How can we use energy changes to describe motion?
- When you push an object and make it move,you are doing work.
- Work done by a force (J) = Force (N) x Distance moved by force (m)
- You transfer energy to the object, your chemical energy store decreases and the energy of the object increases.
- Amount of energy transferred(J) = Work done(J)
- The moving object has kinetic energy.
- Kinetic energy(J) = 1/2 x Mass(Kg) x (Velocity)2 (m/s)2
- The faster the object moves and the greater its mass, the more kinetic energy it has.
- If you push it with greater force, you do more work and transfer more energy, the object moves faster and its kinetic energy increases.
Gravitational Potential Energy
- When you lift an object up, you are doing Work and the Gravitational potential energy of the object increases.
- Change in GPE (J) = Weight (N) x Vertical height difference (m).
- When you let go of the object, its kinetic energy increases and its GPE decreases.
- GPE lost(J) = KInetic energy gained(J).
- If an object gets faster, it is accelerating. The acceleration of an object is its change in speed, or change in velocity, in a given time period.
- Equation to caculate acceleration,
- Acceleration (m/s) = Change in velocity (m/s) / Time taken (s)
- If two cars collide,their momentum changes until it becomes 0.
- The more time the change takes, the smaller the resultant force.
- This idea is used in road safety for:
- Crumple zones- squash slowly in a collision = longer time and smaller resultant force.
- Seat belts -stretch in a collision = longer change in momentum and lesser force.
- Helmets- change shape when hit = head stops moving slowly and force is less.
- Air bags - increase time for the change of momentum.
How are forces and motion connected?
- All moving objects have Momentum.
- Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
- When a resultant force acts on an object, the momentum of the object changes in the direction of the force:
- Change of momentum (kg m/s) = Resultant force (N) x Time for which it acts (s).
- If the resultant force = 0, the Momentum doesn't change:
- If it is stationary it remains still.
- If it was already moving, it continues at a steady speed in a straight line.
Speed-time and Velocity-time graphs
- Speed-time = how speed varies with time.
- Velocity-time = velocity of an object at every instance of its journey.
- The gradient of a section of a velocity-time graph = to the object's acceleration.
Why do objects keep moving?
Driving and counter forces
- When you push a skateboard forwards, you are exerting the driving force on it. The counter force, in the opposite direction, is due to air resistance and friction.
- If the driving force is greater than the counter force, the skateboard speeds up.
- If the driving force is equal to the counter force, the skateboard moves at a constant speed in a straight line.
- If the driving force is less than the counter force, the skateboard slows down.
Speed and velocity
Equation to calculate average speed:
Speed(m/s) = Distance(m) / Time(s)
- If a horse runs 20m in 10s: average speed = 20m/10s = 2m/s
The instantaneous velocity of an object is its instantaneous speed in a certain direction.
How do we describe motion?
Distance-time and Displacement-time graphs
- Distance-time graphs describe movement.
- Speed can be calculated using one = the gradient between 2 points on the line.
- Steeper gradient = Greater speed.
- Displacement- the straight line distance from its starting point, with an indication of direction.
What are Forces?
- Forces arise from an interacton between two objects.
- Each force in an Interaction pair acts on a different object.
- The forces are equal in size and opposite in direction.
- The resultant force on an object is the sum of the individual forces that act on it, taking their direction into account.
Reaction of surfaces
- When you push down on the floor with your feet, the floor pushes up on your feet with an equal force. This force is the reaction of the surface.
- When you push something forwards, the opposing force of friction if equal will stop the object from moving.
- The harder you push, the greater the size of the friction force becomes.
- Eventually the friction force reaches its limit and the object can now move.
How do objects start moving?
- When a car engine starts, the wheels turn; exerting a backwards force on the road surface.
- The other force in the interaction pair, the forward force, is the same size. This gets the car moving.
Rockets and Jet engines
- A rocket pushes out hot gases as its fuel burns; the rocket pushes down on those gases. The escaping gases exert an equal and opposite force on the rocket, and push it upwards.
- A jet engine draws in air at the front and pushes it out at the back. An equal and opposite force pushes the engine forwards.