P4

Forces occur when there is an interaction between 2 objects. These forces always happen in pairs - when 1 object exerts a force on another, it always experiences a force in return.

green arrow= upwards force, red arrow=downwards force

These 2 forces are an interaction pair. They are equal in size and opposite in direction. You can only have an interaction pair if the forces are caused by the interaction. In this case, the compression in the weightlifter's muscles is caused by the weight pushing down, and the upwards force on the weight is caused by the weightlifter's muscles.

Sometimes a force is produced as a response to another force - these are not the same as interaction pairs. 

A book on a table has a downwards force (its weight) due to gravity. Thie downwards force, pushing on the table produces an upwards force called reaction. The weight and the reaction of the surface are the same sizes and in opposite directions. They are not an interaction pair because the weight of the book is caused by the earth's gravity, not by the table.

Another common force is friction. When 2 surfaces slide past each other, the interaction between them produces a force of friction.

The book is moving to the right across the table as shown by the red arrow. The blue and green arrow show the interaction pair of friction forces. The book experiences a backward force. This will tend to slow it down. The table experiences a forwards force. This will tend to move it forwards with the book.

When you push backward on the floor with your foot, the friction between your foot and the floor exerts a backward force on the floor. The other force of the interaction pair is the floor pushing your foot forwards. The result is that you move forwards, but the floor stays still.

The momentum of an object is its mass multiplied by its velocity. The larger the mass and velocity the larger the momentum. Forces change momentum - the larger the force the more quickly the momentum changes. The resultant force is the overall result of all forces acting on an object.

Resultant Force;

There are several forces acting on the moving car, shown by arrows;

• Gravity pulls down on the car (green), The reaction force from the road pushes up on the wheels (blue), The driving force from the engine pushes the car along (yellow), There is friction between the road and the tires (red), Air resistance acts on the front of the car (purple)

The resultant force is the sum of all different forces acting on the car. You have to take account of the directions - the reaction forces on the wheels (blue) add up to the same as the weight (green), so these cancel out. The driving force from the engine (yellow) is in the opposite direction to the counter forces of friction (red) and air resistance (purple). When the car is increasing its speed then all these forces add to give a single resultant force forwards.

Movement with balanced and unbalanced forces;

A car or bicycle has a driving force pushing it forwards. There are always counter forces of air resistance and friction pushing backward. You need to know how these forces compare if you are to predict what will happen to the speed of a moving object.

• If the driving force is greater than the counter forces, there is a resultant force forwards. This will make the car speed up
• If the driving force is less than the counter forces, there is a resultant force backward. This will make the car slow down.

If the driving force is the same as the counter forces, there is no resultant force, and so no change in velocity.

• If the car is already moving, it will carry on at a steady speed in a straight line
• If the car is not moving, it will stay still

Vertical Motion;

If a ball is released from a height then forces begin to act on it. It will start to fall due to the force of gravity acting on it. The ball will begin to accelerate. 

If an object is dropped that is light relative to its size, like a feather, it will speed up when released at first but then fall at a steady speed. This is due to air resistance.

The faster an object moves, the greater the force of air resistance on it becomes. The light object will reach a steady speed when the force of air resistance balances out the force of gravity.

Force and Momentum Change;

Momentum (kgm/s) = Mass (kg) x Velocity (m/s)

Change of Momentum (kgm/s) = Resultant Force (N) x Time (s)

The speed of a moving object can be calculated if the distance traveled and the time taken are known. The faster an object moves, the steeper is the line representing it on a distance-time graph.

The velocity of an object is its speed in a particular direction. In velocity-time graphs sloping lines represent steadily increasing or decreasing velocities. Horizontal lines represent the movement at constant velocities.

Speed, Distance and Time;

Speed (m/s) = Distance Travelled (m) / Time Taken (s)

Work done and energy transferred are measured in joules (J). The work done on an object can be calculated if the force and distance moved are known. Objects raised against the force of gravity increase gravitational potential energy. The more mass an object has and the faster it moves, the more kinetic energy it has.

Work, Force, and Distance;

Work and Energy are measured in the same unit, the Joule (J)

Work Done (J) = Force (N) x Distance (m)

Weight;

Weight (N) = Mass (kg) x Gravitational Field Strength (kg) 

The gravitational field strength on the Earth's surface is about 10 N/kg

Kinetic Energy;

Kinetic Energy = 1/2 x Mass x Speed2 (squared)