- Velocity = speed in a given direction
Speed(m/s)=distance travelled(m) / time taken (secs)
Stationary = horizontal line
The steeper the line the greater the speed of the object
Acceleration (m/s) = change in velocity (m/s) / time taken (s)
To calculate the gradient of the line on a graph, divide the change in the vertical axis by the change in the horizontal axis.
-the gradient of a velocity-time graph represents the acceleration
the area under a velocity-time graph represents the distance covered
mass and acceleration
- When all the forces are balanced, the resultant force is zero. In this case:
- a stationary object remains stationary
- a moving object keeps on moving at the same speed in the same direction
- When all the forces are not balanced, the resultant force is not zero. In this case:
- A stationary object begins to move in the direction of the resultant force.
- A moving object speeds up, slows down or changes direction depending on the direction of the resultant force.
- resultant force (newton, N) = mass (kg) × acceleration (m/s2
Air resistance - drag
- When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.
- This happens when two objects are pushed together. They exert equal and opposite forces on each other. The contact force from the ground pushes up on your feet even as you stand still. This is the force you feel in your feet. You feel the ground pushing back against your weight pushing down.
- This is the force that resists movement between two surfaces which are in contact.
- This is the force that pulls objects towards the Earth. We call the force of gravity on an object its weight. The Earth pulls with a force of about 10 newtons on every kilogram of mass.
Weight and Friction
- weight(N) = mass (kg) x gravitational field strength (N/kg)
- When an object is dropped, we can identify three stages before it hits the ground:
- At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
- As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
- Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.
- Forces affecting a falling object:
- The weight of the object. This is a force acting downwards, caused by the object’s mass the Earth’s gravitational field.
- Air resistance. This is a frictional force acting in the opposite direction to the movement of the object.
Kinectic and momentum
- work done (joule, J) = force (newton, N) × distance (metre, m)
- Any object that is raised against the force of gravity stores gravitational potential energy.
Every moving object has kinetic energy The more mass an object has, and the faster it is moving, the more kinetic energy it has.
momentum (kg m/s) = mass (kg) × velocity (m/s)
- kinetic energy = 1⁄2 × mass × speed2