Forces: GCSE AQA Physics

What is a force?→a push or pull that acts on an object due to the interaction with another object.

What are forces measured in?→ Newtons (N)

What can forces make an object do?

• Change the speed

• Change the direction

• Change the shape

What is a contact force?→ When the objects are physically touching

What is a non-contact force→ When the objects are physically separated

What are the examples of contact forces?

• Friction (two surfaces touching)

• Tension

• Air Resistance (an object moves through a fluid, liquid or a gas)

• Normal contact force/reaction force (when object rests on a surface)

• Lift

What are the examples of non-contact forces?

• Magnetic force

• Electrostatic force

• Gravity/gravitational forces

What are scalar quantities?→they have magnitude only (size)

What are vector quantities?→they have magnitude & an associated (positive/negative) direction

What are the examples of scalar quantities?

• Mass

• Time

• Distance (the distance covered in a straight line has a direction) - the displacement at the end of a journey is less than the distance travelled.

• Temperature

• Energy

• Speed (how fast an object moves)

What are the examples of vector quantities?

• Velocity (speed in a specific direction)

• Acceleration

• Forces

• Displacement

Weight, Mass & Gravity:

What is 1KN (kilonewtons) equal to?→1000N

What is a gravitational force?→The area around an object where another body experiences a force.

What is the difference between mass & weight?

• Mass - kilograms (kg)

• Weight - Newtons (N)

• Weight acts at the centre of mass (weight is a downwards force)

Is mass & weight directly proportional or inversely proportional to each other→directly proportional

What are the units for mass?→Kilograms (Kg)

What are the units for weight?→Newtons (N)

What is mass?→The amount of matter in an object

What is weight?→The effect of gravity on a object

How much is the gravitational field strength of the earth?→9.8N/Kg

What is the centre of mass?→When weight acts through a single point of an object.

What equation links to gravitational field strength, weight & mass?

• weight = mass x gravitational strength

• w = m g

• weight, w, in newtons, N

• mass, m, in kilograms, kg

• gravitational field strength, g, in newtons per kilogram, N/kg

Resultant Forces:

What is a resultant force→The sum of all forces acting on an object

When forces are balanced what happens to the resultant force & it's effect upon the object?

• Resultant force = equals zero

• Effect = constant velocity (velocity does not change) or is stationary (no movement)

• Bullet 2 ⇒ links to: newtons first law

When the forces are not balanced what happens to the resultant force & it's effect upon the object?

• Resultant force = does not equal zero

• Effect = acceleration or deceleration

If there is a resultant force upon the object what will it do?→the object will accelerate in the direction of that resultant force

If there is no resultant force there is?→No Acceleration

If the resultant force opposes the direction of the movement of object→Decelerate (slow down)

Speed:

What is speed?→How fast an object moves

What is the units for speed?→m/s (metres per second)

What is the average speed of a person walking?→1.5 m/s

What is the average speed of a person running?→3 m/s

What is the average speed of a sound?→330 m/s

What type of quantity is speed?→Scalar

What is the equation linking to distance, speed & time?

• Distance = speed x time

• s = v x t

• Distance (s) = m (metres)

• Speed (v) = m/s (metres per second)

• Time (t) = s (seconds)

Acceleration:

What is acceleration?→The rate of change of velocity

What equation is linked to acceleration, velocity & time?

• acceleration = change in velocity/time

• change in velocity = (final velocity - initial velocity)

• a/acceleration= m/s squared (metres per second, squared)

• v/velocity = m/s (metres per second)

• t/time = s (seconds)

What is uniform acceleration?→when the acceleration is constant

What is the Uniform acceleration equation?

• (final velocity)^2 - (initial velocity)^2 = 2 x acceleration x distance

• V^2 - u^2 = 2 a s

• a = m/s^2

• v/final velocity = m/s

• u/initial velocity = m/s

• s/distance = m (metres)

Newton's Laws:

What is Newton's First Law?

• If the resultant force acting on an object is zero and:

• The object is stationary, the object remains stationary

• If the resultant force acting on an object is zero and:

• The object is moving, the object continues to move at the same speed & same direction (the object continues to move at the same velocity)

What is Newton's Second Law?

• The acceleration of an object is proportional to the resultant force acting on the object, & inversely proportional to the mass of the object.

What's the equation for Newton's Second Law?

• Resultant force = mass x acceleration

• F = m a

• Force = N (newtons)

• Mass = Kg (kilograms)

• Acceleration = m/s^2 (metres per second squared)

What would happen when increasing the resultant force on an object?→It would increase the acceleration

What would happen when increasing the mass of an object?→It would decrease the acceleration

What is Newton's Third Law?→Whenever two objects interact, the forces they exert on each other are equal and opposite (equilibrium).

Explain Newton's third law (the action & reaction) of bouncing on a trampoline.

• The person exerts a downwards contact force on the trampoline,

• The equal and opposite force from the trampoline rebounds the person upwards.

Explain Newton's third law (the action & reaction) of swimming.

• The person swimming pushes the water backwards, the water has an equal and opposite reaction force propelling the swimmer forwards.

Explain Newton's third law (the action & reaction) of an owl flying.

• The owl's wings push down on the air, the air pushes back in an equal and opposite direction (upwards) to keep the bird in the air.

What is Inertia→The tendency of objects to continue in their state of the rest or of uniform motion

What is Inertial mass?

• a measure of how difficult it is to change the velocity of an object:

• It is defined as the ratio of force over acceleration.

Conservation of momentum

What is momentum?

• is the product of the mass & velocity of an object, p = m x v

• Objects have property called momentum

The momentum of an object is what?→proportional to it's mass & proportional to it's velocity (the greater the velocity upon an object, the greater the momentum will be)

What is the equation for momentum?

• momentum = mass x velocity

• p = m x v

• momentum/p = kg m/s (kilograms per second)

• mass/m = kilograms (kg)

• velocity/v, is measure in metres per second (m/s)

What type of quantity is momentum?→vector quantity (has size & magnitude)

Objects that travel in opposite directions can be described as what?→having positive or negative momentum.

What is the principle of conservation of momentum?→The momentum in a closed system is always conserved (maintained) in any interaction

What is a closed system?

• is a collection of objects that are not affected by any external factors:

• there are no outside forces affecting the system like friction, air resistance, electrostatic attraction,

• there is no transfer of energy into or out of the system.

Why is momentum conversed?

• Because of Newton's Third Law

• If an object A exerts a force on object B, then object B will exert an equal sized force in the opposite direction on object B.

When the pairs of forces cause equal & opposite changes in momentum what will happen?→There is no change in total momentum.

What is an explosion?→when objects which are combined break apart and move off in opposite directions.

In collisions the total?→the total momentum before = total momentum after