# P4: Explaining Motion

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- Created by: Nnenna2kool
- Created on: 04-04-16 11:38

## 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**.

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## 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)**.**

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## 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)

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## Road Safety

- 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**.

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## 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**.

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## 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**.

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## 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.**

** **

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## 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**.

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## 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.

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## 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**.

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