Forces and Motion

• Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
• Speed is defined as the distance travelled per unit time.
• If the speed of something is changing, it is accelerating.
• The acceleration of free fall near to the Earth is constant.
• Velocity is the speed in a given direction.
• Acceleration is the change in velocity per unit time.
• The gradient of a distance - time graph is velocity.
• A negative gradient on a distance - time graph means returning back to the starting point.
• A horizontal line on a distance - time graph means it is stationary.
• If the distance on a distance - time graph is zero, it is back at the starting point.
• A curved line on a distance - time graph means that the velocity is changing, and it is accelerating.
• The gradient of a velocity - time graph is acceleration.
• A negative gradient on a velocity - time graph means deceleration.
• If the speed on a velocity - time graph is zero, it is at rest.
• A horizontal line on a velocity - time graph means constant speed.
• The area under a curve on a velocity - time graph is the distance travelled.
• A curved line on a velocity - time graph means that the acceleration is changing.
• Safety features in cars work by increasing the time taken for the people in the car to come to rest, so the force is reduced.
• Hooke’s law states that for a spring, 𝐹 = 𝑘𝑥 where F is the force applied to the spring, k is the spring constant, and x is the extension.
• For a horizontal beam supported at its ends, the upwards forces at the supports change with the position of a heavy object placed on the beam.
• Elastic deformation is when the object returns to its original shape when the load has been removed, an example being a spring being stretched under normal usage.
• A non-linear (curved line) force-extension graph is when the deformation does not follow Hooke’s law and after this region, it will fracture.
• A linear (straight line) force-extension graph is when the deformation follows Hooke’s law and the point it stops being linear is called the limit of proportionality.
• As the body falls, it accelerates which increases its speed and hence air resistance.
• Mass is a measure of how much matter is in an object, measured in kilograms (kg).
• Newton’s second law states that force = mass × acceleration F=ma
• There is no acceleration and the terminal velocity is reached.
• Reaction time can also be increased by caffeine, which reduces the thinking distance.
• Newton’s third law states that every action force has an equal and opposite reaction force.
• Factors which increase the stopping distance include greater speed or mass, poor road conditions (icy, wet) or car conditions (worn tires, worn brake pads).
• The resultant of two or more forces acting along the same line is the sum of their individual forces if in the same direction and the difference of their individual forces if in the opposite direction.
• This causes the resultant force downwards to decrease.
• The force of the Earth’s gravity on an object is equal and opposite to the force of the object’s gravity on the Earth.
• The distance travelled in the time between pressing the brakes and the vehicle coming to a stop is called the braking distance.
• Factors which increase the thinking distance include greater speed and slower reaction time due to alcohol, tiredness or distractions.
• Newton’s first law states that an object has a constant velocity unless acted on by a resultant force.
• In a motion of a body falling in a uniform gravitational field, initially there is no air resistance and the only force acting on it is weight.
• A force may produce a change in size and shape of a body.
• The stopping distance is the sum of the thinking distance and braking distance.
• The acceleration decreases, so the body is not speeding up as quickly.