# P4: Explaining Motion

HideShow resource information

## 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.
1 of 10

## 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)
2 of 10

## Acceleration

Acceleration

• 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)
3 of 10

• 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.
4 of 10

## 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.
5 of 10

## 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.
6 of 10

## 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.

7 of 10

## 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.
8 of 10

## What are Forces?

Interaction Pairs

• 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.

Resultant force

• 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.

Friction

• 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.
9 of 10

## How do objects start moving?

Using friction

• 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.
10 of 10