P3c(i) Forces and Motion

You will find out aboutHow a force speeds up or slows down an object

How Force, Mass and Acceleration are linkedHow to use the F=ma equation

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Forces all around usTo make this car move faster all the driver has to do is press harder on the accelerator pedal. This causes an increase in PULL of the engine that provides that FORWARD force. Without that pull force there would be no forward force.

To calculate force you need to use the equation:Force = mass x acceleration F = m x aThe unit is the Newton, N, named after Isaac Newton.

Example:What force is needed to move a car of 1,500kg to give it an acceleration of 5m/s2?F = maF = 1,500kg x 5m/s2 = 7,500N

I wonder if Isaac Newton ever said “Let the force be with you…”!!

F

m X a

REMEMBER: You need to be able to re-arrange the F=ma equation so use the Triangle rule to help.

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F=ma rules

F = m aB has a larger mass than A. To accelerate at the same amount B requires more Force.

B has a larger mass than A. If the forces acting on A and B are the same then A will have a higher acceleration.

A

B

Contextualising:A catapult is stretched back as far as it can go. Therefore the forces acting on A and B are equal.A travels further because it has a smaller mass.

AB

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Balancing of Forces50N 50N

In this instance the object was not moving to begin with, so it started at rest.If 50N push the object from the left and 50N push the object form the right then the RESULTANT (overall) force is:50N – 50N = 0N (The object is still stationary)

50N

50N

In this instance the object was already moving at a steady constant speed with a forward force of 50N. If a push force of 50N acted in the opposite direction then the object could not accelerate so would reach a steady constant speed. It would not be stationary as it started with a steady constant speed provided by the forward force. The RESULTANT force would still be:50N – 50N = 0N (The object is moving at a constant speed)

So if two forces acting on an object are BALANCED then that object would either be stationary (if it started at rest)or it would move at a constant speed (if it was already moving).

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Unbalanced Forces

The box and arrow diagram represents a car moving on a road.The engine provides the forward force of 12,000N.The road causes friction which works against the forward force. This OPPOSITIONAL force is 3,000N.

The UNBALANCED Force is equal to:Forward Force – Backward Force.

If the RESULTANT Force is positive then the car would accelerate FORWARD.If the RESULTANT Force is negative then the car would accelerate BACKWARD (or decelerate).

So our UNBALANCED Force for above:Forward Force – Backward Force12,000N - 3,000N = 9,000N Forward (Recall this has magnitude AND direction so is a VECTOR quantity)

Note that the arrows are to scale; 12,000N is 4x larger than 3,000N so the arrow must be 4x larger too.

12,000N3,000N

Positive so accelerating FORWARD

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Questions1. John plays badminton. He hits the shuttle with a higher Force. What happens

to its motion?2. A football and a tennis ball are kicked with equal Force. Which goes further?

Why?3. A bus has a mass of 32,000kg. What Force is required to accelerate it to 4m/s2?4. Why do skydivers use parachutes as they land?5. A tank of mass 50,000kg has a resultant forward force acting on it of 25,000N.

What is its acceleration?6. Why do you think there are rules about the maximum mass of footballs in

competitions?7. Olivia is driving on a road at 45m/s. She removes her foot from the accelerator

pedal and slows down to 15m/s in 10 seconds. Calculate her deceleration. If the mass of her car was 1,800kg what was

the size of the resistive force acting on the car?

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Questions1. John plays badminton. He hits the shuttle with a higher Force. What happens to its motion?

It accelerates faster2. A football and a tennis ball are kicked with equal Force. Which goes further? Why? Tennis ball

as it has a smaller mass3. A bus has a mass of 32,000kg. What Force is required to accelerate it to 4m/s2? F = ma so

F = 32,000kg x 4m/s2 = 128,000N4. Why do skydivers use parachutes as they land? Increase oppositional force so their

resultant force decreases. This means their acceleration decreases so they fall slower. (No need to discuss mass here as mass does not change)

5. A tank of mass 50,000kg has a resultant forward force acting on it of 25,000N. What is its acceleration? Re-arrange F=ma to make a the object: a= F / m = 25,000N / 50,000kg = 0.5m/s2

6. Why do you think there are rules about the maximum mass of footballs in competitions? Bigger mass means more Force needed for same acceleration. This could cause an injury to the footballer kicking the ball. On the other hand less mass means at the same Force the ball would have a higher acceleration. This could cause an injury if it hit someone. (Discuss small and large mass in answer)

7. Olivia is driving on a road at 45m/s. She removes her foot from the accelerator pedal and slows down to 15m/s in 10 seconds. Calculate her deceleration. Acceleration = -3m/s2(Ensure you got the – sign) If the mass of her car was 1,800kg what was the size of the resistive force acting on the car? F = ma so F= 1,800kg x -3m/s2 = -5,400N (The – sign tells us it was an OPPOSITIONAL Force)

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