2. The Causes of Motion
• Galileo developed the concept of inertia.
• Inertia is that property that causes a body
to RESIST changes in its state of motion.
• Inertia depends on mass.
• If no net force acts on an object, it stays at
a constant velocity.
• Constant velocity can be 0 m/s too.
3. Newton’s 1 Law of Motion
st
• In the absence of a resultant external
force acting upon it, a body will remain at
rest or in uniform rectilinear motion (its
acceleration will be zero).
• It takes an unbalanced force acting on an
object to change its state of rest or motion.
• E.g. an apple on a table has no net force.
• E.g. an apple dropping off the table has a
net force acting on it.
4. Exam Question
A block glides on a perfectly smooth surface.
• v = constant
The block is subjected to the same force (F) in four different directions as shown below.
In all four cases, the force (F) is less than the weight of the block.
F F F
F
Case 1 Case 2 Case 3 Case 4
In which case will the force (F) cause a change in the speed of the block?
A) In cases 1 and 2
B) In cases 2 and 4
C) In cases 3 and 4
D) In all four cases
5. Exam Question
Which of the following situations show an object or person that is not acted on by a
resulting force?
1. A cyclist slowing down
2. Someone sitting on a park bench
3. A sign fixed to the wall
4. A truck moving away after stopping at a stop sign
5. A rocket moving at a constant speed in space
A) Situations 1, 2 and 4
B) Situations 1, 3 and 5
C) Situations 2, 3 and 5
D) None of these situations
6. Exam Question
The diagram below represents the trajectory of an object moving to the right on a
horizontal plane. Each circle represents the position of the object at 0.05 s intervals.
A
A force acts upon the object at point A. What was the effect of this force on its speed?
A) It only changed the speed of the object.
B) It only changed the direction of the object.
C) It changed the speed and the direction of the object.
D) It had no effect on the speed of the object.
9. Newton’s 2nd Law of Motion
• If an unbalanced force acts on an object,
the object accelerates in the direction of
the force.
• The acceleration varies inversely with the
mass.
• The acceleration varies directly with the
force.
• F = ma.
• 1 N is the force required to accelerate 1 kg
by 1m/s2.
11. ♫ ♪ I’m Free Falling…♫
• The acceleration of
both elephants due to
gravity is the same.
12. Activity
• What is the acceleration of a 70 kg skater
pushed by a force of 161 N [W]?
• F = ma or a = F/m = 161 N [W]/ 70 kg
• a = 2.3 m/s2 [W]
13. Exam Question
A net force "F" causes a mass m1 of 4.0 kg to accelerate at a rate of 6.0 m/s2. This same
force "F" causes a second mass m2 to accelerate at 8.0 m/s2.
What is the value of mass m2?
A) 2.5 kg
B) 3.0 kg
C) 3.5 kg
D) 5.3 kg
14. Exam Question
The diagram below shows a cart with mass M acted on by a force F which produces an
acceleration a.
F
Situation I M
A second cart with a mass of 5M is attached to the first, and a force three times bigger
than the first acts on the system.
The diagram below shows this situation.
3 F
Situation II 5 M M
How will the acceleration in Situation II compare with that of Situation I?
A) It must be 3 times is large.
B) It must be 2 times is large.
C) It must be 2 times smaller.
D) It must be 3 times smaller.
15. Exam Question
In the following mechanical system, block 1 has a mass of 200 g. As it falls, it drags cart 2 along
the table top horizontally. The cart has a mass of 300 g. A string attaches the block to the cart
over the pulley. Disregard friction and the mass of the string and the pulley.
What is the acceleration of the block?
A) 2.0 m/s2
B) 4.0 m/s2
C) 6.0 m/s2
D) 10.0 m/s2
18. Newton’s 3 Law of Motion
rd
• Newton’s 3rd Law of Motion: For every
action force, there is an equal and opposite
reaction force.
• OR If object A exerts a force on object B,
then object B exerts a force equal in
magnitude but opposite in direction on
object A.
• OR You cannot touch someone else without
being touched in return.
• OR Forces exist in pairs.
20. Examples of Reaction Forces
• E.g. A tractor exerts a force of 2000 N [W]
on a trailer. The trailer exerts a force of
2000 N [E] on the tractor.
• When we push on a wall it pushes back.
• When we kick a football, the same force
kicks back on us. Our foot decelerates a
bit because our mass is greater. The
football accelerates a lot because its mass
is small.
22. Exam Question
A car with a mass of 1000 kg and moving at a speed of 30 m/s comes to rest over a
distance of 100 metres.
What is the force of friction (acting on the wheels of the car) which causes the car to
stop?
A) 3 000 N
B) 3 500 N
C) 4 000 N
D) 4 500 N
24. Summary
• Newton’s 1st Law of Motion: If no net force
acts on an object, it maintains its state of rest
or its constant speed in a straight line.
• Newton’s 2nd Law of Motion: If an unbalance
force acts on an object, the object
accelerates in the direction of the force. The
acceleration varies inversely with the mass
and directly with the force. F = ma.
• Newton’s 3rd Law of Motion: For every action
force, there is an equal and opposite reaction
force.
