a = change in velocity / t

Change in velocity is the final velocity - starting (m/s)

Explain why an object moving in a circular path at a constant speed is accelerating

An object moving in a circular path at a constant speed is accelerating because velocity is a vector, meaning it has both speed and direction. Since the object's direction is constantly changing, its velocity is also constantly changing, and a change in velocity over time is the definition of acceleration.

Explain what is meant by a force

A force is a push or a pull that acts on an object by its interaction with another object. Forces are vector quantities.

Contact force

Contact forces are forces that act between two objects that are physically touching each other. e.g Friction

Non-Contact Force

Non-contact forces are forces that act between two objects that are not physically touching each other. e.g Magnetic Force

The effects of balanced forces on an object

A balanced force means there is no effect on motion, stationary objects remain stationary and moving objects continue to move at a constant speed and in a constant direction (constant velocity). The resultant force is zero.

The effects of unbalanced forces on an object

An unbalanced force means there is a change in motion. Stationary objects move in the direction of the resultant force. (net force) A moving object will speed up, slow down, or change direction. The resultant force is more than zero.

Resultant force

The resultant force is a single force that has the same effect as two or more forces acting together

Free body force diagram

A free body diagram models the forces acting on an object

Newton's 1st law of motion

Newton's 1st law states that every object maintains its state of rest or uniform straight line motion, unless acted on by a resultant force. (Constant velocity, arrow lengths equal)

Newton's 2nd law of motion

Newton's 2nd law states the acceleration of an object of mass, m, is directly proportional to the resultant force. (kg)

Newton's 3rd law of motion

Newton's 3rd law states that if object A exerts a force on object B, then object B will exert a force on object A, which will be the same size, same type of force and opposite direction.

Explain why the acceleration of an object that experiences air resistance changes, and why the object reaches a terminal velocity

The acceleration of an object experiencing air resistance changes because the air resistance force increases with the object's speed. As the object falls, its velocity increases, which in turn increases the magnitude of the air resistance. This growing force counteracts gravity, resulting in a decreasing net force and thus a decreasing acceleration

Explain that the resultant force is zero at terminal velocity

At terminal velocity, the resultant force on an object is zero because the forces acting on it are balanced and cancel each other out.

Explain the difference between Newton's 3rd law pairs of forces, and balanced forces.

Newton's third law pairs are equal and opposite forces that act on two different objects, while balanced forces are equal and opposite forces that act on the same object.

Draw tangents to find the gradient of a curve

A tangent shows how steep the curve is exactly at that point, so its gradient gives you the instantaneous gradient. Draw a tangent to make a big triangle, then calculate the gradient.

Recognise that the instantaneous speed at a given time is the gradient of a tangent that touches a distance-time graph at that time

You cannot use the whole curve because the speed is not the same everywhere. A distance-time graph shows how far something has travelled as time passes. If the line is straight, the speed is constant .If the graph is curved, the speed is changing.

To find the speed at one exact moment, you use a tangent.

Use the equation Resultant Force = mass × acceleration (F = ma)

Newton's 2nd Law, describes the relationship between force, mass and acceleration. F= Resultant Force acting on an object, measured in N. The resultant force is the overall force when all forces acting on an object are combined. m is the mass of the object, measured in kg. a is the acceleration of the object measured in m/s squared.

Explain terminal velocity

When an object starts to fall, gravity is the only significant force acting on it, causing it to accelerate downwards. As the object's speed increases, it collides with more air molecules per second. This creates a frictional force, or air resistance, that pushes upward against the object's motion. This upward air resistance opposes the downward pull of gravity. As the speed continues to increase, the air resistance force becomes stronger, and the net downward force becomes smaller. This causes the object's acceleration to decrease. Eventually, the object's speed becomes high enough for the force of air resistance to be equal in magnitude to the force of gravity. When the two forces are balanced, the net force on the object is zero, and the object stops accelerating. It then falls at a constant maximum speed, which is its terminal velocity.