Physics Y7 & Y8 Forces and Motion

Question 1

What is acceleration?

Answer 1

Acceleration is the rate at which an object speeds up.

Question 2

How do you calculate speed, distance or time if you
know two of them?

Answer 2

Speed can be calculated using the formula, speed = distance / time
Time can be calculated using the formula time = distance / speed
Distance can be calculated using the formula distance = speed x time

Question 3

What is deceleration?

Answer 3

Deceleration is the rate at which an object slows down.

Question 4

What is the unit for speed?

Answer 4

If distance is in metres and time in seconds then the unit for speed is m/s
If distance is in kilometres and time in hours then the unit for speed is km/h

Question 5

What is the unit for acceleration?

Answer 5

The unit of acceleration is m/s/s (metres per second per second).

Question 6

What does a distance time graph tell us?

Answer 6

A straight line tells us the object is moving with constant speed. The gradient of a line on a
distance time graph tells us the speed. The steeper the line is on a distance time graph the
faster an object is moving. A flat line on a distance time graph tells us the object is
stationary.

Question 7

What does an acceleration of 1m/s/s mean?

Answer 7

An acceleration of 1 m/s/s tells us that every second an object’s speed increases by 1 m/s

Question 8

What is a contact force?

Answer 8

A contact force occurs when two objects touch each other. Friction, normal contact force, and air resistance are examples of contact forces.

Question 9

What does a speed time graph tell us?

Answer 9

A straight line tells us that the object is moving with constant acceleration. The steeper the line on a speed time graph the larger the object’s acceleration. A flat line on a speed-time graph tells us the object is moving at a constant speed.

Question 10

What is a non-contact force?

Answer 10

A non-contact force does not require two objects to touch each other. Gravitational and
magnetic forces are examples of non-contact forces.

Question 11

What happens to an object when a resultant force acts on it in the direction it is moving?

Answer 11

When a resultant force acts on an object in the direction it is moving that object
accelerates and its kinetic store increases.

Question 12

What is the normal contact force?

Answer 12

The normal contact force (sometimes known as reaction force) is the push up of the ground on the object in contact with it

Question 13

What is the relationship between the size of an object’s acceleration and the resultant force acting on it?

Answer 13

The larger the resultant force acting on an object the larger that object’s acceleration. Acceleration is directly proportional to resultant force. This means that if resultant force doubles acceleration doubles.

Question 14

What is a magnetic field?

Answer 14

A magnetic field is a region around a magnet where magnetic materials experience magnetic forces

Question 15

What does a graph of acceleration against resultant force look like?

Answer 15

A graph of acceleration against resultant force will be a straight line through the origin.

Question 16

What happens to an object when a resultant force acts on it in the opposite direction to its motion?

Answer 16

When a resultant force acts on an object in the opposite direction to motion that object will decelerate, its kinetic store will decrease and the thermal store of the surroundings
will increase.

Question 16

What does a magnetic field diagram tell us?

Answer 16

How close the field lines are on a magnetic field diagram tell us how strong the field is
(closer the lines, stronger the field) and the direction of the arrows tells us the direction a
North pole would move in if you put it in the field.

Question 17

How can you determine the shape of a magnetic
field experimentally?

Answer 17

The shape of the field can be determined by using a plotting compass. Place the compass
near to the magnet and put a dot on the page next to where the needle arrowhead points.
Then place the compass so that the tail of the needle is on the dot and place a new dot
where the compass arrowhead now points. Repeat until you go off the page or back round
to the magnet. Join the dots up smoothly. Repeat for a different starting point.

Question 18

When you push an object, and then let go, why does it slow down?

Answer 18

The force you used to push the object is a contact force so as soon as the object is out of
your hands there is no longer any contact and therefore no push force. In the absence of
friction this object would keep on moving at a steady speed because there would be no resultant force on it. In reality the object will slow down because there is a resultant force acting on it in the opposite direction to its motion; this is caused by friction between the object and the surface it is on.

Question 19

What is the Earth’s gravitational field?

Answer 19

The Earth’s gravitational field is the region around the Earth where objects will experience attractive forces towards the centre of the Earth.

Question 20

How do we stop moving vehicles?

Answer 20

When we stop moving objects such as cars, we use the brakes to apply a force on the vehicle in the opposite direction to its motion.

Question 21

How can you measure the gravitational force acting
on an object?

Answer 21

Gravitational forces on objects can be measured by freely suspending the object from a
newton meter and reading the scale

Question 22

Why do we stop moving vehicles over a longer period of time?

Answer 22

We try to apply the braking force over a longer period of time because this means the speed is reduced more slowly and this results in a smaller deceleration. Smaller decelerations require smaller resultant forces.

Question 23

How strong is the Earth’s gravitational field?

Answer 23

On the surface of the Earth the strength of the gravitational field is 10 N/kg. The Earth’s

gravitational field gets weaker as you move further above the surface.

Question 24

Why do objects moving through air experience air resistance?

Answer 24

When objects move through air there are air resistance forces that act on them because the object collides with air molecules.

Question 25

How can you measure the Earth’s gravitational field
strength?

Answer 25

Gravitational field strength can be determined by working out the gradient of a graph of
gravitational force against mass.

Question 26

Why do larger surface areas and faster speeds result in more air resistance?

Answer 26

The larger the surface area of the object and the faster it is moving the more air
molecules it will collide with per second and the larger the air resistance force it will experience.

Question 27

What is weight?

Answer 27

The gravitational force of attraction you experience because you are in the Earth’s
gravitational field is also known as your weight. Weight is a force and is measured in
Newtons (N).

Question 28

What forces act on a falling object?

Answer 28

Air resistance and weight.

Question 29

What is mass?

Answer 29

Mass is the amount of material that makes you up and is measured in kg.

Question 30

What is terminal velocity?

Answer 30

A falling object will accelerate to start with but then reaches a steady speed that we know as terminal velocity.

Question 31

How can you calculate an object’s weight?

Answer 31

Your weight on Earth can be calculated by using the following formula Weight = mass x
gravitational field strength (where the gravitational field strength on Earth is 10 N/kg)

Question 32

Why do falling objects reach terminal velocity?

Answer 32

An object dropped from a height accelerates downwards to start with because its weight
is the only force acting on it and therefore there is a resultant force downwards.
However, it will not accelerate forever as the air resistance force acting on it increases as it gets faster. This reduces the resultant force acting on the object and makes its acceleration smaller. Eventually the air resistance force will equal the weight and at
this point there will be no resultant force so the object cannot accelerate any further – the object has reached a steady speed known as terminal velocity.

Question 33

What are the rules for drawing force diagrams?

Answer 33

When drawing force diagrams force arrows should always start on the object; the size of
the arrow tells us the size of the force and the direction tells us which way the force acts.

Question 34

What is a resultant force?

Answer 34

If two (or more) forces act on an object then these can be replaced by a single force arrow that we call the “resultant force”.

Question 35

How do you calculate the size of a resultant force?

Answer 35

If the forces act in the same direction we add the forces to work out the size of the
resultant force. If the forces act in opposite directions then we take the smallest one away
from the biggest to work out the size of the resultant force.

Question 36

What is Newton’s first law?

Answer 36

Newton’s first law tells us that if no resultant force acts on an object then that object’s
motion will not change; it will either stay still or, if it is already moving, then it will carry on
moving at a steady speed. If there is a resultant force acting on an object then the object’s
motion will change.

Question 37

What happens to an object’s motion if a resultant
force acts on it?

Answer 37

A resultant force in the direction in which an object is moving will cause it to speed up
(accelerate). A resultant force in the opposite direction to the way an object is moving will
cause it to slow down.