PHYSICS Forces 1 LEARNING OBJECTIVES 1.5.1 Effects of forces Core Recognise that a force may produce a change in size and shape of a body Plot and interpret extension-load graphs and describe the associated experimental procedure Describe the ways in which a force may change the motion of a body Find the resultant of two or more forces acting along the same line

Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line Understand friction as the force between two surfaces which impedes motion and results in heating Recognise air resistance as a form of friction Supplement State Hookes Law and recall and use the expression F = k x, where k is the spring constant

Recognise the significance of the limit of proportionality for an extension-load graph Recall and use the relation between force, mass and acceleration (including the direction), F = ma Describe qualitatively motion in a circular path due to a perpendicular force LEARNING OBJECTIVES 1.5.1 Effects of forces Core Recognise that a force may produce a change in size and shape

of a body Plot and interpret extension-load graphs and describe the associated experimental procedure Describe the ways in which a force may change the motion of a body Find the resultant of two or more forces acting along the same line Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line Understand friction as the force between two surfaces which impedes motion and results in

heating Recognise air resistance as a form of friction Supplement State Hookes Law and recall and use the expression F = k x, where k is the spring constant Recognise the significance of the limit of proportionality for an extension-load graph Recall and use the relation between force, mass and acceleration (including the direction), F = ma Describe qualitatively motion in a

circular path due to a perpendicular force What is a force? A force is a push or a pull. Some common examples: WEIGHT pulls things downwards What is a force? A force is a push or a pull. Some common examples: An equal and opposite force,

perpendicular to the surface (at right angles to) prevents the man from penetrating the What is a force? A force is a push or a pull. Some common examples: WEIGHT pulls things downwards FRICTION acts against anything moving

AIR RESISTANCE (drag) acts against anything moving through air UPTHRUST keeps things Forces are vector quantities because they have both size and direction. Forces are vector quantities because they have both size and direction.

SI units Forces are measured in newtons (N) Forces are vector quantities because they have both size and direction. SI units Forces are measured in newtons (N) Small forces can be measured using a spring balance (or newton meter)

Newtons first law of motion If no external force is acting on it, and object will: - If stationary, remain stationary - If moving, keep moving at a steady speed in a straight line. Newtons first law of motion If no external force is acting on it, and object will:

- If stationary, remain stationary - If moving, keep moving at a steady speed in a straight line. In space, where there are no external forces, a satellite will continue to move at a steady speed in a straight line . for ever! Balanced forces If forces are in balance, then

they cancel each other out, and the object behaves as if there is no force on it at all Balanced forces When terminal velocity is reached, the skydiver is falling at a steady speed. The force of air resistance is exactly balanced by the air resistance pushing upwards. If forces are in balance, then they cancel each other out, and

the object behaves as if there is no force on it at all Balanced or unbalanced forces? What will happen in each case? A B C D

Balanced and Unbalanced Forces Balanced forces: If the forces acting on an object are balanced then the object will either remain stationary or continue to move with a constant speed. Balanced and Unbalanced Forces Balanced forces:

If the forces acting on an object are balanced then the object will either remain stationary or continue to move with a constant speed. Unbalanced forces: If the forces acting on an object are unbalanced then the object will change its speed. It will begin to move, speed up, slow down or stop. Friction and Stopping Forces Friction and Stopping Forces Although it is sometimes unwanted, friction can really help us for example in car braking systems, and giving shoes grip on the ground.

Friction and Stopping Forces Although it is sometimes unwanted, friction can really help us for example in car braking systems, and giving shoes grip on the ground. As the block is gently pulled, friction stops it moving increase the force and the block will start to slip = starting or static friction. Friction and Stopping Forces Although it is sometimes unwanted, friction can really help us for example in car braking systems, and giving shoes grip on the ground. When the block starts to move, the friction drops. Moving or dynamic

friction is less than static friction. This friction HEATS materials up. Stopping distance The distance needed for a car, travelling at a given speed, to stop (m). Stopping distance = Thinking distance + Braking Distance Thinking Distance Before we react to a danger our brain takes time to think. The distance travelled during this time is the Thinking Distance (m) Mmh, a level

crossing! I should stop now! 0.6 s Braking Distance Cars dont stop straight away. They travel a certain distance from when you start braking to when they stop. This is the Braking Distance. Just in time! LEARNING OBJECTIVES 1.5.1 Effects of forces

Core Recognise that a force may produce a change in size and shape of a body Plot and interpret extension-load graphs and describe the associated experimental procedure Describe the ways in which a force may change the motion of a body Find the resultant of two or more forces acting along the same line Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line

Understand friction as the force between two surfaces which impedes motion and results in heating Recognise air resistance as a form of friction Supplement State Hookes Law and recall and use the expression F = k x, where k is the spring constant Recognise the significance of the limit of proportionality for an extension-load graph Recall and use the relation between

force, mass and acceleration (including the direction), F = ma Describe qualitatively motion in a circular path due to a perpendicular force Hookes Law and forces acting on a stretched spring. Robert Hooke was born in 1635 and he devised an equation describing elasticity.

Hookes Law and forces acting on a stretched spring. Robert Hooke was born in 1635 and the 1660s he devised an equation describing elasticity. Hookes Law and forces acting on a stretched spring.

Hooke discovered that the amount a spring stretches is proportional to the amount of force applied to it. Robert Hooke was born in 1635 and the 1660s he devised an equation describing elasticity.

Hookes Law and forces acting on a stretched spring. Hooke discovered that the amount a spring stretches is proportional to the amount of force applied to it. That is, if you double the load the extension will double. = Hookes Law Robert Hooke was

born in 1635 and the 1660s he devised an equation describing elasticity. Hooke discovered that the amount a spring stretches is proportional to the amount of force applied to it.

That is, if you double the load the extension will double. = Hookes Law Hookes Law and forces acting on a stretched spring. Robert Hooke was born in 1635 and the 1660s he devised an equation describing elasticity.

Hooke discovered that the amount a spring stretches is proportional to the amount of force applied to it. That is, if you double the load the extension will double.

= Hookes Law Hookes Law and forces acting on a stretched spring. For any spring, dividing the load (force) by the extension gives a value called the spring constant (K), provided that the spring is not stretched beyond its elastic limit.

Robert Hooke was born in 1635 and the 1660s he devised an equation describing elasticity. Hookes Law and forces acting on a stretched spring. Spring constant: Load = spring constant x extension x

F = k x For any spring, dividing the load (force) by the extension gives a value called the spring constant (K), provided that the spring is not stretched beyond its elastic limit. Robert Hooke was born in 1635 and the

1660s he devised an equation describing elasticity. Hookes Law and forces acting on a stretched spring. Spring constant: Load = spring constant x extension x X F = k x

Up to point X the extension is proportional to the load. Point X is the limit or proportionality For any spring, dividing the load (force) by the extension gives a value called the spring constant (K), provided that the spring is not

stretched beyond its elastic limit. Robert Hooke was born in 1635 and the 1660s he devised an equation describing elasticity. Hookes Law and forces acting on a stretched spring. Beyond point X the spring continues to behave elastically and returns to its original length when the force is removed. At the elastic limit the spring behaves in a plastic way and

does not return to its original length it is permanently stretched. X Up to point X the extension is proportional to the load. Point X is the limit or proportionality For any spring, dividing the load (force) by the extension gives a

value called the spring constant (K), provided that the spring is not stretched beyond its elastic limit. LEARNING OBJECTIVES 1.5.1 Effects of forces Core Recognise that a force may produce a change in size and shape of a body Plot and interpret extension-load graphs and describe the associated

experimental procedure Describe the ways in which a force may change the motion of a body Find the resultant of two or more forces acting along the same line Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line Understand friction as the force between two surfaces which impedes motion and results in heating Recognise air resistance as a form of friction

Supplement State Hookes Law and recall and use the expression F = k x, where k is the spring constant Recognise the significance of the limit of proportionality for an extension-load graph Recall and use the relation between force, mass and acceleration (including the direction), F = ma Describe qualitatively motion in a circular path due to a perpendicular force

Force, mass and acceleration Force, mass and acceleration are related by the formula: Force, mass and acceleration are related by the formula: FORCE (N) = MASS (kg) x ACCELERATION (m/s2) Force, mass and acceleration are related by

the formula: FORCE (N) = MASS (kg) x ACCELERATION (m/s2) Newtons second law of motion Force, mass and acceleration are related by the formula: FORCE (N) = MASS (kg) x ACCELERATION (m/s2) F m x a Force, mass and

acceleration are related by the formula: FORCE (N) = MASS (kg) x ACCELERATION (m/s2) Now an example try we must! F m x a Mass = 3kg Frictional force = 12N

Motor force = 20N Mass = 3kg Frictional force = 12N Motor force = 20N Resultant force = 20 12 = 8N (to the right) Acceleration = F / m a = 8 / 3 = 2.67m/s2 LEARNING OBJECTIVES 1.5.1 Effects of forces

Core Recognise that a force may produce a change in size and shape of a body Plot and interpret extension-load graphs and describe the associated experimental procedure Describe the ways in which a force may change the motion of a body Find the resultant of two or more forces acting along the same line Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line

Understand friction as the force between two surfaces which impedes motion and results in heating Recognise air resistance as a form of friction Supplement State Hookes Law and recall and use the expression F = k x, where k is the spring constant Recognise the significance of the limit of proportionality for an extension-load graph Recall and use the relation between

force, mass and acceleration (including the direction), F = ma Describe qualitatively motion in a circular path due to a perpendicular force PHYSICS Forces 1