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Secondary Three Physics SSP 3: Dynamics

Name: …………………………………………… ( ) Class: …………… Date: ……………………….….

SSP Three Dynamics types of forces

free-body diagrams

weight normal friction tension

push pull air resistance engine force

book resting on table metal ball dropped from a height

football kicked and travelling upwards

arrow shot from a bow travelling horizontally

log being dragged through mud circular fluorescent tube suspended by two strings

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newton’s first

when an object is at rest or moving with uniform/constant velocity, the vector sum (add the arrows) of all the forces acting on the object must be a big fat zero.

newton’s second when a net or resultant force is applied to an object, it accelerates (speeds up or slows down).

in general, we can label the forces as shown below:

the relationship between the forces above is:

resultant force Fnet =

newton’s third (not very important)

if object A exerts a force on object B, then object B exerts an equal and opposite force on object A. the action—reaction pairs act on different objects!

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box at rest v = 0

box moving with uniform velocity v = constant

frictional force applied force resultant force opposing force

booktable

earth

fat man


1. An aircraft, flying at a constant height, is gaining speed. The four forces acting are L lift due to the wings R air resistance T thrust due to the engines W weight of the aircraft

Which is correct?

2. A block of mass 6 kg is pulled across a rough surface by a 54 N force, against a friction force F.

The acceleration of the block is 6 m/s2. What is the value of F?

A 9 N B 18 N C 36 N D 54 N

3. A person just supports a mass of 20 kg suspended from a rope. What is the resultant force acting on the mass?

A 0 N B 10 N C 20 N D 200 N

4. A car is accelerating along a road in the direction shown. The wheel shown is connected to the engine. In which direction is the force of friction exerted by the road on the car tyre?

vertical forces horizontal forces

A L = W T = R

B L > W T > R

C L = W T > R

D L > W T = R

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5. The diagram shows a car moving forwards. Four of the forces acting on it are P, Q, R, and S.

Which force, or forces, when increased in value, would cause an increase in speed?

A P and S B Q only C R only D R and Q

6. A car is travelling at constant speed along a road, and drives onto a large patch of oil. The driver applies the brakes to stop the car.

Compared to braking on a dry road, the car ………………………………. . A slows down more quickly because of the reduced friction between the tyres and the road. B speeds up because of the reduced friction between the tyres and the road. C takes longer to slow down because of the reduced friction between the tyres and the road. D takes longer to slow down because of the greater friction between the tyres and the road.

7. A heavy sack is suspended from a ceiling by a rope. The sack is then pulled to one side and held stationary.

Which diagram represents the forces acting on the sack in its new position?

A B C D

8. A trolley weighing 5.0 N is pulled along a level bench by a horizontal force of 10 N. The force of friction acting on the wheels of the trolley is 2.0 N. What is the size of the resultant force causing the trolley to accelerate?

A 3.0 N B 8.0 N C 10 N D 13 N

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9. A bus has a total mass of 12 000 kg. It moves along a horizontal stretch of road at a speed of 10 m/s. It then accelerates, reaching a final speed of 30 m/s after 16 s.

What is the size of the average resultant force acting on teh bus when it is accelerating?

A 7500 N B 9600 N C 15 000 N D 22 500 N

10. A small submarine of mass 1000 kg sinks in water with a uniform speed of 2 m/s. What is the resultant force exerted on the submarines it sinks?

A 0 N B 500 N C 2000 N D 10 000 N

11. The diagram shows a runner’s shoe at the start of a race. Which arrow indicates the direction of the force of friction acting on the runner’s shoe?

12. An object of weight W is pulled along a rough, horizontal surface by a force P. The force of friction is F. Which diagram correctly shows the directions of these forces on the object?

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13. A block of wood is at rest on a sloping ramp. The forces acting on the block are the weight W of the block, the friction F between the block and the ramp, and the normal force N exerted by the ramp. Which free-body diagram is correct?

8. A rock of mass 1500 kg lies on a flat grassy surface, and is pulled by a tractor using a rope. The tractor exerts a force of 5000 N, and the frictional force opposing the motion has a magnitude of 500 N. Find the acceleration of the rock.

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